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  5. PLANETARY VISION OUTER SPACE PUZZLE Ravensburger 1000 jigsaw 27" x 20" Germany

PLANETARY VISION OUTER SPACE PUZZLE Ravensburger 1000 jigsaw 27" x 20" Germany 1 of 3Only 1 left — 6 watchersPLANETARY VISION OUTER SPACE PUZZLE Ravensburger 1000 jigsaw 27" x 20" Germany 2 2 of 3PLANETARY VISION OUTER SPACE PUZZLE Ravensburger 1000 jigsaw 27" x 20" Germany 3 3 of 3 See More
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PLANETARY VISION OUTER SPACE PUZZLE Ravensburger 1000 jigsaw 27" x 20" Germany

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Seller: sidewaysstairsco ✉️ (1,183) 100%, Location: Santa Ana, California, US, Ships to: US & many other countries, Item: 203601147247 PLANETARY VISION OUTER SPACE PUZZLE Ravensburger 1000 jigsaw 27" x 20" Germany. Check out my other new & used items>>>>>>HERE! (click me) FOR SALE: A high-quality puzzle featuring part of our universe 2018 RAVENSBURGER "PLANETARY VISION" JIGSAW PUZZLE DETAILS: Challenge yourself with this 1000 piece Ravensburger puzzle! When you assemble the pieces you reveal a "Planetary Vision" of our solar system, complete with planets, stars, the sun and other elements of the Milky Way Galaxy. The puzzle features Earth surrounded by Jupiter, Saturn, Mars, Venus, Neptune, moons, and stars. "Planetary Vision" shows that space is filled with fantastic colors and textures. The art for this out-of-this-world puzzle was originally titled "Firmament" and was created by digital artist Adrian Chesterman.  Chesterman is known for creating the art for the world’s largest jigsaw puzzle (commissioned by Educa Boras) which had 33,600 total pieces. This 1008 (to be exact) piece puzzle is a challenging and satisfying puzzling experience for most. A quality product from Germany! Original Ravensburger Quality jigsaw puzzles are premium grade puzzles crafted in Germany. The pieces are steel-cut for a perfect, interlocking fit with no tearing. Each piece in the puzzle is unique.  The image is printed on linen-structured paper that reduces glare.  The extra thick cardboard paper ensures that your puzzle will last for years. Piece Count: 1008 Size: 27 x 20 inches Made In: Germany Catalog Number: No. 19 858 0 CONDITION: In excellent, pre-owned condition. Please see photos.  *To ensure safe delivery all items are carefully packaged before shipping out.* THANK YOU FOR LOOKING. QUESTIONS? JUST ASK. *ALL PHOTOS AND TEXT ARE INTELLECTUAL PROPERTY OF SIDEWAYS STAIRS CO. ALL RIGHTS RESERVED.* "Ravensburger AG is a German game and toy company, publishing house and market leader in the European jigsaw puzzle market..... History The company was founded by Otto Robert Maier with seat in Ravensburg, a town in Upper Swabia in southern Germany. He began publishing in 1883 with his first author contract. He started publishing instruction folders for craftsmen and architects, which soon acquired him a solid financial basis. His first board game appeared in 1884, named "Journey around the world". At the turn of the 20th century, his product line broadened to include picture books, books, children’s activity books, Art Instruction manuals, non-fiction books, and reference books as well as children’s games, Happy Families and activity kits. In 1900, the Ravensburger blue triangle trademark was registered with the Imperial Patent office. As of 1912, many board and activity games had an export version that was distributed to Western Europe, the countries of the Danube Monarchy as well as Russia. Before the First World War, Ravensburger had around 800 products. The publishing house was damaged during the Second World War and continued to produce games in the years of the reconstruction. The company focused on children's games and books and specialized books for art, architecture and hobbies, and from 1962 grew strongly. The company started to produce jigsaw puzzle games in 1964, and in the same year opened subsidiaries in Austria, France, Italy, the Netherlands, Switzerland and the United Kingdom. In 1977 the company split into a book publishing arm and a game publishing arm. Today there are approximately 1800 available books and 850 games as well as puzzles, hobby products and CD-ROM titles at Ravensburger and its subsidiaries, which include Alea for "hobby and ardent game players" and F.X. Schmid for games, playing cards and children's books. Ravensburger products are exported to more than fifty countries. Ravensburger also expanded to video games in the late 1990s by forming Ravensburger Interactive, which they sold in May 2002 to JoWooD Productions. Under the label, F.X. Schmid, Ravensburger produce one of the only two packs of true Tarock cards in Germany: a 54-card pack of the Tarot Nouveau pattern with genre scenes and used for playing the Tarot game of Cego popular in the Black Forest region. In September 2010, Ravensburger broke Educa's record for the world's largest jigsaw puzzle of 24,000 pieces.[1] Ravensburger's new puzzle design by late pop artist Keith Haring titled, "Keith Haring: Double Retrospect" breaks the Guinness Book of World Records measuring 17' × 6' (5.18 m x 1.82 m) built from 32,256 pieces and comes with its own dolly cart for toting. Currently the largest commercial puzzle is Kodak's "27 Wonders from Around The World".[2][3] Ravensburger's currently largest puzzle is "Memorable Disney Moments" with 40,320 pieces.[4] Swedish toy train company BRIO was acquired by the Ravensburger Group on 8 January 2015.[5] In 2017, Ravensburger acquired American game company Wonder Forge.[6] The company's North American division, Ravensburger NA, is based in Seattle and releases approximately 25 games per year, the most successfully of which so far is Villainous, based on various Disney properties.[7] Ravensbuger NA sold about 3 million copies of games in 2018.[7] Notable board games Games sold under the "Ravensburger" imprint:     Dingbats     Emoji     Enchanted Forest     Havannah     Java     Journey through Europe     Know Interactive Board Game     Labyrinth (board game)     Make 'n' Break     Malefiz     Mexica     The Name of the Rose (2008)[8]     Nobody is perfect     Quest     Reversi     Rivers, Roads & Rails     Scotland Yard     Star Wars     Tactil     Take It Easy     Tikal     Top Secret Spies     Villainous     What Do You Hear? Games sold under the "Alea" label:     Broom Service     Castles of Burgundy     Chinatown     Las Vegas     Princes of Florence     Puerto Rico     Ra     San Juan Games sold under the F.X. Schmid label:     Auf Achse     Torres Games sold under the "Ravensburger Digital" label:     Concentration in various editions" (wikipedia.org) "Outer space is the expanse that exists beyond Earth and between celestial bodies. Outer space is not completely empty—it is a hard vacuum containing a low density of particles, predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, neutrinos, dust, and cosmic rays. The baseline temperature of outer space, as set by the background radiation from the Big Bang, is 2.7 kelvins (−270.45 °C; −454.81 °F).[1] The plasma between galaxies is thought to account for about half of the baryonic (ordinary) matter in the universe, having a number density of less than one hydrogen atom per cubic metre and a temperature of millions of kelvins.[2] Local concentrations of matter have condensed into stars and galaxies. Studies indicate that 90% of the mass in most galaxies is in an unknown form, called dark matter, which interacts with other matter through gravitational but not electromagnetic forces.[3][4] Observations suggest that the majority of the mass-energy in the observable universe is dark energy, a type of vacuum energy that is poorly understood.[5][6] Intergalactic space takes up most of the volume of the universe, but even galaxies and star systems consist almost entirely of empty space. Outer space does not begin at a definite altitude above the Earth's surface. The Kármán line, an altitude of 100 km (62 mi) above sea level,[7][8] is conventionally used as the start of outer space in space treaties and for aerospace records keeping. The framework for international space law was established by the Outer Space Treaty, which entered into force on 10 October 1967. This treaty precludes any claims of national sovereignty and permits all states to freely explore outer space. Despite the drafting of UN resolutions for the peaceful uses of outer space, anti-satellite weapons have been tested in Earth orbit. Humans began the physical exploration of space during the 20th century with the advent of high-altitude balloon flights. This was followed by crewed rocket flights and, then, crewed Earth orbit, first achieved by Yuri Gagarin of the Soviet Union in 1961. Due to the high cost of getting into space, human spaceflight has been limited to low Earth orbit and the Moon. On the other hand, uncrewed spacecraft have reached all of the known planets in the Solar System. Outer space represents a challenging environment for human exploration because of the hazards of vacuum and radiation. Microgravity also has a negative effect on human physiology that causes both muscle atrophy and bone loss. In addition to these health and environmental issues, the economic cost of putting objects, including humans, into space is very high.... Formation and state This is an artist's concept of the metric expansion of space, where a volume of the Universe is represented at each time interval by the circular sections. At left is depicted the rapid inflation from the initial state, followed thereafter by steadier expansion to the present day, shown at right. Main article: Big Bang The size of the whole universe is unknown, and it might be infinite in extent.[9] According to the Big Bang theory, the very early Universe was an extremely hot and dense state about 13.8 billion years ago[10] which rapidly expanded. About 380,000 years later the Universe had cooled sufficiently to allow protons and electrons to combine and form hydrogen—the so-called recombination epoch. When this happened, matter and energy became decoupled, allowing photons to travel freely through the continually expanding space.[11] Matter that remained following the initial expansion has since undergone gravitational collapse to create stars, galaxies and other astronomical objects, leaving behind a deep vacuum that forms what is now called outer space.[12] As light has a finite velocity, this theory also constrains the size of the directly observable universe.[11] The present day shape of the universe has been determined from measurements of the cosmic microwave background using satellites like the Wilkinson Microwave Anisotropy Probe. These observations indicate that the spatial geometry of the observable universe is "flat", meaning that photons on parallel paths at one point remain parallel as they travel through space to the limit of the observable universe, except for local gravity.[13] The flat Universe, combined with the measured mass density of the Universe and the accelerating expansion of the Universe, indicates that space has a non-zero vacuum energy, which is called dark energy.[14] Estimates put the average energy density of the present day Universe at the equivalent of 5.9 protons per cubic meter, including dark energy, dark matter, and baryonic matter (ordinary matter composed of atoms). The atoms account for only 4.6% of the total energy density, or a density of one proton per four cubic meters.[15] The density of the Universe is clearly not uniform; it ranges from relatively high density in galaxies—including very high density in structures within galaxies, such as planets, stars, and black holes—to conditions in vast voids that have much lower density, at least in terms of visible matter.[16] Unlike matter and dark matter, dark energy seems not to be concentrated in galaxies: although dark energy may account for a majority of the mass-energy in the Universe, dark energy's influence is 5 orders of magnitude smaller than the influence of gravity from matter and dark matter within the Milky Way.[17] Environment See also: Planetary habitability A black background with luminous shapes of various sizes scattered randomly about. They typically have white, red or blue hues. Part of the Hubble Ultra-Deep Field image showing a typical section of space containing galaxies interspersed by deep vacuum. Given the finite speed of light, this view covers the past 13 billion years of the history of outer space. Outer space is the closest known approximation to a perfect vacuum. It has effectively no friction, allowing stars, planets, and moons to move freely along their ideal orbits, following the initial formation stage. The deep vacuum of intergalactic space is not devoid of matter, as it contains a few hydrogen atoms per cubic meter.[18] By comparison, the air humans breathe contains about 1025 molecules per cubic meter.[19][20] The low density of matter in outer space means that electromagnetic radiation can travel great distances without being scattered: the mean free path of a photon in intergalactic space is about 1023 km, or 10 billion light years.[21] In spite of this, extinction, which is the absorption and scattering of photons by dust and gas, is an important factor in galactic and intergalactic astronomy.[22] Stars, planets, and moons retain their atmospheres by gravitational attraction. Atmospheres have no clearly delineated upper boundary: the density of atmospheric gas gradually decreases with distance from the object until it becomes indistinguishable from outer space.[23] The Earth's atmospheric pressure drops to about 0.032 Pa at 100 kilometres (62 miles) of altitude,[24] compared to 100,000 Pa for the International Union of Pure and Applied Chemistry (IUPAC) definition of standard pressure. Above this altitude, isotropic gas pressure rapidly becomes insignificant when compared to radiation pressure from the Sun and the dynamic pressure of the solar wind. The thermosphere in this range has large gradients of pressure, temperature and composition, and varies greatly due to space weather.[25] The temperature of outer space is measured in terms of the kinetic activity of the gas, as it is on Earth. The radiation of outer space has a different temperature than the kinetic temperature of the gas, meaning that the gas and radiation are not in thermodynamic equilibrium.[26][27] All of the observable universe is filled with photons that were created during the Big Bang, which is known as the cosmic microwave background radiation (CMB). (There is quite likely a correspondingly large number of neutrinos called the cosmic neutrino background.[28]) The current black body temperature of the background radiation is about 3 K (−270 °C; −454 °F).[29] The gas temperatures in outer space can vary widely. For example, the temperature in the Boomerang Nebula is 1 K,[30] while the solar corona reaches temperatures over 1.2–2.6 million K.[31] Magnetic fields have been detected in the space around just about every class of celestial object. Star formation in spiral galaxies can generate small-scale dynamos, creating turbulent magnetic field strengths of around 5–10 μG. The Davis–Greenstein effect causes elongated dust grains to align themselves with a galaxy's magnetic field, resulting in weak optical polarization. This has been used to show ordered magnetic fields exist in several nearby galaxies. Magneto-hydrodynamic processes in active elliptical galaxies produce their characteristic jets and radio lobes. Non-thermal radio sources have been detected even among the most distant, high-z sources, indicating the presence of magnetic fields.[32] Outside a protective atmosphere and magnetic field, there are few obstacles to the passage through space of energetic subatomic particles known as cosmic rays. These particles have energies ranging from about 106 eV up to an extreme 1020 eV of ultra-high-energy cosmic rays.[33] The peak flux of cosmic rays occurs at energies of about 109 eV, with approximately 87% protons, 12% helium nuclei and 1% heavier nuclei. In the high energy range, the flux of electrons is only about 1% of that of protons.[34] Cosmic rays can damage electronic components and pose a health threat to space travelers.[35] According to astronauts, like Don Pettit, space has a burned/metallic odor that clings to their suits and equipment, similar to the scent of an arc welding torch.[36][37] Effect on biology and human bodies See also: Astrobiology, Astrobotany, Plants in space, Animals in space, Effect of spaceflight on the human body, Bioastronautics, and Weightlessness The lower half shows a blue planet with patchy white clouds. The upper half has a man in a white spacesuit and maneuvering unit against a black background. Because of the hazards of a vacuum, astronauts must wear a pressurized space suit while off-Earth and outside their spacecraft. Despite the harsh environment, several life forms have been found that can withstand extreme space conditions for extended periods. Species of lichen carried on the ESA BIOPAN facility survived exposure for ten days in 2007.[38] Seeds of Arabidopsis thaliana and Nicotiana tabacum germinated after being exposed to space for 1.5 years.[39] A strain of bacillus subtilis has survived 559 days when exposed to low-Earth orbit or a simulated martian environment.[40] The lithopanspermia hypothesis suggests that rocks ejected into outer space from life-harboring planets may successfully transport life forms to another habitable world. A conjecture is that just such a scenario occurred early in the history of the Solar System, with potentially microorganism-bearing rocks being exchanged between Venus, Earth, and Mars.[41] Even at relatively low altitudes in the Earth's atmosphere, conditions are hostile to the human body. The altitude where atmospheric pressure matches the vapor pressure of water at the temperature of the human body is called the Armstrong line, named after American physician Harry G. Armstrong. It is located at an altitude of around 19.14 km (11.89 mi). At or above the Armstrong line, fluids in the throat and lungs boil away. More specifically, exposed bodily liquids such as saliva, tears, and liquids in the lungs boil away. Hence, at this altitude, human survival requires a pressure suit, or a pressurized capsule.[42] Out in space, sudden exposure of an unprotected human to very low pressure, such as during a rapid decompression, can cause pulmonary barotrauma—a rupture of the lungs, due to the large pressure differential between inside and outside the chest.[citation needed] Even if the subject's airway is fully open, the flow of air through the windpipe may be too slow to prevent the rupture.[43] Rapid decompression can rupture eardrums and sinuses, bruising and blood seep can occur in soft tissues, and shock can cause an increase in oxygen consumption that leads to hypoxia.[citation needed] As a consequence of rapid decompression, oxygen dissolved in the blood empties into the lungs to try to equalize the partial pressure gradient. Once the deoxygenated blood arrives at the brain, humans lose consciousness after a few seconds and die of hypoxia within minutes.[44] Blood and other body fluids boil when the pressure drops below 6.3 kPa, and this condition is called ebullism.[45] The steam may bloat the body to twice its normal size and slow circulation, but tissues are elastic and porous enough to prevent rupture. Ebullism is slowed by the pressure containment of blood vessels, so some blood remains liquid.[46][47] Swelling and ebullism can be reduced by containment in a pressure suit. The Crew Altitude Protection Suit (CAPS), a fitted elastic garment designed in the 1960s for astronauts, prevents ebullism at pressures as low as 2 kPa.[48] Supplemental oxygen is needed at 8 km (5 mi) to provide enough oxygen for breathing and to prevent water loss, while above 20 km (12 mi) pressure suits are essential to prevent ebullism.[49] Most space suits use around 30–39 kPa of pure oxygen, about the same as on the Earth's surface. This pressure is high enough to prevent ebullism, but evaporation of nitrogen dissolved in the blood could still cause decompression sickness and gas embolisms if not managed.[50] Humans evolved for life in Earth gravity, and exposure to weightlessness has been shown to have deleterious effects on human health. Initially, more than 50% of astronauts experience space motion sickness. This can cause nausea and vomiting, vertigo, headaches, lethargy, and overall malaise. The duration of space sickness varies, but it typically lasts for 1–3 days, after which the body adjusts to the new environment. Longer-term exposure to weightlessness results in muscle atrophy and deterioration of the skeleton, or spaceflight osteopenia. These effects can be minimized through a regimen of exercise.[51] Other effects include fluid redistribution, slowing of the cardiovascular system, decreased production of red blood cells, balance disorders, and a weakening of the immune system. Lesser symptoms include loss of body mass, nasal congestion, sleep disturbance, and puffiness of the face.[52] During long-duration space travel, radiation can pose an acute health hazard. Exposure to high-energy, ionizing cosmic rays can result in fatigue, nausea, vomiting, as well as damage to the immune system and changes to the white blood cell count. Over longer durations, symptoms include an increased risk of cancer, plus damage to the eyes, nervous system, lungs and the gastrointestinal tract.[53] On a round-trip Mars mission lasting three years, a large fraction of the cells in an astronaut's body would be traversed and potentially damaged by high energy nuclei.[54] The energy of such particles is significantly diminished by the shielding provided by the walls of a spacecraft and can be further diminished by water containers and other barriers. The impact of the cosmic rays upon the shielding produces additional radiation that can affect the crew. Further research is needed to assess the radiation hazards and determine suitable countermeasures.[55] Regions Space is a partial vacuum: its different regions are defined by the various atmospheres and "winds" that dominate within them, and extend to the point at which those winds give way to those beyond. Geospace extends from Earth's atmosphere to the outer reaches of Earth's magnetic field, whereupon it gives way to the solar wind of interplanetary space.[56] Interplanetary space extends to the heliopause, whereupon the solar wind gives way to the winds of the interstellar medium.[57] Interstellar space then continues to the edges of the galaxy, where it fades into the intergalactic void.[58] Geospace The lower half is the blue-white planet in low illumination. Nebulous red streamers climb upward from the limb of the disk toward the black sky. The Space Shuttle is visible along the left edge. Aurora australis observed from the Space Shuttle Discovery, on STS-39, May 1991 (orbital altitude: 260 km) Geospace is the region of outer space near Earth, including the upper atmosphere and magnetosphere.[56] The Van Allen radiation belts lie within the geospace. The outer boundary of geospace is the magnetopause, which forms an interface between the Earth's magnetosphere and the solar wind. The inner boundary is the ionosphere.[59] The variable space-weather conditions of geospace are affected by the behavior of the Sun and the solar wind; the subject of geospace is interlinked with heliophysics—the study of the Sun and its impact on the planets of the Solar System.[60] The day-side magnetopause is compressed by solar-wind pressure—the subsolar distance from the center of the Earth is typically 10 Earth radii. On the night side, the solar wind stretches the magnetosphere to form a magnetotail that sometimes extends out to more than 100–200 Earth radii.[61][62] For roughly four days of each month, the lunar surface is shielded from the solar wind as the Moon passes through the magnetotail.[63] Geospace is populated by electrically charged particles at very low densities, the motions of which are controlled by the Earth's magnetic field. These plasmas form a medium from which storm-like disturbances powered by the solar wind can drive electrical currents into the Earth's upper atmosphere. Geomagnetic storms can disturb two regions of geospace, the radiation belts and the ionosphere. These storms increase fluxes of energetic electrons that can permanently damage satellite electronics, interfering with shortwave radio communication and GPS location and timing.