Planet

This article is about the astronomical object. For otheruses, see Planet (disambiguation).

A planet (from Ancient Greek , astrplants, or , plns astr, meaning wan-dering star)[1] is an astronomical object orbiting a star orstellar remnant that

is massive enough to be rounded by its own gravity,

is not massive enough to cause thermonuclear fu-sion, and

has cleared its neighbouring region ofplanetesimals.[lower-alpha 1][2][3]

The term planet is ancient, with ties to history, science,mythology, and religion. Several planets in the SolarSystem can be seen with the naked eye. These wereregarded by many early cultures as divine, or as emis-saries of deities. As scientific knowledge advanced, hu-man perception of the planets changed, incorporating anumber of disparate objects. In 2006, the InternationalAstronomical Union (IAU) officially adopted a resolutiondefining planets within the Solar System. This defini-tion is controversial because it excludes many objects ofplanetary mass based on where or what they orbit. Al-though eight of the planetary bodies discovered before1950 remain planets under the modern definition, somecelestial bodies, such as Ceres, Pallas, Juno, Vesta (eachan object in the solar asteroid belt), and Pluto (the firsttrans-Neptunian object discovered), that were once con-sidered planets by the scientific community are no longerviewed as such.The planets were thought by Ptolemy to orbit Earth indeferent and epicycle motions. Although the idea that theplanets orbited the Sun had been suggested many times,it was not until the 17th century that this view was sup-ported by evidence from the first telescopic astronomicalobservations, performed by Galileo Galilei. By carefulanalysis of the observation data, Johannes Kepler foundthe planets orbits were not circular but elliptical. Asobservational tools improved, astronomers saw that, likeEarth, the planets rotated around tilted axes, and someshared such features as ice caps and seasons. Since thedawn of the Space Age, close observation by space probeshas found that Earth and the other planets share char-acteristics such as volcanism, hurricanes, tectonics, andeven hydrology.

Planets are generally divided into two main types: largelow-density giant planets, and smaller rocky terrestrials.Under IAU definitions, there are eight planets in the SolarSystem. In order of increasing distance from the Sun,they are the four terrestrials, Mercury, Venus, Earth, andMars, then the four giant planets, Jupiter, Saturn, Uranus,and Neptune. Six of the planets are orbited by one ormore natural satellites.More than a thousand planets around other stars("extrasolar planets" or exoplanets) have been discov-ered in the Milky Way: as of 10 July 2015, 1932 knownextrasolar planets in 1222 planetary systems (including484 multiple planetary systems), ranging in size fromjust above the size of the Moon to gas giants abouttwice as large as Jupiter.[4] On December 20, 2011, theKepler Space Telescope team reported the discovery ofthe first Earth-sized extrasolar planets, Kepler-20e[5] andKepler-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 forevery star in the Milky Way.[10] Around one in fiveSun-like[lower-alpha 2] stars is thought to have an Earth-sized[lower-alpha 3] planet in its habitable[lower-alpha 4] zone.

1 History

Further information: History of astronomy andDefinitionof planetSee also: Timeline of Solar System astronomyThe idea of planets has evolved over its history, from thedivine wandering stars of antiquity to the earthly objectsof the scientific age. The concept has expanded to in-clude worlds not only in the Solar System, but in hundredsof other extrasolar systems. The ambiguities inherent indefining planets have led to much scientific controversy.The five classical planets, being visible to the naked eye,have been known since ancient times and have had asignificant impact on mythology, religious cosmology,and ancient astronomy. In ancient times, astronomersnoted how certain lights moved across the sky in re-lation to the other stars. Ancient Greeks called theselights (plantes asteres, wanderingstars) or simply (plantai, wanderers),[11]from which todays word planet was derived.[12][13] Inancient Greece, China, Babylon, and indeed all pre-modern civilizations,[14][15] it was almost universally be-lieved that Earth was the center of the Universe and thatall the planets circled Earth. The reasons for this per-

1

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2 1 HISTORY

Printed rendition of a geocentric cosmological model from Cos-mographia, Antwerp, 1539

ception were that stars and planets appeared to revolvearound Earth each day[16] and the apparently common-sense perceptions that Earth was solid and stable and thatit was not moving but at rest.