[64] Magnetic storms can also be a hazard to astronauts, even in low Earth orbit. They also create aurorae seen at high latitudes in an oval surrounding the geomagnetic poles.[65] Although it meets the definition of outer space, the atmospheric density within the first few hundred kilometers above the Kármán line is still sufficient to produce significant drag on satellites.[66] This region contains material left over from previous crewed and uncrewed launches that are a potential hazard to spacecraft. Some of this debris re-enters Earth's atmosphere periodically.[67] Cislunar space Lunar Gateway, one of the planned space stations for crewed cislunar travel in the 2020s Earth's gravity keeps the Moon in orbit at an average distance of 384,403 km (238,857 mi). The region outside Earth's atmosphere and extending out to just beyond the Moon's orbit, including the Lagrange points, is sometimes referred to as cislunar space.[68] Deep space is defined by the United States government and others as any region beyond cislunar space.[69][70][71][72] The International Telecommunication Union responsible for radio communication (including satellites) defines the beginning of deep space at about 5 times that distance (2×106 km).[73] The region where Earth's gravity remains dominant against gravitational perturbations from the Sun is called the Hill sphere.[74] This extends into translunar space to a distance of roughly 1% of the mean distance from Earth to the Sun,[75] or 1.5 million km (0.93 million mi). Interplanetary space Main article: Interplanetary medium At lower left, a white coma stands out against a black background. Nebulous material streams away to the top and left, slowly fading with distance. The sparse plasma (blue) and dust (white) in the tail of comet Hale–Bopp are being shaped by pressure from solar radiation and the solar wind, respectively Interplanetary space is defined by the solar wind, a continuous stream of charged particles emanating from the Sun that creates a very tenuous atmosphere (the heliosphere) for billions of kilometers into space. This wind has a particle density of 5–10 protons/cm3 and is moving at a velocity of 350–400 km/s (780,000–890,000 mph).[76] Interplanetary space extends out to the heliopause where the influence of the galactic environment starts to dominate over the magnetic field and particle flux from the Sun.[57] The distance and strength of the heliopause varies depending on the activity level of the solar wind.[77] The heliopause in turn deflects away low-energy galactic cosmic rays, with this modulation effect peaking during solar maximum.[78] The volume of interplanetary space is a nearly total vacuum, with a mean free path of about one astronomical unit at the orbital distance of the Earth. This space is not completely empty, and is sparsely filled with cosmic rays, which include ionized atomic nuclei and various subatomic particles. There is also gas, plasma and dust,[79] small meteors, and several dozen types of organic molecules discovered to date by microwave spectroscopy.[80] A cloud of interplanetary dust is visible at night as a faint band called the zodiacal light.[81] Interplanetary space contains the magnetic field generated by the Sun.[76] There are also magnetospheres generated by planets such as Jupiter, Saturn, Mercury and the Earth that have their own magnetic fields. These are shaped by the influence of the solar wind into the approximation of a teardrop shape, with the long tail extending outward behind the planet. These magnetic fields can trap particles from the solar wind and other sources, creating belts of charged particles such as the Van Allen radiation belts. Planets without magnetic fields, such as Mars, have their atmospheres gradually eroded by the solar wind.[82] Interstellar space Main article: Interstellar medium "Interstellar space" redirects here. For the album, see Interstellar Space. Patchy orange and blue nebulosity against a black background, with a curved orange arc wrapping around a star at the center. Bow shock formed by the magnetosphere of the young star LL Orionis (center) as it collides with the Orion Nebula flow Interstellar space is the physical space within a galaxy beyond the influence each star has upon the encompassed plasma.[58] The contents of interstellar space are called the interstellar medium. Approximately 70% of the mass of the interstellar medium consists of lone hydrogen atoms; most of the remainder consists of helium atoms. This is enriched with trace amounts of heavier atoms formed through stellar nucleosynthesis. These atoms are ejected into the interstellar medium by stellar winds or when evolved stars begin to shed their outer envelopes such as during the formation of a planetary nebula.[83] The cataclysmic explosion of a supernova generates an expanding shock wave consisting of ejected materials that further enrich the medium.[84] The density of matter in the interstellar medium can vary considerably: the average is around 106 particles per m3,[85] but cold molecular clouds can hold 108–1012 per m3.[26][83] A number of molecules exist in interstellar space, as can tiny 0.1 μm dust particles.[86] The tally of molecules discovered through radio astronomy is steadily increasing at the rate of about four new species per year. Large regions of higher density matter known as molecular clouds allow chemical reactions to occur, including the formation of organic polyatomic species. Much of this chemistry is driven by collisions. Energetic cosmic rays penetrate the cold, dense clouds and ionize hydrogen and helium, resulting, for example, in the trihydrogen cation. An ionized helium atom can then split relatively abundant carbon monoxide to produce ionized carbon, which in turn can lead to organic chemical reactions.[87] The local interstellar medium is a region of space within 100 parsecs (pc) of the Sun, which is of interest both for its proximity and for its interaction with the Solar System. This volume nearly coincides with a region of space known as the Local Bubble, which is characterized by a lack of dense, cold clouds. It forms a cavity in the Orion Arm of the Milky Way galaxy, with dense molecular clouds lying along the borders, such as those in the constellations of Ophiuchus and Taurus. (The actual distance to the border of this cavity varies from 60 to 250 pc or more.) This volume contains about 104–105 stars and the local interstellar gas counterbalances the astrospheres that surround these stars, with the volume of each sphere varying depending on the local density of the interstellar medium. The Local Bubble contains dozens of warm interstellar clouds with temperatures of up to 7,000 K and radii of 0.5–5 pc.[88] When stars are moving at sufficiently high peculiar velocities, their astrospheres can generate bow shocks as they collide with the interstellar medium. For decades it was assumed that the Sun had a bow shock. In 2012, data from Interstellar Boundary Explorer (IBEX) and NASA's Voyager probes showed that the Sun's bow shock does not exist. Instead, these authors argue that a subsonic bow wave defines the transition from the solar wind flow to the interstellar medium.[89][90] A bow shock is the third boundary of an astrosphere after the termination shock and the astropause (called the heliopause in the Solar System).[90] Intergalactic space Structure of the Universe Matter distribution in a cubic section of the universe. The blue fiber structures represent the matter and the empty regions in between represent the cosmic voids of the intergalactic medium. A star-forming region in the Large Magellanic Cloud, perhaps the closest Galaxy to Earth's Milky Way Main articles: Warm–hot intergalactic medium, Intracluster medium, and Intergalactic dust Intergalactic space is the physical space between galaxies. Studies of the large scale distribution of galaxies show that the Universe has a foam-like structure, with groups and clusters of galaxies lying along filaments that occupy about a tenth of the total space. The remainder forms huge voids that are mostly empty of galaxies. Typically, a void spans a distance of (10–40) h−1 Mpc, where h is the Hubble constant in units of 100 km s−1 Mpc−1, or the dimensionless Hubble constant.[91] Surrounding and stretching between galaxies, there is a rarefied plasma[92] that is organized in a galactic filamentary structure.[93] This material is called the intergalactic medium (IGM). The density of the IGM is 5–200 times the average density of the Universe.[94] It consists mostly of ionized hydrogen; i.e. a plasma consisting of equal numbers of electrons and protons. As gas falls into the intergalactic medium from the voids, it heats up to temperatures of 105 K to 107 K,[2] which is high enough so that collisions between atoms have enough energy to cause the bound electrons to escape from the hydrogen nuclei; this is why the IGM is ionized. At these temperatures, it is called the warm–hot intergalactic medium (WHIM). (Although the plasma is very hot by terrestrial standards, 105 K is often called "warm" in astrophysics.) Computer simulations and observations indicate that up to half of the atomic matter in the Universe might exist in this warm–hot, rarefied state.[94][95][96] When gas falls from the filamentary structures of the WHIM into the galaxy clusters at the intersections of the cosmic filaments, it can heat up even more, reaching temperatures of 108 K and above in the so-called intracluster medium (ICM).[97] Earth orbit Main article: Geocentric orbit A spacecraft enters orbit when its centripetal acceleration due to gravity is less than or equal to the centrifugal acceleration due to the horizontal component of its velocity. For a low Earth orbit, this velocity is about 7,800 m/s (28,100 km/h; 17,400 mph);[98] by contrast, the fastest piloted airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was 2,200 m/s (7,900 km/h; 4,900 mph) in 1967 by the North American X-15.[99] To achieve an orbit, a spacecraft must travel faster than a sub-orbital spaceflight. The energy required to reach Earth orbital velocity at an altitude of 600 km (370 mi) is about 36 MJ/kg, which is six times the energy needed merely to climb to the corresponding altitude.[100] Spacecraft with a perigee below about 2,000 km (1,200 mi) are subject to drag from the Earth's atmosphere,[101] which decreases the orbital altitude. The rate of orbital decay depends on the satellite's cross-sectional area and mass, as well as variations in the air density of the upper atmosphere. Below about 300 km (190 mi), decay becomes more rapid with lifetimes measured in days. Once a satellite descends to 180 km (110 mi), it has only hours before it vaporizes in the atmosphere.[66] The escape velocity required to pull free of Earth's gravitational field altogether and move into interplanetary space is about 11,200 m/s (40,300 km/h; 25,100 mph).[102] Boundary For the boundary of the universe, see observable universe. A white rocketship with oddly-shaped wings at rest on a runway. SpaceShipOne completed the first human private spaceflight in 2004, reaching an altitude of 100.12 km (62.21 mi).[103] There is no clear boundary between Earth's atmosphere and space, as the density of the atmosphere gradually decreases as the altitude increases. There are several standard boundary designations, namely:     The Fédération Aéronautique Internationale has established the Kármán line at an altitude of 100 km (62 mi) as a working definition for the boundary between aeronautics and astronautics. This is used because at an altitude of about 100 km (62 mi), as Theodore von Kármán calculated, a vehicle would have to travel faster than orbital velocity to derive sufficient aerodynamic lift from the atmosphere to support itself.[7][8]     The United States designates people who travel above an altitude of 50 mi (80 km) as astronauts.[104]     NASA's Space Shuttle used 400,000 feet (122 km, 76 mi) as its re-entry altitude (termed the Entry Interface), which roughly marks the boundary where atmospheric drag becomes noticeable, thus beginning the process of switching from steering with thrusters to maneuvering with aerodynamic control surfaces.[105] In 2009, scientists reported detailed measurements with a Supra-Thermal Ion Imager (an instrument that measures the direction and speed of ions), which allowed them to establish a boundary at 118 km (73.3 mi) above Earth. The boundary represents the midpoint of a gradual transition over tens of kilometers from the relatively gentle winds of the Earth's atmosphere to the more violent flows of charged particles in space, which can reach speeds well over 268 m/s (600 mph).[106][107] Legal status Main article: Space law See also: Bogota Declaration At top, a dark rocket is emitting a bright plume of flame against a blue sky. Underneath, a column of smoke is partly concealing a navy ship. 2008 launch of the SM-3 missile used to destroy American reconnaissance satellite USA-193 The Outer Space Treaty provides the basic framework for international space law. It covers the legal use of outer space by nation states, and includes in its definition of outer space the Moon and other celestial bodies. The treaty states that outer space is free for all nation states to explore and is not subject to claims of national sovereignty, calling outer space the "province of all mankind". This status as a common heritage of mankind has been used, though not without opposition, to enforce the right to access and shared use of outer space for all nations equally, particularly non-spacefaring nations.[108] It also prohibits the development of nuclear weapons in outer space. The treaty was passed by the United Nations General Assembly in 1963 and signed in 1967 by the USSR, the United States of America and the United Kingdom. As of 2017, 105 state parties have either ratified or acceded to the treaty. An additional 25 states signed the treaty, without ratifying it.[109][110] Since 1958, outer space has been the subject of multiple United Nations resolutions. Of these, more than 50 have been concerning the international co-operation in the peaceful uses of outer space and preventing an arms race in space.[111] Four additional space law treaties have been negotiated and drafted by the UN's Committee on the Peaceful Uses of Outer Space. Still, there remains no legal prohibition against deploying conventional weapons in space, and anti-satellite weapons have been successfully tested by the US, USSR, China,[112] and in 2019, India.[113] The 1979 Moon Treaty turned the jurisdiction of all heavenly bodies (including the orbits around such bodies) over to the international community. The treaty has not been ratified by any nation that currently practices human spaceflight.[114] In 1976, eight equatorial states (Ecuador, Colombia, Brazil, Congo, Zaire, Uganda, Kenya, and Indonesia) met in Bogotá, Colombia. With their "Declaration of the First Meeting of Equatorial Countries", or "the Bogotá Declaration", they claimed control of the segment of the geosynchronous orbital path corresponding to each country.[115] These claims are not internationally accepted.[116] Discovery, exploration and applications See also: Space science Discovery In 350 BCE, Greek philosopher Aristotle suggested that nature abhors a vacuum, a principle that became known as the horror vacui. This concept built upon a 5th-century BCE ontological argument by the Greek philosopher Parmenides, who denied the possible existence of a void in space.[117] Based on this idea that a vacuum could not exist, in the West it was widely held for many centuries that space could not be empty.[118] As late as the 17th century, the French philosopher René Descartes argued that the entirety of space must be filled.[119] In ancient China, the 2nd-century astronomer Zhang Heng became convinced that space must be infinite, extending well beyond the mechanism that supported the Sun and the stars. The surviving books of the Hsüan Yeh school said that the heavens were boundless, "empty and void of substance". Likewise, the "sun, moon, and the company of stars float in the empty space, moving or standing still".[120] The Italian scientist Galileo Galilei knew that air had mass and so was subject to gravity. In 1640, he demonstrated that an established force resisted the formation of a vacuum. It would remain for his pupil Evangelista Torricelli to create an apparatus that would produce a partial vacuum in 1643. This experiment resulted in the first mercury barometer and created a scientific sensation in Europe. The French mathematician Blaise Pascal reasoned that if the column of mercury was supported by air, then the column ought to be shorter at higher altitude where the air pressure is lower.[121] In 1648, his brother-in-law, Florin Périer, repeated the experiment on the Puy de Dôme mountain in central France and found that the column was shorter by three inches. This decrease in pressure was further demonstrated by carrying a half-full balloon up a mountain and watching it gradually expand, then contract upon descent.[122] A glass display case holds a mechanical device with a lever arm, plus two metal hemispheres attached to draw ropes The original Magdeburg hemispheres (lower left) used to demonstrate Otto von Guericke's vacuum pump (right) In 1650, German scientist Otto von Guericke constructed the first vacuum pump: a device that would further refute the principle of horror vacui. He correctly noted that the atmosphere of the Earth surrounds the planet like a shell, with the density gradually declining with altitude. He concluded that there must be a vacuum between the Earth and the Moon.[123] Back in the 15th century, German theologian Nicolaus Cusanus speculated that the Universe lacked a center and a circumference. He believed that the Universe, while not infinite, could not be held as finite as it lacked any bounds within which it could be contained.[124] These ideas led to speculations as to the infinite dimension of space by the Italian philosopher Giordano Bruno in the 16th century. He extended the Copernican heliocentric cosmology to the concept of an infinite Universe filled with a substance he called aether, which did not resist the motion of heavenly bodies.[125] English philosopher William Gilbert arrived at a similar conclusion, arguing that the stars are visible to us only because they are surrounded by a thin aether or a void.[126] This concept of an aether originated with ancient Greek philosophers, including Aristotle, who conceived of it as the medium through which the heavenly bodies move.[127] The concept of a Universe filled with a luminiferous aether retained support among some scientists until the early 20th century. This form of aether was viewed as the medium through which light could propagate.[128] In 1887, the Michelson–Morley experiment tried to detect the Earth's motion through this medium by looking for changes in the speed of light depending on the direction of the planet's motion. The null result indicated something was wrong with the concept. The idea of the luminiferous aether was then abandoned. It was replaced by Albert Einstein's theory of special relativity, which holds that the speed of light in a vacuum is a fixed constant, independent of the observer's motion or frame of reference.[129][130] The first professional astronomer to support the concept of an infinite Universe was the Englishman Thomas Digges in 1576.[131] But the scale of the Universe remained unknown until the first successful measurement of the distance to a nearby star in 1838 by the German astronomer Friedrich Bessel. He showed that the star system 61 Cygni had a parallax of just 0.31 arcseconds (compared to the modern value of 0.287″). This corresponds to a distance of over 10 light years.[132] In 1917, Heber Curtis noted that novae in spiral nebulae were, on average, 10 magnitudes fainter than galactic novae, suggesting that the former are 100 times further away.[133] The distance to the Andromeda Galaxy was determined in 1923 by American astronomer Edwin Hubble by measuring the brightness of cepheid variables in that galaxy, a new technique discovered by Henrietta Leavitt.[134] This established that the Andromeda galaxy, and by extension all galaxies, lay well outside the Milky Way.[135] The modern concept of outer space is based on the "Big Bang" cosmology, first proposed in 1931 by the Belgian physicist Georges Lemaître.[136] This theory holds that the universe originated from a very dense form that has since undergone continuous expansion. The earliest known estimate of the temperature of outer space was by the Swiss physicist Charles É. Guillaume in 1896. Using the estimated radiation of the background stars, he concluded that space must be heated to a temperature of 5–6 K. British physicist Arthur Eddington made a similar calculation to derive a temperature of 3.18 K in 1926. German physicist Erich Regener used the total measured energy of cosmic rays to estimate an intergalactic temperature of 2.8 K in 1933.[137] American physicists Ralph Alpher and Robert Herman predicted 5 K for the temperature of space in 1948, based on the gradual decrease in background energy following the then-new Big Bang theory.[137] The modern measurement of the cosmic microwave background is about 2.7K. The term outward space was used in 1842 by the English poet Lady Emmeline Stuart-Wortley in her poem "The Maiden of Moscow".[138] The expression outer space was used as an astronomical term by Alexander von Humboldt in 1845.[139] It was later popularized in the writings of H. G. Wells in 1901.[140] The shorter term space is older, first used to mean the region beyond Earth's sky in John Milton's Paradise Lost in 1667.[141][142] Exploration and application Main articles: Space exploration and Human presence in space See also: Astronautics, Spaceflight, Benefits of space exploration, Earth observation, Commercialization of space, Human spaceflight, and Space habitation The first image taken by a human of the whole Earth, probably photographed by William Anders of Apollo 8.[143] South is up; South America is in the middle. For most of human history, space was explored by observations made from the Earth's surface—initially with the unaided eye and then with the telescope. Before reliable rocket technology, the closest that humans had come to reaching outer space was through balloon flights. In 1935, the U.S. Explorer II crewed balloon flight reached an altitude of 22 km (14 mi).[144] This was greatly exceeded in 1942 when the third launch of the German A-4 rocket climbed to an altitude of about 80 km (50 mi). In 1957, the uncrewed satellite Sputnik 1 was launched by a Russian R-7 rocket, achieving Earth orbit at an altitude of 215–939 kilometres (134–583 mi).[145] This was followed by the first human spaceflight in 1961, when Yuri Gagarin was sent into orbit on Vostok 1. The first humans to escape low-Earth orbit were Frank Borman, Jim Lovell and William Anders in 1968 on board the U.S. Apollo 8, which achieved lunar orbit[146] and reached a maximum distance of 377,349 km (234,474 mi) from the Earth.[147] The first spacecraft to reach escape velocity was the Soviet Luna 1, which performed a fly-by of the Moon in 1959.[148] In 1961, Venera 1 became the first planetary probe. It revealed the presence of the solar wind and performed the first fly-by of Venus, although contact was lost before reaching Venus. The first successful planetary mission was the 1962 fly-by of Venus by Mariner 2.[149] The first fly-by of Mars was by Mariner 4 in 1964. Since that time, uncrewed spacecraft have successfully examined each of the Solar System's planets, as well their moons and many minor planets and comets. They remain a fundamental tool for the exploration of outer space, as well as for observation of the Earth.[150] In August 2012, Voyager 1 became the first man-made object to leave the Solar System and enter interstellar space.[151] The absence of air makes outer space an ideal location for astronomy at all wavelengths of the electromagnetic spectrum. This is evidenced by the spectacular pictures sent back by the Hubble Space Telescope, allowing light from more than 13 billion years ago—almost to the time of the Big Bang—to be observed.[152] Not every location in space is ideal for a telescope. The interplanetary zodiacal dust emits a diffuse near-infrared radiation that can mask the emission of faint sources such as extrasolar planets. Moving an infrared telescope out past the dust increases its effectiveness.[153] Likewise, a site like the Daedalus crater on the far side of the Moon could shield a radio telescope from the radio frequency interference that hampers Earth-based observations.[154] Uncrewed spacecraft in Earth orbit are an essential technology of modern civilization. They allow direct monitoring of weather conditions, relay long-range communications like television, provide a means of precise navigation, and allow remote sensing of the Earth. The latter role serves a wide variety of purposes, including tracking soil moisture for agriculture, prediction of water outflow from seasonal snow packs, detection of diseases in plants and trees, and surveillance of military activities.[155] The deep vacuum of space could make it an attractive environment for certain industrial processes, such as those requiring ultraclean surfaces.[156] Like asteroid mining, space manufacturing would require a large financial investment with little prospect of immediate return.[157] An important factor in the total expense is the high cost of placing mass into Earth orbit: $8,000–$26,000 per kg, according to a 2006 estimate (allowing for inflation since then).[158] The cost of access to space has declined since 2013. Partially reusable rockets such as the Falcon 9 have lowered access to space below 3500 dollars per kilogram. With these new rockets the cost to send materials into space remains prohibitively high for many industries. Proposed concepts for addressing this issue include, fully reusable launch systems, non-rocket spacelaunch, momentum exchange tethers, and space elevators.[159] Interstellar travel for a human crew remains at present only a theoretical possibility. The distances to the nearest stars mean it would require new technological developments and the ability to safely sustain crews for journeys lasting several decades. For example, the Daedalus Project study, which proposed a spacecraft powered by the fusion of deuterium and helium-3, would require 36 years to reach the "nearby" Alpha Centauri system. Other proposed interstellar propulsion systems include light sails, ramjets, and beam-powered propulsion. More advanced propulsion systems could use antimatter as a fuel, potentially reaching relativistic velocities." (wikipedia.org) "The Solar System[b] is the gravitationally bound system of the Sun and the objects that orbit it, either directly or indirectly.[c] Of the objects that orbit the Sun directly, the largest are the eight planets,[d] with the remainder being smaller objects, the dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly—the natural satellites—two are larger than the smallest planet, Mercury.[e] The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system's mass is in the Sun, with the majority of the remaining mass contained in Jupiter. The four smaller inner system planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal. The four outer system planets are giant planets, being substantially more massive than the terrestrials. The two largest planets, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles, such as water, ammonia and methane. All eight planets have almost circular orbits that lie within a nearly flat disc called the ecliptic. The Solar System also contains smaller objects.[f] The asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, and beyond them a newly discovered population of sednoids. Within these populations, some objects are large enough to have rounded under their own gravity, though there is considerable debate as to how many there will prove to be.[9][10] Such objects are categorized as dwarf planets. The only certain dwarf planet is Pluto, with another trans-Neptunian object, Eris, expected to be, and the asteroid Ceres at least close to being a dwarf planet.[f] In addition to these two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds, freely travel between regions. Six of the planets, the six largest possible dwarf planets, and many of the smaller bodies are orbited by natural satellites, usually termed "moons" after the Moon. Each of the outer planets is encircled by planetary rings of dust and other small objects. The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region in the interstellar medium known as the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium; it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere. The Solar System is located 26,000 light-years from the center of the Milky Way galaxy in the Orion Arm, which contains most of the visible stars in the night sky. The nearest stars are within the so-called Local Bubble, with the closest Proxima Centauri at 4.25 light-years.... Discovery and exploration Main article: Discovery and exploration of the Solar System Andreas Cellarius's illustration of the Copernican system, from the Harmonia Macrocosmica (1660) For most of history, humanity did not recognize or understand the concept of the Solar System. Most people up to the Late Middle Ages–Renaissance believed Earth to be stationary at the centre of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system.[11][12] In the 17th century, Galileo discovered that the Sun was marked with sunspots, and that Jupiter had four satellites in orbit around it.[13] Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn.[14] Around 1677, Edmond Halley observed a transit of Mercury across the Sun, leading him to realise that observations of the solar parallax of a planet (more ideally using the transit of Venus) could be used to trigonometrically determine the distances between Earth, Venus, and the Sun.[15] In 1705, Halley realised that repeated sightings of a comet were of the same object, returning regularly once every 75–76 years. This was the first evidence that anything other than the planets orbited the Sun,[16] though this had been theorized about comets in the 1st century by Seneca.[17] Around 1704, the term "Solar System" first appeared in English.[18] In 1838, Friedrich Bessel successfully measured a stellar parallax, an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism.[19] Improvements in observational astronomy and the use of uncrewed spacecraft have since enabled the detailed investigation of other bodies orbiting the Sun. Comprehensive overview of the Solar System. The Sun, planets, dwarf planets and moons are at scale for their relative sizes, not for distances. A separate distance scale is at the bottom. Moons are listed near their planets by proximity of their orbits; only the largest moons are shown. Structure and composition The principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99.86% of the system's known mass and dominates it gravitationally.[20] The Sun's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons, asteroids, and comets) together comprise less than 0.002% of the Solar System's total mass.[g] Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic. The planets are very close to the ecliptic, whereas comets and Kuiper belt objects are frequently at significantly greater angles to it.[24][25] As a result of the formation of the Solar System, planets (and most other objects) orbit the Sun in the same direction that the Sun is rotating (counter-clockwise, as viewed from above Earth's north pole).[26] There are exceptions, such as Halley's Comet. Most of the larger moons orbit their planets in this prograde direction (with Triton being the largest retrograde exception) and most larger objects rotate themselves in the same direction (with Venus being a notable retrograde exception). The overall structure of the charted regions of the Solar System consists of the Sun, four relatively small inner planets surrounded by a belt of mostly rocky asteroids, and four giant planets surrounded by the Kuiper belt of mostly icy objects. Astronomers sometimes informally divide this structure into separate regions. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four giant planets.[27] Since the discovery of the Kuiper belt, the outermost parts of the Solar System are considered a distinct region consisting of the objects beyond Neptune.[28] Most of the planets in the Solar System have secondary systems of their own, being orbited by planetary objects called natural satellites, or moons (two of which, Titan and Ganymede, are larger than the planet Mercury). The four giant planets have planetary rings, thin bands of tiny particles that orbit them in unison. Most of the largest natural satellites are in synchronous rotation, with one face permanently turned toward their parent.[29] All planets of the Solar System lie very close to the ecliptic. The closer they are to the Sun, the faster they travel (inner planets on the left, all planets except Neptune on the right). Kepler's laws of planetary motion describe the orbits of objects about the Sun. Following Kepler's laws, each object travels along an ellipse with the Sun at one focus. Objects closer to the Sun (with smaller semi-major axes) travel more quickly because they are more affected by the Sun's gravity. On an elliptical orbit, a body's distance from the Sun varies over the course of its year. A body's closest approach to the Sun is called its perihelion, whereas its most distant point from the Sun is called its aphelion. The orbits of the planets are nearly circular, but many comets, asteroids, and Kuiper belt objects follow highly elliptical orbits. The positions of the bodies in the Solar System can be predicted using numerical models. Although the Sun dominates the system by mass, it accounts for only about 2% of the angular momentum.[30][31] The planets, dominated by Jupiter, account for most of the rest of the angular momentum due to the combination of their mass, orbit, and distance from the Sun, with a possibly significant contribution from comets.[30] The Sun, which comprises nearly all the matter in the Solar System, is composed of roughly 98% hydrogen and helium.[32] Jupiter and Saturn, which comprise nearly all the remaining matter, are also primarily composed of hydrogen and helium.[33][34] A composition gradient exists in the Solar System, created by heat and light pressure from the Sun; those objects closer to the Sun, which are more affected by heat and light pressure, are composed of elements with high melting points. Objects farther from the Sun are composed largely of materials with lower melting points.[35] The boundary in the Solar System beyond which those volatile substances could condense is known as the frost line, and it lies at roughly 5 AU (750 million km; 460 million mi) from the Sun.[4] The objects of the inner Solar System are composed mostly of rock,[36] the collective name for compounds with high melting points, such as silicates, iron or nickel, that remained solid under almost all conditions in the protoplanetary nebula.[37] Jupiter and Saturn are composed mainly of gases, the astronomical term for materials with extremely low melting points and high vapour pressure, such as hydrogen, helium, and neon, which were always in the gaseous phase in the nebula.[37] Ices, like water, methane, ammonia, hydrogen sulfide, and carbon dioxide,[36] have melting points up to a few hundred kelvins.[37] They can be found as ices, liquids, or gases in various places in the Solar System, whereas in the nebula they were either in the solid or gaseous phase.[37] Icy substances comprise the majority of the satellites of the giant planets, as well as most of Uranus and Neptune (the so-called "ice giants") and the numerous small objects that lie beyond Neptune's orbit.[36][38] Together, gases and ices are referred to as volatiles.[39] Distances and scales Size comparison of the Sun and the planets (clickable) The distance from Earth to the Sun is 1 astronomical unit [AU] (150,000,000 km; 93,000,000 mi). For comparison, the radius of the Sun is 0.0047 AU (700,000 km; 400,000 mi). Thus, the Sun occupies 0.00001% (10−5 %) of the volume of a sphere with a radius the size of Earth's orbit, whereas Earth's volume is roughly one millionth (10−6) that of the Sun. Jupiter, the largest planet, is 5.2 astronomical units (780,000,000 km; 480,000,000 mi) from the Sun and has a radius of 71,000 km (0.00047 AU; 44,000 mi), whereas the most distant planet, Neptune, is 30 AU (4.5×109 km; 2.8×109 mi) from the Sun. With a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between its orbit and the orbit of the next nearer object to the Sun. For example, Venus is approximately 0.33 AU farther out from the Sun than Mercury, whereas Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a relationship between these orbital distances (for example, the Titius–Bode law),[40] but no such theory has been accepted. Some Solar System models attempt to convey the relative scales involved in the Solar System on human terms. Some are small in scale (and may be mechanical—called orreries)—whereas others extend across cities or regional areas.[41] The largest such scale model, the Sweden Solar System, uses the 110-metre (361 ft) Ericsson Globe in Stockholm as its substitute Sun, and, following the scale, Jupiter is a 7.5-metre (25-foot) sphere at Stockholm Arlanda Airport, 40 km (25 mi) away, whereas the farthest current object, Sedna, is a 10 cm (4 in) sphere in Luleå, 912 km (567 mi) away.[42][43] If the Sun–Neptune distance is scaled to 100 metres (330 ft), then the Sun would be about 3 cm (1.2 in) in diameter (roughly two-thirds the diameter of a golf ball), the giant planets would be all smaller than about 3 mm (0.12 in), and Earth's diameter along with that of the other terrestrial planets would be smaller than a flea (0.3 mm or 0.012 in) at this scale.[44] The Solar System. Distances are to scale, objects are not. Distances of selected bodies of the Solar System from the Sun. The left and right edges of each bar correspond to the perihelion and aphelion of the body, respectively, hence long bars denote high orbital eccentricity. The radius of the Sun is 0.7 million km, and the radius of Jupiter (the largest planet) is 0.07 million km, both too small to resolve on this image. Formation and evolution Artist's conception of a protoplanetary disk Main article: Formation and evolution of the Solar System The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud.[h] This initial cloud was likely several light-years across and probably birthed several stars.[46] As is typical of molecular clouds, this one consisted mostly of hydrogen, with some helium, and small amounts of heavier elements fused by previous generations of stars. As the region that would become the Solar System, known as the pre-solar nebula,[47] collapsed, conservation of angular momentum caused it to rotate faster. The centre, where most of the mass collected, became increasingly hotter than the surrounding disc.[46] As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a diameter of roughly 200 AU (30 billion km; 19 billion mi)[46] and a hot, dense protostar at the centre.[48][49] The planets formed by accretion from this disc,[50] in which dust and gas gravitationally attracted each other, coalescing to form ever larger bodies. Hundreds of protoplanets may have existed in the early Solar System, but they either merged or were destroyed, leaving the planets, dwarf planets, and leftover minor bodies.[51] The geology of the contact binary object Arrokoth (nicknamed Ultima Thule), the first undisturbed planetesimal visited by a spacecraft, with comet 67P to scale. The eight subunits of the larger lobe, labeled ma to mh, are thought to have been its building blocks. The two lobes came together later, forming a contact binary. Objects such as Arrokoth are believed in turn to have formed protoplanets.[52] Due to their higher boiling points, only metals and silicates could exist in solid form in the warm inner Solar System close to the Sun, and these would eventually form the rocky planets of Mercury, Venus, Earth, and Mars. Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large. The giant planets (Jupiter, Saturn, Uranus, and Neptune) formed further out, beyond the frost line, the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid. The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium, the lightest and most abundant elements. Leftover debris that never became planets congregated in regions such as the asteroid belt, Kuiper belt, and Oort cloud.[51] The Nice model is an explanation for the creation of these regions and how the outer planets could have formed in different positions and migrated to their current orbits through various gravitational interactions.[53] Within 50 million years, the pressure and density of hydrogen in the centre of the protostar became great enough for it to begin thermonuclear fusion.[54] The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved: the thermal pressure equalled the force of gravity. At this point, the Sun became a main-sequence star.[55] The main-sequence phase, from beginning to end, will last about 10 billion years for the Sun compared to around two billion years for all other phases of the Sun's pre-remnant life combined.[56] Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space, ending the planetary formation process. The Sun is growing brighter; early in its main-sequence life its brightness was 70% that of what it is today.[57] The Solar System will remain roughly as we know it today until the hydrogen in the core of the Sun has been entirely converted to helium, which will occur roughly 5 billion years from now. This will mark the end of the Sun's main-sequence life. At that time, the core of the Sun will contract with hydrogen fusion occurring along a shell surrounding the inert helium, and the energy output will be much greater than at present. The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become a red giant. Because of its vastly increased surface area, the surface of the Sun will be considerably cooler (2,600 K (2,330 °C; 4,220 °F) at its coolest) than it is on the main sequence.[56] The expanding Sun is expected to vaporize Mercury and render Earth uninhabitable. Eventually, the core will be hot enough for helium fusion; the Sun will burn helium for a fraction of the time it burned hydrogen in the core. The Sun is not massive enough to commence the fusion of heavier elements, and nuclear reactions in the core will dwindle. Its outer layers will move away into space, leaving a white dwarf, an extraordinarily dense object, half the original mass of the Sun but only the size of Earth.[58] The ejected outer layers will form what is known as a planetary nebula, returning some of the material that formed the Sun—but now enriched with heavier elements like carbon—to the interstellar medium. Sun Main article: Sun The Sun is the Solar System's star and by far its most massive component. Its large mass (332,900 Earth masses),[59] which comprises 99.86% of all the mass in the Solar System,[60] produces temperatures and densities in its core high enough to sustain nuclear fusion of hydrogen into helium, making it a main-sequence star.[61] This releases an enormous amount of energy, mostly radiated into space as electromagnetic radiation peaking in visible light.[62] The Sun is a G2-type main-sequence star. Hotter main-sequence stars are more luminous. The Sun's temperature is intermediate between that of the hottest stars and that of the coolest stars. Stars brighter and hotter than the Sun are rare, whereas substantially dimmer and cooler stars, known as red dwarfs, make up 85% of the stars in the Milky Way.[63][64] The Sun is a population I star; it has a higher abundance of elements heavier than hydrogen and helium ("metals" in astronomical parlance) than the older population II stars.[65] Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, whereas stars born later have more. This high metallicity is thought to have been crucial to the Sun's development of a planetary system because the planets form from the accretion of "metals".[66] Interplanetary medium Main articles: Interplanetary medium and Solar wind The heliospheric current sheet The vast majority of the Solar System consists of a near-vacuum known as the interplanetary medium. Along with light, the Sun radiates a continuous stream of charged particles (a plasma) known as the solar wind. This stream of particles spreads outwards at roughly 1.5 million kilometres per hour (930,000 mph),[67] creating a tenuous atmosphere that permeates the interplanetary medium out to at least 100 AU (15 billion km; 9.3 billion mi) (see § Heliosphere).[68] Activity on the Sun's surface, such as solar flares and coronal mass ejections, disturbs the heliosphere, creating space weather and causing geomagnetic storms.[69] The largest structure within the heliosphere is the heliospheric current sheet, a spiral form created by the actions of the Sun's rotating magnetic field on the interplanetary medium.[70][71] Earth's magnetic field stops its atmosphere from being stripped away by the solar wind.[72] Venus and Mars do not have magnetic fields, and as a result the solar wind is causing their atmospheres to gradually bleed away into space.[73] Coronal mass ejections and similar events blow a magnetic field and huge quantities of material from the surface of the Sun. The interaction of this magnetic field and material with Earth's magnetic field funnels charged particles into Earth's upper atmosphere, where its interactions create aurorae seen near the magnetic poles. The heliosphere and planetary magnetic fields (for those planets that have them) partially shield the Solar System from high-energy interstellar particles called cosmic rays. The density of cosmic rays in the interstellar medium and the strength of the Sun's magnetic field change on very long timescales, so the level of cosmic-ray penetration in the Solar System varies, though by how much is unknown.[74] The interplanetary medium is home to at least two disc-like regions of cosmic dust. The first, the zodiacal dust cloud, lies in the inner Solar System and causes the zodiacal light. It was likely formed by collisions within the asteroid belt brought on by gravitational interactions with the planets.[75] The second dust cloud extends from about 10 AU (1.5 billion km; 930 million mi) to about 40 AU (6.0 billion km; 3.7 billion mi), and was probably created by similar collisions within the Kuiper belt.[76][77] Inner Solar System The inner Solar System is the region comprising the terrestrial planets and the asteroid belt.[78] Composed mainly of silicates and metals, the objects of the inner Solar System are relatively close to the Sun; the radius of this entire region is less than the distance between the orbits of Jupiter and Saturn. This region is also within the frost line, which is a little less than 5 AU (750 million km; 460 million mi) from the Sun. Inner planets Main article: Terrestrial planet The inner planets. From top to bottom rightwards: Earth, Mars, Venus, and Mercury (sizes to scale). Orrery showing the motions of the inner four planets. The small spheres represent the position of each planet on every two Julian days, beginning August 3, 2020 and ending June 21, 2022 (Mars at perihelion). The four terrestrial or inner planets have dense, rocky compositions, few or no moons, and no ring systems. They are composed largely of refractory minerals such as the silicates—which form their crusts and mantles—and metals such as iron and nickel which form their cores. Three of the four inner planets (Venus, Earth and Mars) have atmospheres substantial enough to generate weather; all have impact craters and tectonic surface features, such as rift valleys and volcanoes. The term inner planet should not be confused with inferior planet, which designates those planets that are closer to the Sun than Earth is (i.e. Mercury and Venus). Mercury Main article: Mercury (planet) Mercury (0.4 AU (60 million km; 37 million mi) from the Sun) is the closest planet to the Sun and on average, all seven other planets.[79][80] The smallest planet in the Solar System (0.055 M⊕), Mercury has no natural satellites. Besides impact craters, its only known geological features are lobed ridges or rupes that were probably produced by a period of contraction early in its history.[81] Mercury's very tenuous atmosphere consists of atoms blasted off its surface by the solar wind.[82] Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, or that it was prevented from fully accreting by the young Sun's energy.[83][84] Venus Main article: Venus Venus (0.7 AU (100 million km; 65 million mi) from the Sun) is close in size to Earth (0.815 M⊕) and, like Earth, has a thick silicate mantle around an iron core, a substantial atmosphere, and evidence of internal geological activity. It is much drier than Earth, and its atmosphere is ninety times as dense. Venus has no natural satellites. It is the hottest planet, with surface temperatures over 400 °C (752 °F), most likely due to the amount of greenhouse gases in the atmosphere.[85] No definitive evidence of current geological activity has been detected on Venus, but it has no magnetic field that would prevent depletion of its substantial atmosphere, which suggests that its atmosphere is being replenished by volcanic eruptions.[86] Earth Main article: Earth Earth (1 AU (150 million km; 93 million mi) from the Sun) is the largest and densest of the inner planets, the only one known to have current geological activity, and the only place where life is known to exist.[87] Its liquid hydrosphere is unique among the terrestrial planets, and it is the only planet where plate tectonics has been observed. Earth's atmosphere is radically different from those of the other planets, having been altered by the presence of life to contain 21% free oxygen.[88] It has one natural satellite, the Moon, the only large satellite of a terrestrial planet in the Solar System. Mars Main article: Mars Mars (1.5 AU (220 million km; 140 million mi) from the Sun) is smaller than Earth and Venus (0.107 M⊕). It has an atmosphere of mostly carbon dioxide with a surface pressure of 6.1 millibars (0.088 psi; 0.18 inHg) (roughly 0.6% of that of Earth).[89] Its surface, peppered with vast volcanoes, such as Olympus Mons, and rift valleys, such as Valles Marineris, shows geological activity that may have persisted until as recently as 2 million years ago.[90] Its red colour comes from iron oxide (rust) in its soil.[91] Mars has two tiny natural satellites (Deimos and Phobos) thought to be either captured asteroids,[92] or ejected debris from a massive impact early in Mars's history.[93] Asteroid belt Main article: Asteroid belt The donut-shaped asteroid belt is located between the orbits of Mars and Jupiter.       Sun       Jupiter trojans       Planetary orbit       Asteroid belt       Hilda asteroids       NEOs (selection) Asteroids except for the largest, Ceres, are classified as small Solar System bodies[f] and are composed mainly of refractory rocky and metallic minerals, with some ice.[94][95] They range from a few metres to hundreds of kilometres in size. Asteroids smaller than one meter are usually called meteoroids and micrometeoroids (grain-sized), depending on different, somewhat arbitrary definitions. The asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU (340 and 490 million km; 210 and 310 million mi) from the Sun. It is thought to be remnants from the Solar System's formation that failed to coalesce because of the gravitational interference of Jupiter.[96] The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter.[97] Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of Earth.[23] The asteroid belt is very sparsely populated; spacecraft routinely pass through without incident.[98] Ceres Main article: Ceres (dwarf planet) Ceres – map of gravity fields: red is high; blue, low. Ceres (2.77 AU (414 million km; 257 million mi)) is the largest asteroid, a protoplanet, and a dwarf planet.[f] It has a diameter of slightly under 1,000 km (620 mi), and a mass large enough for its own gravity to pull it into a spherical shape. Ceres was considered a planet when it was discovered in 1801 and was reclassified to asteroid in the 1850s as further observations revealed additional asteroids.[99] It was classified as a dwarf planet in 2006 when the definition of a planet was created. Asteroid groups Asteroids in the asteroid belt are divided into asteroid groups and families based on their orbital characteristics. Asteroid moons are asteroids that orbit larger asteroids. They are not as clearly distinguished as planetary moons, sometimes being almost as large as their partners. The asteroid belt also contains main-belt comets, which may have been the source of Earth's water.[100] Jupiter trojans are located in either of Jupiter's L4 or L5 points (gravitationally stable regions leading and trailing a planet in its orbit); the term trojan is also used for small bodies in any other planetary or satellite Lagrange point. Hilda asteroids are in a 2:3 resonance with Jupiter; that is, they go around the Sun three times for every two Jupiter orbits.[101] The inner Solar System also contains near-Earth asteroids, many of which cross the orbits of the inner planets.[102] Some of them are potentially hazardous objects. Outer Solar System The outer region of the Solar System is home to the giant planets and their large moons. The centaurs and many short-period comets also orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles, such as water, ammonia, and methane than those of the inner Solar System because the lower temperatures allow these compounds to remain solid.[51] Outer planets Main article: Giant planet The outer planets (in the background) Jupiter, Saturn, Uranus and Neptune, compared to the inner planets Earth, Venus, Mars and Mercury (in the foreground) Orrery showing the motions of the outer four planets. The small spheres represent the position of each planet on every 200 Julian days, beginning November 18, 1877 and ending September 3, 2042 (Neptune at perihelion). The four outer planets, or giant planets (sometimes called Jovian planets), collectively make up 99% of the mass known to orbit the Sun.[g] Jupiter and Saturn are together more than 400 times the mass of Earth and consist overwhelmingly of the gases hydrogen and helium, hence their designation as gas giants.[103] Uranus and Neptune are far less massive—less than 20 Earth masses (M⊕) each—and are composed primarily of ices. For these reasons, some astronomers suggest they belong in their own category, ice giants.[104] All four giant planets have rings, although only Saturn's ring system is easily observed from Earth. The term superior planet designates planets outside Earth's orbit and thus includes both the outer planets and Mars. Jupiter Main article: Jupiter Jupiter (5.2 AU (780 million km; 480 million mi)), at 318 M⊕, is 2.5 times the mass of all the other planets put together. It is composed largely of hydrogen and helium. Jupiter's strong internal heat creates semi-permanent features in its atmosphere, such as cloud bands and the Great Red Spot. Jupiter has 79 known satellites. The four largest, Ganymede, Callisto, Io, and Europa, show similarities to the terrestrial planets, such as volcanism and internal heating.[105] Ganymede, the largest satellite in the Solar System, is larger than Mercury. Saturn Main article: Saturn Saturn (9.5 AU (1.42 billion km; 880 million mi)), distinguished by its extensive ring system, has several similarities to Jupiter, such as its atmospheric composition and magnetosphere. Although Saturn has 60% of Jupiter's volume, it is less than a third as massive, at 95 M⊕. Saturn is the only planet of the Solar System that is less dense than water.[106] The rings of Saturn are made up of small ice and rock particles. Saturn has 82 confirmed satellites composed largely of ice. Two of these, Titan and Enceladus, show signs of geological activity.[107] Titan, the second-largest moon in the Solar System, is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere. Uranus Main article: Uranus Uranus (19.2 AU (2.87 billion km; 1.78 billion mi)), at 14 M⊕, is the lightest of the outer planets. Uniquely among the planets, it orbits the Sun on its side; its axial tilt is over ninety degrees to the ecliptic. It has a much colder core than the other giant planets and radiates very little heat into space.[108] Uranus has 27 known satellites, the largest ones being Titania, Oberon, Umbriel, Ariel, and Miranda.[109] Neptune Main article: Neptune Neptune (30.1 AU (4.50 billion km; 2.80 billion mi)), though slightly smaller than Uranus, is more massive (17 M⊕) and hence more dense. It radiates more internal heat, but not as much as Jupiter or Saturn.[110] Neptune has 14 known satellites. The largest, Triton, is geologically active, with geysers of liquid nitrogen.[111] Triton is the only large satellite with a retrograde orbit. Neptune is accompanied in its orbit by several minor planets, termed Neptune trojans, that are in 1:1 resonance with it. Centaurs Main article: Centaur (small Solar System body) The centaurs are icy comet-like bodies whose orbits have semi-major axes greater than Jupiter's (5.5 AU (820 million km; 510 million mi)) and less than Neptune's (30 AU (4.5 billion km; 2.8 billion mi)). The largest known centaur, 10199 Chariklo, has a diameter of about 250 km (160 mi).[112] The first centaur discovered, 2060 Chiron, has also been classified as a comet (95P) because it develops a coma just as comets do when they approach the Sun.[113] Comets Hale–Bopp seen in 1997 Main article: Comet Comets are small Solar System bodies,[f] typically only a few kilometres across, composed largely of volatile ices. They have highly eccentric orbits, generally a perihelion within the orbits of the inner planets and an aphelion far beyond Pluto. When a comet enters the inner Solar System, its proximity to the Sun causes its icy surface to sublimate and ionise, creating a coma: a long tail of gas and dust often visible to the naked eye. Short-period comets have orbits lasting less than two hundred years. Long-period comets have orbits lasting thousands of years. Short-period comets are thought to originate in the Kuiper belt, whereas long-period comets, such as Hale–Bopp, are thought to originate in the Oort cloud. Many comet groups, such as the Kreutz Sungrazers, formed from the breakup of a single parent.[114] Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult.[115] Old comets whose volatiles have mostly been driven out by solar warming are often categorised as asteroids.[116] Trans-Neptunian region Beyond the orbit of Neptune lies the area of the "trans-Neptunian region", with the doughnut-shaped Kuiper belt, home of Pluto and several other dwarf planets, and an overlapping disc of scattered objects, which is tilted toward the plane of the Solar System and reaches much further out than the Kuiper belt. The entire region is still largely unexplored. It appears to consist overwhelmingly of many thousands of small worlds—the largest having a diameter only a fifth that of Earth and a mass far smaller than that of the Moon—composed mainly of rock and ice. This region is sometimes described as the "third zone of the Solar System", enclosing the inner and the outer Solar System.[117] Kuiper belt Main article: Kuiper belt Known objects in the Kuiper belt       Sun       Jupiter trojans       Giant planets       Kuiper belt       Scattered disc       Neptune trojans Size comparison of some large TNOs with Earth: Pluto and its moons, Eris, Makemake, Haumea, Sedna, Gonggong, Quaoar, and Orcus. The Kuiper belt is a great ring of debris similar to the asteroid belt, but consisting mainly of objects composed primarily of ice.[118] It extends between 30 and 50 AU (4.5 and 7.5 billion km; 2.8 and 4.6 billion mi) from the Sun. Though it is estimated to contain anything from dozens to thousands of dwarf planets, it is composed mainly of small Solar System bodies. Many of the larger Kuiper belt objects, such as Quaoar, Varuna, and Orcus, may prove to be dwarf planets with further data. There are estimated to be over 100,000 Kuiper belt objects with a diameter greater than 50 km (30 mi), but the total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of Earth.[22] Many Kuiper belt objects have multiple satellites,[119] and most have orbits that take them outside the plane of the ecliptic.[120] The Kuiper belt can be roughly divided into the "classical" belt and the resonances.[118] Resonances are orbits linked to that of Neptune (e.g. twice for every three Neptune orbits, or once for every two). The first resonance begins within the orbit of Neptune itself. The classical belt consists of objects having no resonance with Neptune, and extends from roughly 39.4 to 47.7 AU (5.89 to 7.14 billion km; 3.66 to 4.43 billion mi).[121] Members of the classical Kuiper belt are classified as cubewanos, after the first of their kind to be discovered, 15760 Albion (which previously had the provisional designation 1992 QB1), and are still in near primordial, low-eccentricity orbits.[122] Pluto and Charon Main articles: Pluto and Charon (moon) The dwarf planet Pluto (with an average orbit of 39 AU (5.8 billion km; 3.6 billion mi)) is the largest known object in the Kuiper belt. When discovered in 1930, it was considered to be the ninth planet; this changed in 2006 with the adoption of a formal definition of planet. Pluto has a relatively eccentric orbit inclined 17 degrees to the ecliptic plane and ranging from 29.7 AU (4.44 billion km; 2.76 billion mi) from the Sun at perihelion (within the orbit of Neptune) to 49.5 AU (7.41 billion km; 4.60 billion mi) at aphelion. Pluto has a 3:2 resonance with Neptune, meaning that Pluto orbits twice round the Sun for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called plutinos.[123] Charon, the largest of Pluto's moons, is sometimes described as part of a binary system with Pluto, as the two bodies orbit a barycentre of gravity above their surfaces (i.e. they appear to "orbit each other"). Beyond Charon, four much smaller moons, Styx, Nix, Kerberos, and Hydra, orbit within the system. Makemake and Haumea Main articles: Makemake and Haumea Makemake (45.79 AU average), although smaller than Pluto, is the largest known object in the classical Kuiper belt (that is, a Kuiper belt object not in a confirmed resonance with Neptune). Makemake is the brightest object in the Kuiper belt after Pluto. It was assigned a naming committee under the expectation that it would prove to be a dwarf planet in 2008.[6] Its orbit is far more inclined than Pluto's, at 29°.[124] Haumea (43.13 AU average) is in an orbit similar to Makemake, except that it is in a temporary 7:12 orbital resonance with Neptune.[125] It was named under the same expectation that it would prove to be a dwarf planet, though subsequent observations have indicated that it may not be a dwarf planet after all.[126] Scattered disc Main article: Scattered disc The scattered disc, which overlaps the Kuiper belt but extends out to about 200 AU, is thought to be the source of short-period comets. Scattered-disc objects are thought to have been ejected into erratic orbits by the gravitational influence of Neptune's early outward migration. Most scattered disc objects (SDOs) have perihelia within the Kuiper belt but aphelia far beyond it (some more than 150 AU from the Sun). SDOs' orbits are also highly inclined to the ecliptic plane and are often almost perpendicular to it. Some astronomers consider the scattered disc to be merely another region of the Kuiper belt and describe scattered disc objects as "scattered Kuiper belt objects".[127] Some astronomers also classify centaurs as inward-scattered Kuiper belt objects along with the outward-scattered residents of the scattered disc.[128] Eris Main article: Eris (dwarf planet) Eris (with an average orbit of 68 AU) is the largest known scattered disc object, and caused a debate about what constitutes a planet, because it is 25% more massive than Pluto[129] and about the same diameter. It is the most massive of the known dwarf planets. It has one known moon, Dysnomia. Like Pluto, its orbit is highly eccentric, with a perihelion of 38.2 AU (roughly Pluto's distance from the Sun) and an aphelion of 97.6 AU, and steeply inclined to the ecliptic plane. Farthest regions From the Sun to the nearest star: The Solar System on a logarithmic scale in astronomical units (AU) The point at which the Solar System ends and interstellar space begins is not precisely defined because its outer boundaries are shaped by two forces, the solar wind and the Sun's gravity. The limit of the solar wind's influence is roughly four times Pluto's distance from the Sun; this heliopause, the outer boundary of the heliosphere, is considered the beginning of the interstellar medium.[68] The Sun's Hill sphere, the effective range of its gravitational dominance, is thought to extend up to a thousand times farther and encompasses the hypothetical Oort cloud.[130] Heliosphere Main article: Heliosphere The bubble-like heliosphere with its various transitional regions moving through the interstellar medium The heliosphere is a stellar-wind bubble, a region of space dominated by the Sun, in which it radiates its solar wind at approximately 400 km/s, a stream of charged particles, until it collides with the wind of the interstellar medium. The collision occurs at the termination shock, which is roughly 80–100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind.[131] Here the wind slows dramatically, condenses and becomes more turbulent,[131] forming a great oval structure known as the heliosheath. This structure is thought to look and behave very much like a comet's tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind; evidence from the Cassini and Interstellar Boundary Explorer spacecraft has suggested that it is forced into a bubble shape by the constraining action of the interstellar magnetic field.[132] The outer boundary of the heliosphere, the heliopause, is the point at which the solar wind finally terminates and is the beginning of interstellar space.[68] Voyager 1 and Voyager 2 are reported to have passed the termination shock and entered the heliosheath, at 94 and 84 AU from the Sun, respectively.[133][134] Voyager 1 is reported to have crossed the heliopause in August 2012.[135] The shape and form of the outer edge of the heliosphere is likely affected by the fluid dynamics of interactions with the interstellar medium as well as solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with the northern hemisphere extending 9 AU farther than the southern hemisphere.[131] Beyond the heliopause, at around 230 AU, lies the bow shock, a plasma "wake" left by the Sun as it travels through the Milky Way.[136] Zooming out the Solar System:     inner Solar System and Jupiter     outer Solar System and Pluto     orbit of Sedna (detached object)     inner part of the Oort Cloud Due to a lack of data, conditions in local interstellar space are not known for certain. It is expected that NASA's Voyager spacecraft, as they pass the heliopause, will transmit valuable data on radiation levels and solar wind to Earth.[137] How well the heliosphere shields the Solar System from cosmic rays is poorly understood. A NASA-funded team has developed a concept of a "Vision Mission" dedicated to sending a probe to the heliosphere.[138][139] Detached objects Main articles: Detached object and Sednoid 90377 Sedna (with an average orbit of 520 AU) is a large, reddish object with a gigantic, highly elliptical orbit that takes it from about 76 AU at perihelion to 940 AU at aphelion and takes 11,400 years to complete. Mike Brown, who discovered the object in 2003, asserts that it cannot be part of the scattered disc or the Kuiper belt because its perihelion is too distant to have been affected by Neptune's migration. He and other astronomers consider it to be the first in an entirely new population, sometimes termed "distant detached objects" (DDOs), which also may include the object 2000 CR105, which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital period of 3,420 years.[140] Brown terms this population the "inner Oort cloud" because it may have formed through a similar process, although it is far closer to the Sun.[141] Sedna is very likely a dwarf planet, though its shape has yet to be determined. The second unequivocally detached object, with a perihelion farther than Sedna's at roughly 81 AU, is 2012 VP113, discovered in 2012. Its aphelion is only half that of Sedna's, at 400–500 AU.[142][143] Oort cloud Main article: Oort cloud Schematic of the hypothetical Oort cloud, with a spherical outer cloud and a disc-shaped inner cloud The Oort cloud is a hypothetical spherical cloud of up to a trillion icy objects that is thought to be the source for all long-period comets and to surround the Solar System at roughly 50,000 AU (around 1 light-year (ly)), and possibly to as far as 100,000 AU (1.87 ly). It is thought to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Oort cloud objects move very slowly, and can be perturbed by infrequent events, such as collisions, the gravitational effects of a passing star, or the galactic tide, the tidal force exerted by the Milky Way.[144][145] Boundaries See also: Vulcanoid, Planets beyond Neptune, and Planet Nine Much of the Solar System is still unknown. The Sun's gravitational field is estimated to dominate the gravitational forces of surrounding stars out to about two light-years (125,000 AU). Lower estimates for the radius of the Oort cloud, by contrast, do not place it farther than 50,000 AU.[146] Despite discoveries such as Sedna, the region between the Kuiper belt and the Oort cloud, an area tens of thousands of AU in radius, is still virtually unmapped. There are also ongoing studies of the region between Mercury and the Sun.[147] Objects may yet be discovered in the Solar System's uncharted regions. Currently, the furthest known objects, such as Comet West, have aphelia around 70,000 AU from the Sun, but as the Oort cloud becomes better known, this may change. Galactic context See also: Location of Earth Position of the Solar System within the Milky Way Diagram of the Milky Way with the position of the Solar System marked by a yellow arrow Close up on the Orion Arm, with major stellar associations (yellow), nebulae (red) and dark nebulae (grey) around the Local Bubble. The Solar System is located in the Milky Way, a barred spiral galaxy with a diameter of about 100,000 light-years containing more than 100 billion stars.[148] The Sun resides in one of the Milky Way's outer spiral arms, known as the Orion–Cygnus Arm or Local Spur.[149] The Sun lies about 26,660 light-years from the Galactic Centre,[150] and its speed around the center of the Milky Way is about 247 km/s, so that it completes one revolution every 210 million years. This revolution is known as the Solar System's galactic year.[151] The solar apex, the direction of the Sun's path through interstellar space, is near the constellation Hercules in the direction of the current location of the bright star Vega.[152] The plane of the ecliptic lies at an angle of about 60° to the galactic plane.[i] The Solar System's location in the Milky Way is a factor in the evolutionary history of life on Earth. Its orbit is close to circular, and orbits near the Sun are at roughly the same speed as that of the spiral arms.[154][155] Therefore, the Sun passes through arms only rarely. Because spiral arms are home to a far larger concentration of supernovae, gravitational instabilities, and radiation that could disrupt the Solar System, this has given Earth long periods of stability for life to evolve.[154] However, the changing position of the Solar System relative to other parts of the Milky Way could explain periodic extinction events on Earth, according to the Shiva hypothesis or related theories. The Solar System lies well outside the star-crowded environs of the galactic centre. Near the centre, gravitational tugs from nearby stars could perturb bodies in the Oort cloud and send many comets into the inner Solar System, producing collisions with potentially catastrophic implications for life on Earth. The intense radiation of the galactic centre could also interfere with the development of complex life.[154] Even at the Solar System's current location, some scientists have speculated that recent supernovae may have adversely affected life in the last 35,000 years, by flinging pieces of expelled stellar core towards the Sun, as radioactive dust grains and larger, comet-like bodies.[156] Logarithmic depiction of the Solar System's location Celestial neighbourhood Beyond the heliosphere is the interstellar medium, consisting of various clouds of gases. The Solar System currently moves through the Local Interstellar Cloud. The Solar System is surrounded by the Local Interstellar Cloud, although it is not clear if it is embedded in the Local Interstellar Cloud or if it is in the region where the cloud interacts with the neighbouring G-Cloud.[157][158] Both spaces are interstellar clouds in a region known as the 300 light-years wide Local Bubble. Within ten light-years of the Sun there are relatively few stars, the closest being the triple star system Alpha Centauri, which is about 4.4 light-years away and in the G-Cloud. Alpha Centauri A and B are a closely tied pair of Sun-like stars, whereas the closest to Earth, the small red dwarf Proxima Centauri, orbits the pair closer at a distance of 0.2 light-year. In 2016, a potentially habitable exoplanet was confirmed to be orbiting Proxima Centauri, called Proxima Centauri b, the closest confirmed exoplanet to the Sun.[159] The next closest known fusors and rogue planets to the Sun are the red dwarf Barnard's Star (at 5.9 ly), the nearest brown dwarfs of the binary Luhman 16 system (6.6 ly), the closest known rogue or free-floating planetary-mass object at less than 10 Jupiter masses the sub-brown dwarf WISE 0855−0714,[160] (7 ly), as well as the red dwarfs Wolf 359 (7.8 ly) and Lalande 21185 (8.3 ly). The next closest at 8.6 ly is Sirius, the brightest star in Earth's night sky, with roughly twice the Sun's mass, orbited by the closest white dwarf to Earth, Sirius B. Other systems within ten light-years are the binary red-dwarf system Luyten 726-8 (8.7 ly) and the solitary red dwarf Ross 154 (9.7 ly).[161] The closest solitary Sun-like star to the Solar System is Tau Ceti at 11.9 light-years. It has roughly 80% of the Sun's mass but only 60% of its luminosity.[162] Distance and angle conformal map of the nearest stars and (sub-) brown dwarfs within 12 light years of the Solar System (Sol). The nearest and unaided-visible group of stars beyond the immediate celestial neighbourhood is the Ursa Major Moving Group at roughly 80 light-years, which is within the Local Bubble, like the nearest as well as unaided-visible star cluster the Hyades, which lie at its edge. The Local Bubble is an hourglass-shaped cavity or superbubble in the interstellar medium roughly 300 light-years across. The bubble is suffused with high-temperature plasma, that suggests it is the product of several recent supernovae.[163] The Local Bubble is a small superbubble compared to the neighbouring wider Gould Belt and Radcliffe wave each of some thousands of light-years in length, all of which are part of the Orion Arm, that contains most unaided-visible stars, of the Milky Way. The closest star forming regions are the Corona Australis Molecular Cloud, Rho Ophiuchi cloud complex and the Taurus Molecular Cloud, the latter lies just beyond the Local Bubble and is part of the Radcliffe wave. Unaided-visible objects within these regions of a thousand light-years towards the 26 thousand light-years far away Galactic Center are objects like Shaula and outward in the galactic plane such as Elnath. Comparison with extrasolar systems Compared to many other planetary systems, the Solar System stands out in lacking planets interior to the orbit of Mercury.[164][165] The known Solar System also lacks super-Earths (Planet Nine could be a super-Earth beyond the known Solar System).[164] Uncommonly, it has only small rocky planets and large gas giants; elsewhere planets of intermediate size are typical—both rocky and gas—so there is no "gap" as seen between the size of Earth and of Neptune (with a radius 3.8 times as large). Also, these super-Earths have closer orbits than Mercury.[164] This led to the hypothesis that all planetary systems start with many close-in planets, and that typically a sequence of their collisions causes consolidation of mass into few larger planets, but in case of the Solar System the collisions caused their destruction and ejection.[166][167] The orbits of Solar System planets are nearly circular. Compared to other systems, they have smaller orbital eccentricity.[164] Although there are attempts to explain it partly with a bias in the radial-velocity detection method and partly with long interactions of a quite high number of planets, the exact causes remain undetermined.[164][168] Visual summary This section is a sampling of Solar System bodies, selected for size and quality of imagery, and sorted by volume. Some large objects are omitted here (notably Eris, Haumea, Makemake, and Nereid) because they have not been imaged in high quality. Solar System Sun white.jpg     Jupiter and its shrunken Great Red Spot.jpg     Jewel of the Solar System.jpg     Uranus2.jpg     Neptune - Voyager 2 (29347980845) flatten crop.jpg     The Earth seen from Apollo 17.jpg     PIA23791-Venus-NewlyProcessedView-20200608.jpg Sun (star)     Jupiter (planet)     Saturn (planet)     Uranus (planet)     Neptune (planet)     Earth (planet)     Venus (planet) OSIRIS Mars true color.jpg     Ganymede g1 true-edit1.jpg     Titan in true color.jpg     Mercury in color - Prockter07-edit1.jpg     Callisto.jpg     Io highest resolution true color.jpg     FullMoon2010.jpg Mars (planet)     Ganymede (moon of Jupiter)     Titan (moon of Saturn)     Mercury (planet)     Callisto (moon of Jupiter)     Io (moon of Jupiter)     Moon (moon of Earth) Europa-moon-with-margins.jpg     Triton moon mosaic Voyager 2 (large).jpg     Pluto in True Color - High-Res.jpg     Titania (moon) color, edited.jpg     PIA07763 Rhea full globe5.jpg     Voyager 2 picture of Oberon.jpg     Iapetus as seen by the Cassini probe - 20071008.jpg Europa (moon of Jupiter)     Triton (moon of Neptune)     Pluto (dwarf planet)     Titania (moon of Uranus)     Rhea (moon of Saturn)     Oberon (moon of Uranus)     Iapetus (moon of Saturn) Charon in True Color - High-Res.jpg     PIA00040 Umbrielx2.47.jpg     Ariel (moon).jpg     Dione in natural light.jpg     PIA18317-SaturnMoon-Tethys-Cassini-20150411.jpg     Ceres - RC3 - Haulani Crater (22381131691) (cropped).jpg     Vesta full mosaic.jpg Charon (moon of Pluto)     Umbriel (moon of Uranus)     Ariel (moon of Uranus)     Dione (moon of Saturn)     Tethys (moon of Saturn)     Ceres (dwarf planet)     Vesta (belt asteroid) Potw1749a Pallas crop.png     PIA17202 - Approaching Enceladus.jpg     Miranda.jpg     Proteus (Voyager 2).jpg     Mimas Cassini.jpg     Hyperion true.jpg     Iris asteroid eso.jpg Pallas (belt asteroid)     Enceladus (moon of Saturn)     Miranda (moon of Uranus)     Proteus (moon of Neptune)     Mimas (moon of Saturn)     Hyperion (moon of Saturn)     Iris (belt asteroid) Phoebe cassini.jpg     PIA12714 Janus crop.jpg     PIA09813 Epimetheus S. polar region.jpg     Rosetta triumphs at asteroid Lutetia.jpg     Prometheus 12-26-09a.jpg     PIA21055 - Pandora Up Close.jpg     (253) mathilde crop.jpg Phoebe (moon of Saturn)     Janus (moon of Saturn)     Epimetheus (moon of Saturn)     Lutetia (belt asteroid)     Prometheus (moon of Saturn)     Pandora (moon of Saturn)     Mathilde (belt asteroid) Leading hemisphere of Helene - 20110618.jpg     243 Ida large.jpg     UltimaThule CA06 color 20190516.png     Phobos colour 2008.jpg     Deimos-MRO.jpg     Comet 67P on 19 September 2014 NavCam mosaic.jpg     Comet Hartley 2 (super crop).jpg Helene (moon of Saturn)     Ida (belt asteroid)     Arrokoth (Kuiper belt object)     Phobos (moon of Mars)     Deimos (moon of Mars)     Churyumov– Gerasimenko (comet)     Hartley 2 (comet)" (wikipedia.org) "A planetary system is a set of gravitationally bound non-stellar objects in or out of orbit around a star or star system. Generally speaking, systems with one or more planets constitute a planetary system, although such systems may also consist of bodies such as dwarf planets, asteroids, natural satellites, meteoroids, comets, planetesimals[1][2] and circumstellar disks. The Sun together with the planetary system revolving around it, including Earth, forms the Solar System.[3][4] The term exoplanetary system is sometimes used in reference to other planetary systems. As of 1 September 2021, there are 4,834 confirmed exoplanets in 3,572 planetary systems, with 795 systems having more than one planet.[5] Debris disks are also known to be common, though other objects are more difficult to observe. Of particular interest to astrobiology is the habitable zone of planetary systems where planets could have surface liquid water, and thus the capacity to support Earth-like life.... History Heliocentrism Historically, heliocentrism (the doctrine that the Sun is at the centre of the universe) was opposed to geocentrism (placing Earth at the centre of the universe). The notion of a heliocentric Solar System with the Sun at its centre is possibly first suggested in the Vedic literature of ancient India, which often refer to the Sun as the "centre of spheres". Some interpret Aryabhatta's writings in Āryabhaṭīya as implicitly heliocentric. The idea was first proposed in Western philosophy and Greek astronomy as early as the 3rd century BC by Aristarchus of Samos,[6] but received no support from most other ancient astronomers. Discovery of the Solar System Main article: Discovery and exploration of the Solar System Heliocentric model of the Solar System in Copernicus' manuscript De revolutionibus orbium coelestium by Nicolaus Copernicus, published in 1543, presented the first mathematically predictive heliocentric model of a planetary system. 17th-century successors Galileo Galilei, Johannes Kepler, and Sir Isaac Newton developed an understanding of physics which led to the gradual acceptance of the idea that Earth moves around the Sun and that the planets are governed by the same physical laws that governed Earth. Speculation on extrasolar planetary systems In the 16th century the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are likewise accompanied by planets. He was burned at the stake for his ideas by the Roman Inquisition.[7] In the 18th century the same possibility was mentioned by Sir Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."[8] His theories gained traction[colloquialism] through the 19th and 20th centuries despite a lack of supporting evidence. Long before their confirmation by astronomers, conjecture on the nature of planetary systems had been a focus of the search for extraterrestrial intelligence and has been a prevalent theme in fiction, particularly science fiction. Detection of exoplanets The first confirmed detection of an exoplanet was in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmed detection of exoplanets of a main-sequence star was made in 1995, when a giant planet, 51 Pegasi b, was found in a four-day orbit around the nearby G-type star 51 Pegasi. The frequency of detections has increased since then, particularly through advancements in methods of detecting extrasolar planets and dedicated planet finding programs such as the Kepler mission. Origin and evolution See also: Nebular hypothesis, Planetary migration, and Formation and evolution of the Solar System An artist's concept of a protoplanetary disk Planetary systems come from protoplanetary disks that form around stars as part of the process of star formation. During formation of a system, much material is gravitationally-scattered into distant orbits, and some planets are ejected completely from the system, becoming rogue planets. Evolved systems High-mass stars Planets orbiting pulsars have been discovered. Pulsars are the remnants of the supernova explosions of high-mass stars, but a planetary system that existed before the supernova would likely be mostly destroyed. Planets would either evaporate, be pushed off of their orbits by the masses of gas from the exploding star, or the sudden loss of most of the mass of the central star would see them escape the gravitational hold of the star, or in some cases the supernova would kick the pulsar itself out of the system at high velocity so any planets that had survived the explosion would be left behind as free-floating objects. Planets found around pulsars may have formed as a result of pre-existing stellar companions that were almost entirely evaporated by the supernova blast, leaving behind planet-sized bodies. Alternatively, planets may form in an accretion disk of fallback matter surrounding a pulsar.[9] Fallback disks of matter that failed to escape orbit during a supernova may also form planets around black holes.[10] Lower-mass stars Protoplanetary discs observed with the Very Large Telescope.[11] As stars evolve and turn into red giants, asymptotic giant branch stars, and planetary nebulae they engulf the inner planets, evaporating or partially evaporating them depending on how massive they are. As the star loses mass, planets that are not engulfed move further out from the star. If an evolved star is in a binary or multiple system, then the mass it loses can transfer to another star, forming new protoplanetary disks and second- and third-generation planets which may differ in composition from the original planets, which may also be affected by the mass transfer. System architectures The Solar System consists of an inner region of small rocky planets and outer region of large gas giants. However, other planetary systems can have quite different architectures. Studies suggest that architectures of planetary systems are dependent on the conditions of their initial formation.[12] Many systems with a hot Jupiter gas giant very close to the star have been found. Theories, such as planetary migration or scattering, have been proposed for the formation of large planets close to their parent stars.[13] At present,[when?] few systems have been found to be analogous to the Solar System with terrestrial planets close to the parent star. More commonly, systems consisting of multiple Super-Earths have been detected.[14] Components Planets and stars Main article: Planet-hosting stars The Morgan-Keenan spectral classification Most known exoplanets orbit stars roughly similar to the Sun: that is, main-sequence stars of spectral categories F, G, or K. One reason is that planet-search programs have tended to concentrate on such stars. In addition, statistical analyses indicate that lower-mass stars (red dwarfs, of spectral category M) are less likely to have planets massive enough to be detected by the radial-velocity method.[15][16] Nevertheless, several tens of planets around red dwarfs have been discovered by the Kepler spacecraft by the transit method, which can detect smaller planets. Circumstellar disks and dust structures Main article: Circumstellar disk Debris disks detected in HST archival images of young stars, HD 141943 and HD 191089, using improved imaging processes (April 24, 2014). After planets, circumstellar disks are one of the most commonly-observed properties of planetary systems, particularly of young stars. The Solar System possesses at least four major circumstellar disks (the asteroid belt, Kuiper belt, scattered disc, and Oort cloud) and clearly-observable disks have been detected around nearby solar analogs including Epsilon Eridani and Tau Ceti. Based on observations of numerous similar disks, they are assumed to be quite common attributes of stars on the main sequence. Interplanetary dust clouds have been studied in the Solar System and analogs are believed to be present in other planetary systems. Exozodiacal dust, an exoplanetary analog of zodiacal dust, the 1–100 micrometre-sized grains of amorphous carbon and silicate dust that fill the plane of the Solar System[17] has been detected around the 51 Ophiuchi, Fomalhaut,[18][19] Tau Ceti,[19][20] and Vega systems. Comets Main article: Comet As of November 2014 there are 5,253 known Solar System comets[21] and they are thought to be common components of planetary systems. The first exocomets were detected in 1987[22][23] around Beta Pictoris, a very young A-type main-sequence star. There are now a total of 11 stars around which the presence of exocomets have been observed or suspected.[24][25][26][27] All discovered exocometary systems (Beta Pictoris, HR 10,[24] 51 Ophiuchi, HR 2174,[25] 49 Ceti, 5 Vulpeculae, 2 Andromedae, HD 21620, HD 42111, HD 110411,[26][28] and more recently HD 172555[27]) are around very young A-type stars. Other components Further information: Circumplanetary disk Computer modelling of an impact in 2013 detected around the star NGC 2547-ID8 by the Spitzer Space Telescope, and confirmed by ground observations, suggests the involvement of large asteroids or protoplanets similar to the events believed to have led to the formation of terrestrial planets like the Earth.[29] Based on observations of the Solar System's large collection of natural satellites, they are believed common components of planetary systems; however, the existence of exomoons has, so far,[when?] not been confirmed. The star 1SWASP J140747.93-394542.6, in the constellation Centaurus, is a strong candidate for a natural satellite.[30] Indications suggest that the confirmed extrasolar planet WASP-12b also has at least one satellite.[31] Orbital configurations Unlike the Solar System, which has orbits that are nearly circular, many of the known planetary systems display much higher orbital eccentricity.[32] An example of such a system is 16 Cygni. Mutual inclination The mutual inclination between two planets is the angle between their orbital planes. Many compact systems with multiple close-in planets interior to the equivalent orbit of Venus are expected to have very low mutual inclinations, so the system (at least the close-in part) would be even flatter than the Solar System. Captured planets could be captured into any arbitrary angle to the rest of the system. As of 2016 there are only a few systems where mutual inclinations have actually been measured[33] One example is the Upsilon Andromedae system: the planets c and d have a mutual inclination of about 30 degrees.[34][35] Orbital dynamics Planetary systems can be categorized according to their orbital dynamics as resonant, non-resonant-interacting, hierarchical, or some combination of these. In resonant systems the orbital periods of the planets are in integer ratios. The Kepler-223 system contains four planets in an 8:6:4:3 orbital resonance.[36] Giant planets are found in mean-motion resonances more often than smaller planets.[37] In interacting systems the planets orbits are close enough together that they perturb the orbital parameters. The Solar System could be described as weakly interacting. In strongly interacting systems Kepler's laws do not hold.[38] In hierarchical systems the planets are arranged so that the system can be gravitationally considered as a nested system of two-bodies, e.g. in a star with a close-in hot jupiter with another gas giant much further out, the star and hot jupiter form a pair that appears as a single object to another planet that is far enough out. Other, as yet unobserved, orbital possibilities include: double planets; various co-orbital planets such as quasi-satellites, trojans and exchange orbits; and interlocking orbits maintained by precessing orbital planes.[39] Number of planets, relative parameters and spacings The spacings between orbits vary widely amongst the different systems discovered by the Kepler spacecraft. [icon]    This section is empty. You can help by adding to it. (August 2021) Planet capture Free-floating planets in open clusters have similar velocities to the stars and so can be recaptured. They are typically captured into wide orbits between 100 and 105 AU. The capture efficiency decreases with increasing cluster size, and for a given cluster size it increases with the host/primary[clarification needed] mass. It is almost independent of the planetary mass. Single and multiple planets could be captured into arbitrary unaligned orbits, non-coplanar with each other or with the stellar host spin, or pre-existing planetary system. Some planet–host metallicity correlation may still exist due to the common origin of the stars from the same cluster. Planets would be unlikely to be captured around neutron stars because these are likely to be ejected from the cluster by a pulsar kick when they form. Planets could even be captured around other planets to form free-floating planet binaries. After the cluster has dispersed some of the captured planets with orbits larger than 106 AU would be slowly disrupted by the galactic tide and likely become free-floating again through encounters with other field stars or giant molecular clouds.[40] Zones Habitable zone Main article: Circumstellar habitable zone Location of habitable zone around different types of stars The habitable zone around a star is the region where the temperature range allows for liquid water to exist on a planet; that is, not too close to the star for the water to evaporate and not too far away from the star for the water to freeze. The heat produced by stars varies depending on the size and age of the star; this means the habitable zone will also vary accordingly. Also, the atmospheric conditions on the planet influence the planet's ability to retain heat so that the location of the habitable zone is also specific to each type of planet. Habitable zones have usually been defined in terms of surface temperature; however, over half of Earth's biomass is from subsurface microbes,[41] and temperature increases as depth underground increases, so the subsurface can be conducive for life when the surface is frozen; if this is considered, the habitable zone extends much further from the star.[42] Studies in 2013 indicated an estimated frequency of 22±8% of Sun-like[a] stars having an Earth-sized[b] planet in the habitable[c] zone.[43][44] Venus zone The Venus zone is the region around a star where a terrestrial planet would have runaway greenhouse conditions like Venus, but not so near the star that the atmosphere completely evaporates. As with the habitable zone, the location of the Venus zone depends on several factors, including the type of star and properties of the planets such as mass, rotation rate, and atmospheric clouds. Studies of the Kepler spacecraft data indicate that 32% of red dwarfs have potentially Venus-like planets based on planet size and distance from star, increasing to 45% for K-type and G-type stars.[d] Several candidates have been identified, but spectroscopic follow-up studies of their atmospheres are required to determine whether they are like Venus.[45][46] Galactic distribution of planets See also: Galactic habitable zone, Extragalactic planet, and Globular cluster § Planets 90% of planets with known distances are within about 2000 light years of Earth, as of July 2014. The Milky Way is 100,000 light-years across, but 90% of planets with known distances are within about 2000 light years of Earth, as of July 2014. One method that can detect planets much further away is microlensing. The WFIRST spacecraft could use microlensing to measure the relative frequency of planets in the galactic bulge versus the galactic disk.[47] So far, the indications are that planets are more common in the disk than the bulge.[48] Estimates of the distance of microlensing events is difficult: the first planet considered with high probability of being in the bulge is MOA-2011-BLG-293Lb at a distance of 7.7 kiloparsecs (about 25,000 light years).[49] Population I, or metal-rich stars, are those young stars whose metallicity is highest. The high metallicity of population I stars makes them more likely to possess planetary systems than older populations, because planets form by the accretion of metals.[citation needed] The Sun is an example of a metal-rich star. These are common in the spiral arms of the Milky Way.[citation needed] Generally, the youngest stars, the extreme population I, are found farther in and intermediate population I stars are farther out, etc. The Sun is considered an intermediate population I star. Population I stars have regular elliptical orbits around the Galactic Center, with a low relative velocity.[50] Population II, or metal-poor stars, are those with relatively low metallicity which can have hundreds (e.g. BD +17° 3248) or thousands (e.g. Sneden's Star) times less metallicity than the Sun. These objects formed during an earlier time of the universe.[citation needed] Intermediate population II stars are common in the bulge near the center of the Milky Way,[citation needed] whereas Population II stars found in the galactic halo are older and thus more metal-poor.[citation needed] Globular clusters also contain high numbers of population II stars.[51] In 2014, the first planets around a halo star were announced around Kapteyn's star, the nearest halo star to Earth, around 13 light years away. However, later research suggests that Kapteyn b is just an artefact of stellar activity and that Kapteyn c needs more study to be confirmed.[52] The metallicity of Kapteyn's star is estimated to be about 8[e] times less than the Sun.[53] Different types of galaxies have different histories of star formation and hence planet formation. Planet formation is affected by the ages, metallicities, and orbits of stellar populations within a galaxy. Distribution of stellar populations within a galaxy varies between the different types of galaxies.[54] Stars in elliptical galaxies are much older than stars in spiral galaxies. Most elliptical galaxies contain mainly low-mass stars, with minimal star-formation activity.[55] The distribution of the different types of galaxies in the universe depends on their location within galaxy clusters, with elliptical galaxies found mostly close to their centers." (wikipedia.org) "A jigsaw puzzle is a tiling puzzle that requires the assembly of often oddly shaped interlocking and mosaiced pieces. Typically, each individual piece has a portion of a picture; when assembled, they produce a complete picture. Beginning in the 18th century, jigsaw puzzles were created by painting a picture on a flat, rectangular piece of wood, then cutting it into small pieces. Despite the name, a jigsaw was never used. John Spilsbury, a London cartographer and engraver, is credited with commercializing jigsaw puzzles around 1760.[1] They have since come to be made primarily of cardboard. Typical images on jigsaw puzzles include scenes from nature, buildings, and repetitive designs—castles and mountains are common, as well as other traditional subjects. However, any kind of picture can be used. Artisanal puzzle-makers and companies using technologies for one-off and small print-run puzzles utilize a wide range of subject matter, including optical illusions, unusual art, and personal photographs. In addition to traditional flat, two-dimensional puzzles, three-dimensional puzzles have entered large-scale production, including spherical puzzles and architectural recreations. In recent years, a range of jigsaw puzzle accessories including boards, cases, frames, and roll-up mats has become available to assist jigsaw puzzle enthusiasts. While most assembled puzzles are disassembled for reuse, they can also be attached to a backing with adhesive and displayed as art.... History John Spilsbury's "Europe divided into its kingdoms, etc." (1766). He created the jigsaw puzzle for educational purposes, and called them "Dissected Maps".[2][3] London engraver and cartographer John Spilsbury is believed to have produced the first jigsaw puzzle around 1760, using a marquetry saw.[1] Early puzzles, known as dissections, were produced by mounting maps on sheets of hardwood and cutting along national boundaries, creating a puzzle useful for teaching geography.[1] Royal governess Lady Charlotte Finch used such "dissected maps" to teach the children of King George III and Queen Charlotte[4][5] British printed puzzle from 1874. The name "jigsaw" came to be associated with the puzzle around 1880 when fretsaws became the tool of choice for cutting the shapes. Since fretsaws are distinct from jigsaws, the name appears to be a misnomer.[1] Cardboard jigsaw puzzles appeared in the late 1800s, but were slow to replace wooden ones because manufacturers felt that cardboard puzzles would be perceived as low-quality, and because profit margins on wooden jigsaws were larger.[1] Wooden jigsaw pieces, cut by hand Jigsaw puzzles soared in popularity during the Great Depression, as they provided a cheap, long-lasting, recyclable form of entertainment.[1][6] It was around this time that jigsaws evolved to become more complex and appealing to adults.[1] They were also given away in product promotions and used in advertising, with customers completing an image of the promoted product.[1][6] Sales of wooden puzzles fell after World War II as improved wages led to price increases, while improvements in manufacturing processes made paperboard jigsaws more attractive.[6] According to the Alzheimer Society of Canada, doing jigsaw puzzles is one of many activities that can help keep the brain active and may reduce the risk of Alzheimer's disease.[7] Most modern jigsaw puzzles are made of paperboard as they are easier and cheaper to mass-produce. An enlarged photograph or printed reproduction of a painting or other two-dimensional artwork is glued to cardboard, which is then fed into a press. The press forces a set of hardened steel blades of the desired pattern, called a puzzle die, through the board until it is fully cut. The puzzle die is a flat board, often made from plywood, with slots cut or burned in the same shape as the knives that are used. The knives are set into the slots and covered in a compressible material, typically foam rubber, which serves to eject the cut puzzle pieces. The cutting process is similar to making shaped cookies with a cookie cutter—however, the forces involved are tremendously greater: A typical 1000-piece puzzle requires upwards of 700 tons of force to push the die through the board. Beginning in the 1930s, jigsaw puzzles were cut using large hydraulic presses which now cost in the hundreds of thousands of dollars. The precise cuts gave a very snug fit, but the cost limited jigsaw puzzle production to large corporations. Recent roller-press methods achieve the same results at lower cost.[citation needed] New technology has also enabled laser-cutting of wooden or acrylic jigsaw puzzles, with the advantage that the puzzle can be custom-cut to any size or shape, with any number or average size of pieces. Many museums have laser-cut acrylic puzzles made of some of their art that so visiting children can assemble puzzles of the images on display. Acrylic pieces are very durable, waterproof, and can withstand continued use without the image degrading. Also, because the print and cut patterns are computer-based, lost pieces can easily be remade. By the early 1960s, Tower Press was the world's largest jigsaw puzzle maker; it was acquired by Waddingtons in 1969.[10] Numerous smaller-scale puzzle makers work in artisanal styles, handcrafting and handcutting their creations.[11][12][13][14] Variations Jigsaw puzzle software allowing rotation of pieces A three-dimensional puzzle composed of several two-dimensional puzzles stacked on top of one another A puzzle without a picture Jigsaw puzzles come in a variety of sizes. Among those marketed to adults, 300-, 500- and 750-piece puzzles are considered "smaller". More sophisticated, but still common, puzzles come in sizes of 1,000, 1,500, 2,000, 3,000, 4,000, 5,000, 6,000, 7,500, 8,000, 9,000, 13,200, 18,000, 24,000, 32,000 and 40,000 pieces. Jigsaw puzzles geared towards children typically have many fewer pieces, and are typically much larger. For very young children, puzzles with as few as 4 to 9 large pieces (so as not to be a choking hazard) are common. They are usually made of wood or plastic, for durability, and can be cleaned without damage. The most common layout for a thousand-piece puzzle is 38 pieces by 27 pieces, for an actual total of 1,026 pieces. Most 500-piece puzzles are 27 pieces by 19 pieces. A few puzzles are double-sided so they can be solved from either side—adding complexity, as the enthusiast must determine if they are looking at the correct side of each piece. "Family puzzles" of 100–550 pieces use a combination of small-, medium- and large-sized pieces, with each size going in one direction or towards the middle of the puzzle. This allows a family of different skill levels and hand sizes to work on the puzzle together. Companies like Springbok, Cobble Hill, Ravensburger and Suns Out make this type of specialty puzzle. There are also three-dimensional jigsaw puzzles. Many are made of wood or styrofoam and require the puzzle to be solved in a certain order, as some pieces will not fit if others are already in place. Also common are puzzle boxes, simple three-dimensional puzzles with a small drawer or box in the center for storage. Another type of 3-D jigsaw puzzle is a puzzle globe, often made of plastic. Like 2-D puzzles, the assembled pieces form a single layer, but the final form is three-dimensional. Most globe puzzles have designs representing spherical shapes such as the Earth, the Moon, and historical globes of the Earth. Jigsaw puzzles can vary greatly in price depending on their complexity, number of pieces, and brand. In the US, children's puzzles can start around $5, while larger ones can be closer to $50. The most expensive puzzle to date was sold for $US27,000 in 2005 at a charitable auction for The Golden Retriever Foundation.[15] Several word-puzzle games use pieces similar to those in jigsaw puzzles. Examples include Alfa-Lek, Jigsaw Words, Nab-It!, Puzzlage, Typ-Dom, Word Jigsaw, and Yottsugo.[16][citation needed] Puzzle pieces A "whimsy" piece in a wooden jigsaw puzzle A 3D jigsaw puzzle Many puzzles are termed "fully interlocking", meaning that adjacent pieces are connected in such a way that if one is moved horizontally, the others stay attached to it. Sometimes the connection is tight enough to pick up a solved part by holding one piece. Some fully interlocking puzzles have pieces all of a similar shape, with rounded tabs (interjambs) on opposite ends, and corresponding indentations—called blanks—on the other two sides to receive the tabs. Other fully interlocking puzzles may have tabs and blanks variously arranged on each piece; but they usually have four sides, and the numbers of tabs and blanks thus add up to four. Uniformly shaped fully interlocking puzzles, sometimes called "Japanese Style", are the most difficult, because the differences in the pieces' shapes is most subtle.[citation needed] Most jigsaw puzzles are square, rectangular or round, with edge pieces with one straight or smoothly curved side, plus four corner pieces (if the puzzle is square or rectangular). However, some puzzles have edge and corner pieces cut like the rest, with no straight sides, making it more challenging to identify them. Other puzzles utilize more complex edge pieces to form special shapes when assembled, such as profiles of animals. The pieces of spherical jigsaw, like immersive panorama jigsaw, can be triangular shaped, according to the rules of tessellation of the geoid primitive. The designer Yuu Asaka created "Jigsaw Puzzle 29" which has not four corner pieces but five corner pieces, and is made from pale blue acrylic without a picture. [17] It was awarded the Jury Honorable Mention of 2018 Puzzle Design Competition. [18] Because many puzzlers had solved it easily, he created "Jigsaw Puzzle 19" which composed only with corner pieces as revenge. [19] It was made with transparent green acrylic pieces without a picture. [20] World records Largest commercially available jigsaw puzzles Pieces     Name of puzzle     Company     Year     Size [cm]     Area [m2] 54,000     Travel by Art     Grafika     2020     864 × 204     17.65 52,110     (No title: collage of animals)     MartinPuzzle     2018     696 × 202     14.06 51,300     27 Wonders from Around the World     Kodak     2019     869 × 191     16.60 48,000     Around the World     Grafika     2017     768 × 204     15.67 42,000     La vuelta al Mundo     Educa Borras     2017     749 × 157     11.76 40,320     Making Mickey Magic     Ravensburger     2018     680 × 192     13.06 40,320     Memorable Disney Moments     Ravensburger     2016     680 × 192     13.06 33,600     Wild Life     Educa Borras     2014     570 × 157     8.95 32,000     New York City Window     Ravensburger     2014     544 × 192     10.45 32,000     Double Retrospect     Ravensburger     2010     544 × 192     10.45 24,000     Life, The greatest puzzle     Educa Borras     2007     428 × 157     6.72 Calculating the number of border pieces before starting Jigsaw puzzlers often want to know in advance how many border pieces they are looking for to verify they have found all of them. Puzzle sizes are typically listed on commercially distributed puzzles, but usually just include the total number of pieces in the puzzle, and do not list the count of edge or interior pieces. Puzzlers therefore calculate the number of border pieces. To calculate B (border pieces) from P (the total piece count), follow this method:     List the prime factors of P.     For example: For a 513-piece jigsaw, the prime factorization tree is 3×3×3×19=513.     Take the square root of P and round off.     The square root of 513 is about 22.6, so round to 23.     Look for numbers in the prime factor list within +/- 20 percent of the square root of P.         Calculate 20% of the square root of P.         20% of 23 = 4.6.         Develop the range, +/- 20%, from the square root of P.         The square root is about 23. 23 +/- 4.6 = 18.4 to 27.6         Compare the range with the factor list. Define this as E1.         The factor list shows 19 in the range.     Determine the horizontal / vertical dimensions.         Divide P (the total number of pieces) by E1 to determine the horizontal / vertical dimensions, E1xE2.         513 / 19 = 27. This is probably a 19x27 puzzle.         alternate approach: Take the remaining numbers from the prime factorization tree.         3x3x3 = 27     Add the four sides and subtract "4" to correct for the corner pieces, which would otherwise be counted in both the horizontal and vertical.     27 + 27 + 19 + 19 -4 = 88. These 88 border pieces include 4 corners, 17 pieces between corners on the short sides, and 25 between corners on the long sides. Common puzzle dimensions:     1000 piece puzzle: 1026 pieces, 126 border pieces (38x27)[21] Largest-sized jigsaw puzzles The world's largest-sized jigsaw puzzle measured 5,428.8 m2 (58,435 sq ft) with 21,600 pieces, each measuring a Guinness World Records maximum size of 50 cm by 50 cm. It was assembled on 3 November 2002 by 777 people at the former Kai Tak Airport in Hong Kong.[22] Largest jigsaw puzzle – most pieces The Guinness record of CYM Group in 2011 with 551,232 pieces The jigsaw with the greatest number of pieces had 551,232 pieces and measured 14.85 × 23.20 m (48 ft 8.64 in × 76 ft 1.38 in). It was assembled on 25 September 2011 at Phú Thọ Indoor Stadium in Ho Chi Minh City, Vietnam, by students of the University of Economics, Ho Chi Minh City. It is listed by the Guinness World Records for the "Largest Jigsaw Puzzle – most pieces", but as the intact jigsaw had been divided into 3,132 sections, each containing 176 pieces, which were reassembled and then connected, the claim is controversial.[23][24] Cultural references The logo of Wikipedia is a globe made out of jigsaw pieces. The incomplete sphere appears to have some pieces missing, symbolizing the room to add new knowledge.[citation needed] In the logo of the Colombian Office of the Attorney General appears a jigsaw puzzle piece in foreground. They named it as "The Key Piece": "The piece of a puzzle is the proper symbol to visually represent the Office of the Attorney General because it includes the concepts of search, solution and answer that the entity pursues through the investigative activity."[25] Art and entertainment The central antagonist in the Saw film franchise is named Jigsaw.[26] In the 1933 Laurel and Hardy short Me and My Pal, several characters attempt to complete a large jigsaw puzzle.[27] "Lost in Translation" is not only a poem about a child putting together a jigsaw puzzle, it is itself an interpretive puzzle. Life: A User's Manual, Georges Perec's most famous novel, tells as pieces of a puzzle a story about a jigsaw puzzle maker. Symbol for autism An "autism awareness" ribbon, featuring red, blue, and yellow jigsaw pieces Jigsaw puzzle pieces were first used as a symbol for autism in 1963 by the United Kingdom's National Autistic Society.[28] The organization chose jigsaw pieces for their logo to represent the "puzzling" nature of autism and the inability to "fit in" due to social differences, and also because jigsaw pieces were recognizable and otherwise unused. Puzzle pieces have since been incorporated into the logos and promotional materials of many organizations, including the Autism Society of America and Autism Speaks. Proponents of the autism rights movement oppose the jigsaw puzzle iconography, stating that metaphors such as "puzzling" and "incomplete" are harmful to autistic people. Critics of the puzzle piece symbol instead advocate for a rainbow-colored infinity symbol representing diversity.[29] In 2017, the journal Autism concluded that the use of the jigsaw puzzle evoked negative public perception towards autistic individuals, and in February 2018 removed the puzzle piece from their cover." (wikipedia.org) "A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and – according to the International Astronomical Union but not all planetary scientists – has cleared its neighbouring region of planetesimals.[b][1][2] The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Apart from Earth itself, five planets in the Solar System are often visible to the naked eye. These were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union (IAU) officially adopted a resolution defining planets within the Solar System. This definition is controversial because it excludes many objects of planetary mass based on where or what they orbit. Although eight of the planetary bodies discovered before 1950 remain "planets" under the current definition, some celestial bodies, such as Ceres, Pallas, Juno and Vesta (each an object in the solar asteroid belt), and Pluto (the first trans-Neptunian object discovered), that were once considered planets by the scientific community, are no longer viewed as planets under the current definition of planet. The planets were thought by Ptolemy to orbit Earth in deferent and epicycle motions. Although the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. About the same time, by careful analysis of pre-telescopic observational data collected by Tycho Brahe, Johannes Kepler found the planets' orbits were elliptical rather than circular. As observational tools improved, astronomers saw that, like Earth, each of the planets rotated around an axis tilted with respect to its orbital pole, and some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the other planets share characteristics such as volcanism, hurricanes, tectonics, and even hydrology. Planets in the Solar System are divided into two main types: large low-density giant planets, and smaller rocky terrestrials. There are eight planets in the Solar System according to the IAU definition.[1] In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or more natural satellites, the two exceptions being Mercury and Venus. Several thousands of planets around other stars ("extrasolar planets" or "exoplanets") have been discovered in the Milky Way. As of 1 September 2021, 4,834 known extrasolar planets in 3,572 planetary systems (including 795 multiple planetary systems), ranging in size from just above the size of the Moon to gas giants about twice as large as Jupiter, have been discovered, out of which more than 100 planets are the same size as Earth, nine of which are at the same relative distance from their star as Earth from the Sun, i.e. in the circumstellar habitable zone.[3][4] On 20 December 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e[5] and Kepler-20f,[6] orbiting a Sun-like star, Kepler-20.[7][8][9] A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.[10] Around one in five Sun-like[c] stars is thought to have an Earth-sized[d] planet in its habitable[e] zone.... History Further information: History of astronomy, Definition of planet, and Timeline of Solar System astronomy Printed rendition of a geocentric cosmological model from Cosmographia, Antwerp, 1539 The idea of planets has evolved over its history, from the divine lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. The ambiguities inherent in defining planets have led to much scientific controversy. The five classical planets of the Solar System, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, and ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the "fixed stars", which maintained a constant relative position in the sky.[13] Ancient Greeks called these lights πλάνητες ἀστέρες (planētes asteres, "wandering stars") or simply πλανῆται (planētai, "wanderers"),[14] from which today's word "planet" was derived.[15][16][17] In ancient Greece, China, Babylon, and indeed all pre-modern civilizations,[18][19] it was almost universally believed that Earth was the center of the Universe and that all the "planets" circled Earth. The reasons for this perception were that stars and planets appeared to revolve around Earth each day[20] and the apparently common-sense perceptions that Earth was solid and stable and that it was not moving but at rest. Babylon Main article: Babylonian astronomy The first civilization known to have a functional theory of the planets were the Babylonians, who lived in Mesopotamia in the first and second millennia BC. The oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa, a 7th-century BC copy of a list of observations of the motions of the planet Venus, that probably dates as early as the second millennium BC.[21] The MUL.APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun, Moon, and planets over the course of the year.[22] The Babylonian astrologers also laid the foundations of what would eventually become Western astrology.[23] The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC,[24] comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.[25][26] Venus, Mercury, and the outer planets Mars, Jupiter, and Saturn were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope in early modern times.[27] Greco-Roman astronomy See also: Greek astronomy Ptolemy's 7 planetary spheres 1 Moon ☾     2 Mercury ☿     3 Venus ♀     4 Sun ☉     5 Mars ♂     6 Jupiter ♃     7 Saturn ♄ The ancient Greeks initially did not attach as much significance to the planets as the Babylonians. The Pythagoreans, in the 6th and 5th centuries BC appear to have developed their own independent planetary theory, which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe. Pythagoras or Parmenides is said to have been the first to identify the evening star (Hesperos) and morning star (Phosphoros) as one and the same (Aphrodite, Greek corresponding to Latin Venus),[28] though this had long been known by the Babylonians. In the 3rd century BC, Aristarchus of Samos proposed a heliocentric system, according to which Earth and the planets revolved around the Sun. The geocentric system remained dominant until the Scientific Revolution. By the 1st century BC, during the Hellenistic period, the Greeks had begun to develop their own mathematical schemes for predicting the positions of the planets. These schemes, which were based on geometry rather than the arithmetic of the Babylonians, would eventually eclipse the Babylonians' theories in complexity and comprehensiveness, and account for most of the astronomical movements observed from Earth with the naked eye. These theories would reach their fullest expression in the Almagest written by Ptolemy in the 2nd century CE. So complete was the domination of Ptolemy's model that it superseded all previous works on astronomy and remained the definitive astronomical text in the Western world for 13 centuries.[21][29] To the Greeks and Romans there were seven known planets, each presumed to be circling Earth according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order and using modern names): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.[17][29][30] Cicero, in his De Natura Deorum, enumerated the planets known during the 1st century BCE using the names for them in use at the time:[31]     "But there is most matter for wonder in the movements of the five stars which are falsely called wandering; falsely, because nothing wanders which through all eternity preserves its forward and retrograde courses, and its other movements, constant and unaltered. ... For instance, the star which is farthest from the earth, which is known as the star of Saturn, and is called by the Greeks Φαίνων (Phainon), accomplishes its course in about thirty years, and though in that course it does much that is wonderful, first preceding the sun, and then falling off in speed, becoming invisible at the hour of evening, and returning to view in the morning, it never through the unending ages of time makes any variation, but performs the same movements at the same times. Beneath it, and nearer to the earth, moves the planet of Jupiter, which is called in Greek Φαέθων (Phaethon); it completes the same round of the twelve signs in twelve years, and performs in its course the same variations as the planet of Saturn. The circle next below it is held by Πυρόεις (Pyroeis), which is called the planet of Mars, and traverses the same round as the two planets above it in four and twenty months, all but, I think, six days. Beneath this is the planet of Mercury, which is called by the Greeks Στίλβων (Stilbon); it traverses the round of the zodiac in about the time of the year's revolution, and never withdraws more than one sign's distance from the sun, moving at one time in advance of it, and at another in its rear. The lowest of the five wandering stars, and the one nearest the earth, is the planet of Venus, which is called Φωσϕόρος (Phosphoros) in Greek, and Lucifer in Latin, when it is preceding the sun, but Ἕσπερος (Hesperos) when it is following it; it completes its course in a year, traversing the zodiac both latitudinally and longitudinally, as is also done by the planets above it, and on whichever side of the sun it is, it never departs more than two signs' distance from it." India Main articles: Indian astronomy and Hindu cosmology In 499 CE, the Indian astronomer Aryabhata propounded a planetary model that explicitly incorporated Earth's rotation about its axis, which he explains as the cause of what appears to be an apparent westward motion of the stars. He also believed that the orbits of planets are elliptical.[32] Aryabhata's followers were particularly strong in South India, where his principles of the diurnal rotation of Earth, among others, were followed and a number of secondary works were based on them.[33] In 1500, Nilakantha Somayaji of the Kerala school of astronomy and mathematics, in his Tantrasangraha, revised Aryabhata's model.[34] In his Aryabhatiyabhasya, a commentary on Aryabhata's Aryabhatiya, he developed a planetary model where Mercury, Venus, Mars, Jupiter and Saturn orbit the Sun, which in turn orbits Earth, similar to the Tychonic system later proposed by Tycho Brahe in the late 16th century. Most astronomers of the Kerala school who followed him accepted his planetary model.[34][35] Medieval Muslim astronomy Main articles: Astronomy in the medieval Islamic world and Cosmology in medieval Islam In the 11th century, the transit of Venus was observed by Avicenna, who established that Venus was, at least sometimes, below the Sun.[36] In the 12th century, Ibn Bajjah observed "two planets as black spots on the face of the Sun", which was later identified as a transit of Mercury and Venus by the Maragha astronomer Qotb al-Din Shirazi in the 13th century.[37] Ibn Bajjah could not have observed a transit of Venus, because none occurred in his lifetime.[38] European Renaissance Renaissance planets, c. 1543 to 1610 and c. 1680 to 1781 1 Mercury ☿     2 Venus ♀     3 Earth ⊕     4 Mars ♂     5 Jupiter ♃     6 Saturn ♄ See also: Heliocentrism With the advent of the Scientific Revolution, use of the term "planet" changed from something that moved across the sky (in relation to the star field); to a body that orbited Earth (or that was believed to do so at the time); and by the 18th century to something that directly orbited the Sun when the heliocentric model of Copernicus, Galileo and Kepler gained sway. Thus, Earth became included in the list of planets,[39] whereas the Sun and Moon were excluded. At first, when the first satellites of Jupiter and Saturn were discovered in the 17th century, the terms "planet" and "satellite" were used interchangeably – although the latter would gradually become more prevalent in the following century.[40] Until the mid-19th century, the number of "planets" rose rapidly because any newly discovered object directly orbiting the Sun was listed as a planet by the scientific community. 19th century Eleven planets, 1807–1845 1 Mercury ☿     2 Venus ♀     3 Earth ⊕     4 Mars ♂     5 Vesta ⚶     6 Juno ⚵     7 Ceres ⚳     8 Pallas ⚴     9 Jupiter ♃     10 Saturn ♄     11 Uranus ♅ In the 19th century astronomers began to realize that recently discovered bodies that had been classified as planets for almost half a century (such as Ceres, Pallas, Juno, and Vesta) were very different from the traditional ones. These bodies shared the same region of space between Mars and Jupiter (the asteroid belt), and had a much smaller mass; as a result they were reclassified as "asteroids". In the absence of any formal definition, a "planet" came to be understood as any "large" body that orbited the Sun. Because there was a dramatic size gap between the asteroids and the planets, and the spate of new discoveries seemed to have ended after the discovery of Neptune in 1846, there was no apparent need to have a formal definition.[41] 20th century Planets 1854–1930, Solar planets 2006–present 1 Mercury ☿     2 Venus ♀     3 Earth ⊕     4 Mars ♂     5 Jupiter ♃     6 Saturn ♄     7 Uranus ♅     8 Neptune ♆ In the 20th century, Pluto was discovered. After initial observations led to the belief that it was larger than Earth,[42] the object was immediately accepted as the ninth planet. Further monitoring found the body was actually much smaller: in 1936, Ray Lyttleton suggested that Pluto may be an escaped satellite of Neptune,[43] and Fred Whipple suggested in 1964 that Pluto may be a comet.[44] As it was still larger than all known asteroids and the population of dwarf planets & other trans-Neptunian objects was not well observed,[45] it kept its status until 2006. (Solar) planets 1930–2006 1 Mercury ☿     2 Venus ♀     3 Earth ⊕     4 Mars ♂     5 Jupiter ♃     6 Saturn ♄     7 Uranus ♅     8 Neptune ♆     9 Pluto ♇ In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar, PSR B1257+12.[46] This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on October 6, 1995, Michel Mayor and Didier Queloz of the Geneva Observatory announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi).[47] The discovery of extrasolar planets led to another ambiguity in defining a planet: the point at which a planet becomes a star. Many known extrasolar planets are many times the mass of Jupiter, approaching that of stellar objects known as brown dwarfs. Brown dwarfs are generally considered stars due to their ability to fuse deuterium, a heavier isotope of hydrogen. Although objects more massive than 75 times that of Jupiter fuse hydrogen, objects of only 13 Jupiter masses can fuse deuterium. Deuterium is quite rare, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.[48] 21st century With the discovery during the latter half of the 20th century of more objects within the Solar System and large objects around other stars, disputes arose over what should constitute a planet. There were particular disagreements over whether an object should be considered a planet if it was part of a distinct population such as a belt, or if it was large enough to generate energy by the thermonuclear fusion of deuterium. A growing number of astronomers argued for Pluto to be declassified as a planet, because many similar objects approaching its size had been found in the same region of the Solar System (the Kuiper belt) during the 1990s and early 2000s. Pluto was found to be just one small body in a population of thousands. Some of them, such as Quaoar, Sedna, and Eris, were heralded in the popular press as the tenth planet, failing to receive widespread scientific recognition. The announcement of Eris in 2005, an object then thought of as 27% more massive than Pluto, created the necessity and public desire for an official definition of a planet. Acknowledging the problem, the IAU set about creating the definition of planet, and produced one in August 2006. The number of planets dropped to the eight significantly larger bodies that had cleared their orbit (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune), and a new class of dwarf planets was created, initially containing three objects (Ceres, Pluto and Eris).[49] Extrasolar planets There is no official definition of extrasolar planets. In 2003, the International Astronomical Union (IAU) Working Group on Extrasolar Planets issued a position statement, but this position statement was never proposed as an official IAU resolution and was never voted on by IAU members. The positions statement incorporates the following guidelines, mostly focused upon the boundary between planets and brown dwarfs:[2]     Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 times the mass of Jupiter for objects with the same isotopic abundance as the Sun[50]) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass and size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System.     Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed or where they are located.     Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate). This working definition was amended by the IAU's Commission F2: Exoplanets and the Solar System in August 2018.[51] The official working definition of an exoplanet is now as follows:         Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the L4/L5 instability (M/Mcentral < 2/(25+√621) are "planets" (no matter how they formed).         The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System. The IAU noted that this definition could be expected to evolve as knowledge improves. One definition of a sub-brown dwarf is a planet-mass object that formed through cloud collapse rather than accretion. This formation distinction between a sub-brown dwarf and a planet is not universally agreed upon; astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification.[52] One reason for the dissent is that often it may not be possible to determine the formation process. For example, a planet formed by accretion around a star may get ejected from the system to become free-floating, and likewise a sub-brown dwarf that formed on its own in a star cluster through cloud collapse may get captured into orbit around a star. One study suggests that objects above 10 MJup formed through gravitational instability and should not be thought of as planets.[53] The 13 Jupiter-mass cutoff represents an average mass rather than a precise threshold value. Large objects will fuse most of their deuterium and smaller ones will fuse only a little, and the 13 MJ value is somewhere in between. In fact, calculations show that an object fuses 50% of its initial deuterium content when the total mass ranges between 12 and 14 MJ.[54] The amount of deuterium fused depends not only on mass but also on the composition of the object, on the amount of helium and deuterium present.[55] As of 2011 the Extrasolar Planets Encyclopaedia included objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around 13 MJup in the observed mass spectrum reinforces the choice to forget this mass limit".[56] As of 2016 this limit was increased to 60 Jupiter masses[57] based on a study of mass–density relationships.[58] The Exoplanet Data Explorer includes objects up to 24 Jupiter masses with the advisory: "The 13 Jupiter-mass distinction by the IAU Working Group is physically unmotivated for planets with rocky cores, and observationally problematic due to the sin i ambiguity."[59] The NASA Exoplanet Archive includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses.[60] Another criterion for separating planets and brown dwarfs, rather than deuterium fusion, formation process or location, is whether the core pressure is dominated by coulomb pressure or electron degeneracy pressure.[61][62] 2006 IAU definition of planet Euler diagram showing the types of bodies in the Solar System. Main article: IAU definition of planet The matter of the lower limit was addressed during the 2006 meeting of the IAU's General Assembly. After much debate and one failed proposal, a large majority of those remaining at the meeting voted to pass a resolution. The 2006 resolution defines planets within the Solar System as follows:[1]     A "planet"[1] is a celestial body that        (a)     is in orbit around the Sun,        (b)                has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and        (c)     has cleared the neighbourhood around its orbit.          [1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Under this definition, the Solar System is considered to have eight planets. Bodies that fulfill the first two conditions but not the third (such as Ceres, Pluto, and Eris) are classified as dwarf planets, provided they are not also natural satellites of other planets. Originally an IAU committee had proposed a definition that would have included a much larger number of planets as it did not include (c) as a criterion.[63] After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.[64] This definition is based in theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects. As described by astronomer Steven Soter:     The end product of secondary disk accretion is a small number of relatively large bodies (planets) in either non-intersecting or resonant orbits, which prevent collisions between them. Minor planets and comets, including KBOs [Kuiper belt objects], differ from planets in that they can collide with each other and with planets.[65] The 2006 IAU definition presents some challenges for exoplanets because the language is specific to the Solar System and because the criteria of roundness and orbital zone clearance are not presently observable. Margot's criterion Astronomer Jean-Luc Margot proposed a mathematical criterion that determines whether an object can clear its orbit during the lifetime of its host star, based on the mass of the planet, its semimajor axis, and the mass of its host star.[66][67] The formula produces a value[f] called π that is greater than 1 for planets. The eight known planets and all known exoplanets have π values above 100, while Ceres, Pluto, and Eris have π values of 0.1, or less. Objects with π values of 1 or more are also expected to be approximately spherical, so that objects that fulfill the orbital zone clearance requirement automatically fulfill the roundness requirement.[68] Objects formerly considered planets See also: List of former planets The table below lists Solar System bodies once considered to be planets but no longer considered as such by the IAU, as well as whether they would be considered planets under alternative definitions, such as Soter's 2006 definition[65] that favors dynamical dominance or Stern's 2002[69] and 2017 definitions[70] that favor having a shape dominated by gravity. Body     IAU classification     Dynamical dominance     Gravitational rounding     Notes Sun     Star     N/A[g]     N/A[h]     Classified as a classical planet (Ancient Greek πλανῆται, wanderers) in classical antiquity and medieval Europe, in accordance with the now-disproved geocentric model.[71] Moon     Natural satellite     No     Yes Io, Europa, Ganymede, Callisto     Natural satellites     No     Yes     The four largest moons of Jupiter, known as the Galilean moons after their discoverer Galileo Galilei. He referred to them as the "Medicean Planets" in honor of his patron, the Medici family. They were known as secondary planets.[72] Titan,[i] Rhea,[j] Iapetus,[j] Tethys,[k] Dione[k]     Natural satellites     No     Yes     Five of Saturn's larger moons, discovered by Christiaan Huygens and Giovanni Domenico Cassini. As with Jupiter's major moons, they were known as secondary planets.[72] Titania, Oberon[l]     Natural satellites     No     Yes     Two of Uranus' larger moons, discovered by William Herschel and called secondary planets. Juno     Asteroid     No     No     Regarded as planets from their discoveries between 1801 and 1807 until they were reclassified as asteroids during the 1850s.[75] Ceres was subsequently classified by the IAU as a dwarf planet in 2006. Pallas and Vesta     Asteroid     No     Maybe Ceres     Dwarf planet and asteroid     No     Yes Hygiea     Asteroid     No     Maybe     More asteroids, discovered between 1845 and 1851. The rapidly expanding list of bodies between Mars and Jupiter prompted their reclassification as asteroids, which was widely accepted by 1854.[76] Astraea, Hebe, Iris, Flora, Metis, Parthenope, Victoria, Egeria, Irene, Eunomia     Asteroids     No     No Pluto     Dwarf planet and Kuiper belt object     No     Yes     The first known trans-Neptunian object (i.e. minor planet with a semi-major axis beyond Neptune). Regarded as a planet from its discovery in 1930 until it was reclassified as a dwarf planet in 2006. The reporting of newly discovered large Kuiper belt objects as planets – particularly Eris – triggered the August 2006 IAU decision on what a planet is. Mythology and naming See also: Weekday names and Naked-eye planet The Greek gods of Olympus, after whom the Solar System's Roman names of the planets are derived The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. In ancient Greece, the two great luminaries the Sun and the Moon were called Helios and Selene, two ancient Titanic deities; the slowest planet (Saturn) was called Phainon, the shiner; followed by Phaethon (Jupiter), "bright"; the red planet (Mars) was known as Pyroeis, the "fiery"; the brightest (Venus) was known as Phosphoros, the light bringer; and the fleeting final planet (Mercury) was called Stilbon, the gleamer. The Greeks also assigned each planet to one among their pantheon of gods, the Olympians and the earlier Titans:     Helios and Selene were the names of both planets and gods, both of them Titans (later supplanted by Olympians Apollo and Artemis);     Phainon was sacred to Cronus, the Titan who fathered the Olympians;     Phaethon was sacred to Zeus, Cronus's son who deposed him as king;     Pyroeis was given to Ares, son of Zeus and god of war;     Phosphoros was ruled by Aphrodite, the goddess of love; and     Stilbon with its speedy motion, was ruled over by Hermes, messenger of the gods and god of learning and wit.[21] The Greek practice of grafting their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphoros [Venus] after their goddess of love, Ishtar; Pyroeis [Mars] after their god of war, Nergal, Stilbon [Saturn] after their god of wisdom Nabu, and Phaethon [Jupiter] after their chief god, Marduk.[77] There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.[21] The translation was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. Unlike Ares, Nergal was also god of pestilence and the underworld.[78] Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods. Although modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the Roman Empire and, later, the Catholic Church, use the Roman (Latin) names rather than the Greek ones. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable.[79] When the Romans studied Greek astronomy, they gave the planets their own gods' names: Mercurius (for Hermes), Venus (Aphrodite), Mars (Ares), Iuppiter (Zeus) and Saturnus (Cronus). When subsequent planets were discovered in the 18th and 19th centuries, the naming practice was retained with Neptūnus (Poseidon). Uranus is unique in that it is named for a Greek deity rather than his Roman counterpart. Some Romans, following a belief possibly originating in Mesopotamia but developed in Hellenistic Egypt, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet).[80] Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Because each day was named by the god that started it, this is also the order of the days of the week in the Roman calendar after the Nundinal cycle was rejected – and still preserved in many modern languages.[81] In English, Saturday, Sunday, and Monday are straightforward translations of these Roman names. The other days were renamed after Tīw (Tuesday), Wōden (Wednesday), Þunor (Thursday), and Frīġ (Friday), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus, respectively. Earth is the only planet whose name in English is not derived from Greco-Roman mythology. Because it was only generally accepted as a planet in the 17th century,[39] there is no tradition of naming it after a god. (The same is true, in English at least, of the Sun and the Moon, though they are no longer generally considered planets.) The name originates from the Old English word eorþe, which was the word for "ground" and "dirt" as well as the Earth itself.[82] As with its equivalents in the other Germanic languages, it derives ultimately from the Proto-Germanic word erþō, as can be seen in the English earth, the German Erde, the Dutch aarde, and the Scandinavian jord. Many of the Romance languages retain the old Roman word terra (or some variation of it) that was used with the meaning of "dry land" as opposed to "sea".[83] The non-Romance languages use their own native words. The Greeks retain their original name, Γή (Ge). Non-European cultures use other planetary-naming systems. India uses a system based on the Navagraha, which incorporates the seven traditional planets (Surya for the Sun, Chandra for the Moon, Budha for Mercury, Shukra for Venus, Mangala for Mars, Bṛhaspati for Jupiter, and Shani for Saturn) and the ascending and descending lunar nodes Rahu and Ketu. China and the countries of eastern Asia historically subject to Chinese cultural influence (such as Japan, Korea and Vietnam) use a naming system based on the five Chinese elements: water (Mercury), metal (Venus), fire (Mars), wood (Jupiter) and earth (Saturn).[81] In traditional Hebrew astronomy, the seven traditional planets have (for the most part) descriptive names – the Sun is חמה Ḥammah or "the hot one," the Moon is לבנה Levanah or "the white one," Venus is כוכב נוגה Kokhav Nogah or "the bright planet," Mercury is כוכב Kokhav or "the planet" (given its lack of distinguishing features), Mars is מאדים Ma'adim or "the red one," and Saturn is שבתאי Shabbatai or "the resting one" (in reference to its slow movement compared to the other visible planets).[84] The odd one out is Jupiter, called צדק Tzedeq or "justice". Steiglitz suggests that this may be a euphemism for the original name of כוכב בעל Kokhav Ba'al or "Baal's planet", seen as idolatrous and euphemized in a similar manner to Ishbosheth from II Samuel.[84] In Arabic, Mercury is عُطَارِد (ʿUṭārid, cognate with Ishtar / Astarte), Venus is الزهرة (az-Zuhara, "the bright one",[85] an epithet of the goddess Al-'Uzzá[86]), Earth is الأرض (al-ʾArḍ, from the same root as eretz), Mars is اَلْمِرِّيخ (al-Mirrīkh, meaning "featherless arrow" due to its retrograde motion[87]), Jupiter is المشتري (al-Muštarī, "the reliable one", from Akkadian[88]) and Saturn is زُحَل (Zuḥal, "withdrawer"[89]).[90][91] Formation Main article: Nebular hypothesis An artist's impression of protoplanetary disk It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk. Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form protoplanets.[92] After a planet reaches a mass somewhat larger than Mars' mass, it begins to accumulate an extended atmosphere,[93] greatly increasing the capture rate of the planetesimals by means of atmospheric drag.[94][95] Depending on the accretion history of solids and gas, a giant planet, an ice giant, or a terrestrial planet may result.[96][97][98] Asteroid collision - building planets (artist concept). When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation, the solar wind, Poynting–Robertson drag and other effects.[99][100] Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb.[101] Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small bodies. The energetic impacts of the smaller planetesimals (as well as radioactive decay) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, developing a denser core.[102] Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of comets.[103] (Smaller planets will lose any atmosphere they gain through various escape mechanisms.) With the discovery and observation of planetary systems around stars other than the Sun, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity—an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 (helium)—is now thought to determine the likelihood that a star will have planets.[104] Hence, it is thought that a metal-rich population I star will likely have a more substantial planetary system than a metal-poor, population II star. Supernova remnant ejecta producing planet-forming material. Solar System Solar System – sizes but not distances are to scale The Sun and the eight planets of the Solar System The inner planets, Mercury, Venus, Earth, and Mars The four giant planets Jupiter, Saturn, Uranus, and Neptune against the Sun and some sunspots Main article: Solar System See also: List of gravitationally rounded objects of the Solar System According to the IAU definition, there are eight planets in the Solar System, which are in increasing distance from the Sun:     ☿ Mercury     ♀ Venus     ⊕ Earth     ♂ Mars     ♃ Jupiter     ♄ Saturn     ♅ Uranus     ♆ Neptune Jupiter is the largest, at 318 Earth masses, whereas Mercury is the smallest, at 0.055 Earth masses. The planets of the Solar System can be divided into categories based on their composition:     Terrestrials: Planets that are similar to Earth, with bodies largely composed of rock: Mercury, Venus, Earth and Mars. At 0.055 Earth masses, Mercury is the smallest terrestrial planet (and smallest planet) in the Solar System. Earth is the largest terrestrial planet.     Giant planets (Jovians): Massive planets significantly more massive than the terrestrials: Jupiter, Saturn, Uranus, Neptune.         Gas giants, Jupiter and Saturn, are giant planets primarily composed of hydrogen and helium and are the most massive planets in the Solar System. Jupiter, at 318 Earth masses, is the largest planet in the Solar System, and Saturn is one third as massive, at 95 Earth masses.         Ice giants, Uranus and Neptune, are primarily composed of low-boiling-point materials such as water, methane, and ammonia, with thick atmospheres of hydrogen and helium. They have a significantly lower mass than the gas giants (only 14 and 17 Earth masses). The number of geophysical planets in the Solar System is unknown - previously considered to be potentially in the hundreds, but now only estimated at only the low double digits.[105] Planetary attributes Comparison of the rotation period (sped up 10 000 times, negative values denoting retrograde), flattening and axial tilt of the planets and the Moon (SVG animation)     Name     Equatorial diameter[m]     Mass[m]     Semi-major axis (AU)     Orbital period (years)     Inclination to Sun's equator (°)     Orbital eccentricity     Rotation period (days)     Confirmed moons     Axial tilt (°)     Rings     Atmosphere 1.     Mercury     0.383     0.06     0.39     0.24     3.38     0.206     58.65     0     0.10     no     minimal 2.     Venus     0.949     0.81     0.72     0.62     3.86     0.007     −243.02     0     177.30     no     CO2, N2 3.     Earth(a)     1.000     1.00     1.00     1.00     7.25     0.017     1.00     1     23.44     no     N2, O2, Ar 4.     Mars     0.532     0.11     1.52     1.88     5.65     0.093     1.03     2     25.19     no     CO2, N2, Ar 5.     Jupiter     11.209     317.83     5.20     11.86     6.09     0.048     0.41     79     3.12     yes     H2, He 6.     Saturn     9.449     95.16     9.54     29.45     5.51     0.054     0.44     82     26.73     yes     H2, He 7.     Uranus     4.007     14.54     19.19     84.02     6.48     0.047     −0.72     27     97.86     yes     H2, He, CH4 8.     Neptune     3.883     17.15     30.07     164.79     6.43     0.009     0.67     14     29.60     yes     H2, He, CH4 Color legend:   terrestrial planets   gas giants   ice giants (both are giant planets). (a) Find absolute values in article Earth Exoplanets Main article: Exoplanet Exoplanets, by year of discovery, through September 2014. An exoplanet (extrasolar planet) is a planet outside the Solar System. As of 1 September 2021, there are 4,834 confirmed exoplanets in 3,572 planetary systems, with 795 systems having more than one planet.[106][107][108][109] In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.[46] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. These pulsar planets are believed to have formed from the unusual remnants of the supernova that produced the pulsar, in a second round of planet formation, or else to be the remaining rocky cores of giant planets that survived the supernova and then decayed into their current orbits. Sizes of Kepler Planet Candidates – based on 2,740 candidates orbiting 2,036 stars as of 4 November 2013 (NASA). The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of an exoplanet around 51 Pegasi. From then until the Kepler mission most known extrasolar planets were gas giants comparable in mass to Jupiter or larger as they were more easily detected. The catalog of Kepler candidate planets consists mostly of planets the size of Neptune and smaller, down to smaller than Mercury. There are types of planets that do not exist in the Solar System: super-Earths and mini-Neptunes, which could be rocky like Earth or a mixture of volatiles and gas like Neptune—a radius of 1.75 times that of Earth is a possible dividing line between the two types of planet.[110] There are hot Jupiters that orbit very close to their star and may evaporate to become chthonian planets, which are the leftover cores. Another possible type of planet is carbon planets, which form in systems with a higher proportion of carbon than in the Solar System. A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.[10] On 20 December 2011, the Kepler Space Telescope team reported the discovery of the first Earth-size exoplanets, Kepler-20e[5] and Kepler-20f,[6] orbiting a Sun-like star, Kepler-20.[7][8][9] Around 1 in 5 Sun-like stars have an "Earth-sized"[d] planet in the habitable[e] zone, so the nearest would be expected to be within 12 light-years distance from Earth.[11][111] The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation, which estimates the number of intelligent, communicating civilizations that exist in the Milky Way.[112] There are exoplanets that are much closer to their parent star than any planet in the Solar System is to the Sun, and there are also exoplanets that are much farther from their star. Mercury, the closest planet to the Sun at 0.4 AU, takes 88 days for an orbit, but the shortest known orbits for exoplanets take only a few hours, see Ultra-short period planet. The Kepler-11 system has five of its planets in shorter orbits than Mercury's, all of them much more massive than Mercury. Neptune is 30 AU from the Sun and takes 165 years to orbit, but there are exoplanets that are hundreds of AU from their star and take more than a thousand years to orbit, e.g. 1RXS1609 b. Planetary-mass objects Main article: Geophysical definition of planet See also: List of gravitationally rounded objects of the Solar System A planetary-mass object (PMO), planemo,[113] or planetary body is a celestial object with a mass that falls within the range of the definition of a planet: massive enough to achieve hydrostatic equilibrium (to be rounded under its own gravity), but not enough to sustain core fusion like a star.[114][115] By definition, all planets are planetary-mass objects, but the purpose of this term is to refer to objects that do not conform to typical expectations for a planet. These include dwarf planets, which are rounded by their own gravity but not massive enough to clear their own orbit, planetary-mass moons, and free-floating planemos, which may have been ejected from a system (rogue planets) or formed through cloud-collapse rather than accretion (sometimes called sub-brown dwarfs). Dwarf planets Main article: Dwarf planet The dwarf planet Pluto A dwarf planet is a planetary-mass object that is neither a true planet nor a natural satellite; it is in direct orbit of a star, and is massive enough for its gravity to compress it into a hydrostatically equilibrious shape (usually a spheroid), but has not cleared the neighborhood of other material around its orbit. Planetary scientist and New Horizons principal investigator Alan Stern, who proposed the term 'dwarf planet', has argued that location should not matter and that only geophysical attributes should be taken into account, and that dwarf planets are thus a subtype of planet. The IAU accepted the term (rather than the more neutral 'planetoid') but decided to classify dwarf planets as a separate category of object.[116] Rogue planets Main article: Rogue planet See also: Five-planet Nice model Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space.[117] Such objects are typically called rogue planets. Sub-brown dwarfs Main article: Sub-brown dwarf Artist's impression of a super-Jupiter around the brown dwarf 2M1207.[118] Stars form via the gravitational collapse of gas clouds, but smaller objects can also form via cloud-collapse. Planetary-mass objects formed this way are sometimes called sub-brown dwarfs. Sub-brown dwarfs may be free-floating such as Cha 110913-773444[119] and OTS 44,[120] or orbiting a larger object such as 2MASS J04414489+2301513. Binary systems of sub-brown dwarfs are theoretically possible; Oph 162225-240515 was initially thought to be a binary system of a brown dwarf of 14 Jupiter masses and a sub-brown dwarf of 7 Jupiter masses, but further observations revised the estimated masses upwards to greater than 13 Jupiter masses, making them brown dwarfs according to the IAU working definitions.[121][122][123] Former stars In close binary star systems one of the stars can lose mass to a heavier companion. Accretion-powered pulsars may drive mass loss. The shrinking star can then become a planetary-mass object. An example is a Jupiter-mass object orbiting the pulsar PSR J1719-1438.[124] These shrunken white dwarfs may become a helium planet or carbon planet. Satellite planets Main article: Satellite planet Titan, the largest moon of Saturn (and larger than the planet Mercury) Some large satellites (moons) are of similar size or larger than the planet Mercury, e.g. Jupiter's Galilean moons and Titan. Proponents of the geophysical definition of planets argue that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet. Alan Stern proposes the term satellite planet for a planet-sized satellite.[125] Captured planets Rogue planets in stellar clusters have similar velocities to the stars and so can be recaptured. They are typically captured into wide orbits between 100 and 105 AU. The capture efficiency decreases with increasing cluster volume, and for a given cluster size it increases with the host/primary mass. It is almost independent of the planetary mass. Single and multiple planets could be captured into arbitrary unaligned orbits, non-coplanar with each other or with the stellar host spin, or pre-existing planetary system.[126] Attributes Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in the Solar System, whereas others are also commonly observed in extrasolar planets. Dynamic characteristics Orbit Main articles: Orbit and Orbital elements See also: Kepler's laws of planetary motion and Exoplanetology § Orbital parameters The orbit of the planet Neptune compared to that of Pluto. Note the elongation of Pluto's orbit in relation to Neptune's (eccentricity), as well as its large angle to the ecliptic (inclination). According to current definitions, all planets must revolve around stars; thus, any potential "rogue planets" are excluded. In the Solar System, all the planets orbit the Sun in the same direction as the Sun rotates (counter-clockwise as seen from above the Sun's north pole). At least one extrasolar planet, WASP-17b, has been found to orbit in the opposite direction to its star's rotation.[127] The period of one revolution of a planet's orbit is known as its sidereal period or year.[128] A planet's year depends on its distance from its star; the farther a planet is from its star, not only the longer the distance it must travel, but also the slower its speed, because it is less affected by its star's gravity. No planet's orbit is perfectly circular, and hence the distance of each varies over the course of its year. The closest approach to its star is called its periastron (perihelion in the Solar System), whereas its farthest separation from the star is called its apastron (aphelion). As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy, just as a falling object on Earth accelerates as it falls; as the planet reaches apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of its trajectory.[129] Each planet's orbit is delineated by a set of elements:     The eccentricity of an orbit describes how elongated a planet's orbit is. Planets with low eccentricities have more circular orbits, whereas planets with high eccentricities have more elliptical orbits. The planets in the Solar System have very low eccentricities, and thus nearly circular orbits.[128] Comets and Kuiper belt objects (as well as several extrasolar planets) have very high eccentricities, and thus exceedingly elliptical orbits.[130][131]     Illustration of the semi-major axis     The semi-major axis is the distance from a planet to the half-way point along the longest diameter of its elliptical orbit (see image). This distance is not the same as its apastron, because no planet's orbit has its star at its exact centre.[128]     The inclination of a planet tells how far above or below an established reference plane its orbit lies. In the Solar System, the reference plane is the plane of Earth's orbit, called the ecliptic. For extrasolar planets, the plane, known as the sky plane or plane of the sky, is the plane perpendicular to the observer's line of sight from Earth.[132] The eight planets of the Solar System all lie very close to the ecliptic; comets and Kuiper belt objects like Pluto are at far more extreme angles to it.[133] The points at which a planet crosses above and below its reference plane are called its ascending and descending nodes.[128] The longitude of the ascending node is the angle between the reference plane's 0 longitude and the planet's ascending node. The argument of periapsis (or perihelion in the Solar System) is the angle between a planet's ascending node and its closest approach to its star.[128] Axial tilt Main article: Axial tilt Earth's axial tilt is about 23.4°. It oscillates between 22.1° and 24.5° on a 41,000-year cycle and is currently decreasing. Planets also have varying degrees of axial tilt; they lie at an angle to the plane of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from its star, the southern hemisphere points towards it, and vice versa. Each planet therefore has seasons, changes to the climate over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its solstice. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice, when its day is longest, the other has its winter solstice, when its day is shortest. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the planet. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either perpetually in sunlight or perpetually in darkness around the time of its solstices.[134] Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to have negligible to no axial tilt as a result of their proximity to their stars.[135] Rotation See also: Exoplanetology § Rotation and axial tilt The planets rotate around invisible axes through their centres. A planet's rotation period is known as a stellar day. Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the Sun's north pole, the exceptions being Venus[136] and Uranus,[137] which rotate clockwise, though Uranus's extreme axial tilt means there are differing conventions on which of its poles is "north", and therefore whether it is rotating clockwise or anti-clockwise.[138] Regardless of which convention is used, Uranus has a retrograde rotation relative to its orbit. The rotation of a planet can be induced by several factors during formation. A net angular momentum can be induced by the individual angular momentum contributions of accreted objects. The accretion of gas by the giant planets can also contribute to the angular momentum. Finally, during the last stages of planet building, a stochastic process of protoplanetary accretion can randomly alter the spin axis of the planet.[139] There is great variation in the length of day between the planets, with Venus taking 243 days to rotate, and the giant planets only a few hours.[140] The rotational periods of extrasolar planets are not known. However, for "hot" Jupiters, their proximity to their stars means that they are tidally locked (i.e., their orbits are in sync with their rotations). This means, they always show one face to their stars, with one side in perpetual day, the other in perpetual night.[141] Orbital clearing Main article: Clearing the neighbourhood The defining dynamic characteristic of a planet is that it has cleared its neighborhood. A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the planetesimals in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. This characteristic was mandated as part of the IAU's official definition of a planet in August, 2006. This criterion excludes such planetary bodies as Pluto, Eris and Ceres from full-fledged planethood, making them instead dwarf planets.[1] Although to date this criterion only applies to the Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs.[142] Physical characteristics Size and shape See also: Earth § Size and shape, Astronomical body § Size, and Planetary coordinate system [icon]    This section needs expansion. You can help by adding to it. (April 2021) A planet's size is defined at least by an average radius (e.g., Earth radius, Jupiter radius, etc.); polar and equatorial radii of a spheroid or more general triaxial ellipsoidal shapes are often estimated (e.g., reference ellipsoid). Derived quantities include the flattening, surface area, and volume. Knowing further the rotation rate and mass, allows the calculation of normal gravity. Mass Main article: Planetary mass A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the electromagnetic forces binding its physical structure, leading to a state of hydrostatic equilibrium. This effectively means that all planets are spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own centre of mass until the object collapses into a sphere.[143] Mass is also the prime attribute by which planets are distinguished from stars. While the lower stellar mass limit is estimated to be around 75 times that of Jupiter (MJ), the upper planetary mass limit for planethood is only roughly 13 MJ for objects with solar-type isotopic abundance, beyond which it achieves conditions suitable for nuclear fusion. Other than the Sun, no objects of such mass exist in the Solar System; but there are exoplanets of this size. The 13 MJ limit is not universally agreed upon and the Extrasolar Planets Encyclopaedia includes objects up to 60 MJ,[57] and the Exoplanet Data Explorer up to 24 MJ.[144] The smallest known planet is PSR B1257+12A, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a pulsar. Its mass is roughly half that of the planet Mercury.[4] The smallest known planet orbiting a main-sequence star other than the Sun is Kepler-37b, with a mass (and radius) slightly higher than that of the Moon. Internal differentiation Main article: Planetary differentiation Illustration of the interior of Jupiter, with a rocky core overlaid by a deep layer of metallic hydrogen Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface. Each therefore has a differentiated interior consisting of a dense planetary core surrounded by a mantle that either is or was a fluid. The terrestrial planets are sealed within hard crusts,[145] but in the giant planets the mantle simply blends into the upper cloud layers. The terrestrial planets have cores of elements such as iron and nickel, and mantles of silicates. Jupiter and Saturn are believed to have cores of rock and metal surrounded by mantles of metallic hydrogen.[146] Uranus and Neptune, which are smaller, have rocky cores surrounded by mantles of water, ammonia, methane and other ices.[147] The fluid action within these planets' cores creates a geodynamo that generates a magnetic field.[145] Atmosphere Main articles: Atmosphere and Extraterrestrial atmospheres See also: Extraterrestrial skies Earth's atmosphere All of the Solar System planets except Mercury[148] have substantial atmospheres because their gravity is strong enough to keep gases close to the surface. The larger giant planets are massive enough to keep large amounts of the light gases hydrogen and helium, whereas the smaller planets lose these gases into space.[149] The composition of Earth's atmosphere is different from the other planets because the various life processes that have transpired on the planet have introduced free molecular oxygen.[150] Planetary atmospheres are affected by the varying insolation or internal energy, leading to the formation of dynamic weather systems such as hurricanes, (on Earth), planet-wide dust storms (on Mars), a greater-than-Earth-sized anticyclone on Jupiter (called the Great Red Spot), and holes in the atmosphere (on Neptune).[134] At least one extrasolar planet, HD 189733 b, has been claimed to have such a weather system, similar to the Great Red Spot but twice as large.[151] Hot Jupiters, due to their extreme proximities to their host stars, have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets.[152][153] These planets may have vast differences in temperature between their day and night sides that produce supersonic winds,[154] although the day and night sides of HD 189733 b appear to have very similar temperatures, indicating that that planet's atmosphere effectively redistributes the star's energy around the planet.[151] Magnetosphere Main article: Magnetosphere Earth's magnetosphere (diagram) One important characteristic of the planets is their intrinsic magnetic moments, which in turn give rise to magnetospheres. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of electrically conducting material in their interiors, which generate their magnetic fields. These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity in the solar wind around itself called the magnetosphere, which the wind cannot penetrate. The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which cannot effectively protect the planet.[155] Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field.[155] In addition, the moon of Jupiter Ganymede also has one. Of the magnetized planets the magnetic field of Mercury is the weakest, and is barely able to deflect the solar wind. Ganymede's magnetic field is several times larger, and Jupiter's is the strongest in the Solar System (so strong in fact that it poses a serious health risk to future manned missions to its moons). The magnetic fields of the other giant planets are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. The magnetic fields of Uranus and Neptune are strongly tilted relative the rotational axis and displaced from the centre of the planet.[155] In 2004, a team of astronomers in Hawaii observed an extrasolar planet around the star HD 179949, which appeared to be creating a sunspot on the surface of its parent star. The team hypothesized that the planet's magnetosphere was transferring energy onto the star's surface, increasing its already high 7,760 °C temperature by an additional 400 °C.[156] Secondary characteristics Main articles: Natural satellite and Planetary ring The rings of Saturn Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in resonance with each other or with smaller bodies (this is also common in satellite systems). All except Mercury and Venus have natural satellites, often called "moons". Earth has one, Mars has two, and the giant planets have numerous moons in complex planetary-type systems. Many moons of the giant planets have features similar to those on the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life (especially Europa).[157][158][159] The four giant planets are also orbited by planetary rings of varying size and complexity. The rings are composed primarily of dust or particulate matter, but can host tiny 'moonlets' whose gravity shapes and maintains their structure. Although the origins of planetary rings is not precisely known, they are believed to be the result of natural satellites that fell below their parent planet's Roche limit and were torn apart by tidal forces.[160][161] No secondary characteristics have been observed around extrasolar planets. The sub-brown dwarf Cha 110913-773444, which has been described as a rogue planet, is believed to be orbited by a tiny protoplanetary disc[119] and the sub-brown dwarf OTS 44 was shown to be surrounded by a substantial protoplanetary disk of at least 10 Earth masses." (wikipedia.org)
  • Condition: Used
  • Condition: In excellent, pre-owned condition. Please see photos and description.
  • Country/Region of Manufacture: Germany
  • Material: Cardboard
  • Theme: Space
  • Number of Pieces: 1000 - 1999 Pieces
  • Year: 2018
  • Color: Multi-Color
  • MPN: 19 858 0
  • Brand: Ravensburger

PicClick Insights - PLANETARY VISION OUTER SPACE PUZZLE Ravensburger 1000 jigsaw 27" x 20" Germany PicClick Exclusive

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