1.1 Babylon

Main article: Babylonian astronomy

The first civilization known to have a functional the-ory of the planets were the Babylonians, who lived inMesopotamia in the first and second millennia BC. Theoldest surviving planetary astronomical text is the Baby-lonian Venus tablet of Ammisaduqa, a 7th-century BCcopy of a list of observations of the motions of the planetVenus, that probably dates as early as the second mil-lennium BC.[17] The MUL.APIN is a pair of cuneiformtablets dating from the 7th century BC that lays out themotions of the Sun, Moon and planets over the course ofthe year.[18] TheBabylonian astrologers also laid the foun-dations of what would eventually becomeWestern astrol-ogy.[19] The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC,[20] comprises alist of omens and their relationships with various celestialphenomena including the motions of the planets.[21][22]Venus, Mercury and the outer planets Mars, Jupiter andSaturn were all identified by Babylonian astronomers.These would remain the only known planets until the in-vention of the telescope in early modern times.[23]

1.2 Greco-Roman astronomy

See also: Greek astronomy

The ancient Greeks initially did not attach as muchsignificance to the planets as the Babylonians. ThePythagoreans, in the 6th and 5th centuries BC appear tohave developed their own independent planetary theory,which consisted of the Earth, Sun, Moon, and planets re-volving around a Central Fire at the center of the Uni-verse. Pythagoras or Parmenides is said to have been thefirst to identify the evening star (Hesperos) and morningstar (Phosphoros) as one and the same (Aphrodite, Greekcorresponding to Latin Venus).[24] In the 3rd century BC,Aristarchus of Samos proposed a heliocentric system, ac-cording to which Earth and the planets revolved aroundthe Sun. The geocentric system remained dominant untilthe Scientific Revolution.By the 1st century BC, during the Hellenistic period,the Greeks had begun to develop their own mathemat-ical schemes for predicting the positions of the planets.These schemes, which were based on geometry ratherthan the arithmetic of the Babylonians, would eventu-ally eclipse the Babylonians theories in complexity andcomprehensiveness, and account for most of the astro-nomical movements observed from Earth with the nakedeye. These theories would reach their fullest expressionin the Almagest written by Ptolemy in the 2nd centuryCE. So complete was the domination of Ptolemys modelthat it superseded all previous works on astronomy andremained the definitive astronomical text in the Westernworld for 13 centuries.[17][25] To the Greeks and Romansthere were seven known planets, each presumed to becircling Earth according to the complex laws laid out byPtolemy. They were, in increasing order from Earth (inPtolemys order): the Moon, Mercury, Venus, the Sun,Mars, Jupiter, and Saturn.[13][25][26]

1.3 India

Main articles: Indian astronomy and Hindu cosmology

In 499 CE, the Indian astronomer Aryabhata propoundeda planetary model that explicitly incorporated Earthsrotation about its axis, which he explains as the causeof what appears to be an apparent westward motion ofthe stars. He also believed that the orbits of planetsare elliptical.[27] Aryabhatas followers were particularlystrong in South India, where his principles of the diur-nal rotation of Earth, among others, were followed and anumber of secondary works were based on them.[28]

In 1500, Nilakantha Somayaji of the Kerala school ofastronomy and mathematics, in his Tantrasangraha, re-vised Aryabhatas model.[29] In his Aryabhatiyabhasya, acommentary on Aryabhatas Aryabhatiya, he developeda planetary model where Mercury, Venus, Mars, Jupiterand Saturn orbit the Sun, which in turn orbits Earth,similar to the Tychonic system later proposed by TychoBrahe in the late 16th century. Most astronomers of theKerala school who followed him accepted his planetary

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1.7 20th century 3

model.[29][30]

1.4 Medieval Muslim astronomy

Main articles: Astronomy in medieval Islam and Islamiccosmology

In the 11th century, the transit of Venus was observed byAvicenna, who established that Venus was, at least some-times, below the Sun.[31] In the 12th century, Ibn Bajjahobserved two planets as black spots on the face of theSun, which was later identified as a transit of Mercuryand Venus by the Maragha astronomer Qotb al-Din Shi-razi in the 13th century.[32] Ibn Bajjah could not haveobserved a transit of Venus, because none occurred in hislifetime.[33]

1.5 European Renaissance

See also: Heliocentrism

With the advent of the Scientific Revolution, use of theterm planet changed from something that moved acrossthe sky (in relation to the star field); to a body that orbitedEarth (or that were believed to do so at the time); and bythe 18th century to something that directly orbited theSun when the heliocentric model of Copernicus, Galileoand Kepler gained sway.Thus, Earth became included in the list of planets,[34]whereas the Sun and Moon were excluded. At first, whenthe first satellites of Jupiter and Saturn were discovered inthe 17th century, the terms planet and satellite wereused interchangeably although the latter would gradu-ally become more prevalent in the following century.[35]Until the mid-19th century, the number of planets roserapidly because any newly discovered object directly or-biting the Sun was listed as a planet by the scientific com-munity.

1.6 19th century

In the 19th century astronomers began to realize thatrecently discovered bodies that had been classified asplanets for almost half a century (such as Ceres, Pallas,and Vesta) were very different from the traditionalones. These bodies shared the same region of space be-tween Mars and Jupiter (the asteroid belt), and had amuch smaller mass; as a result they were reclassified as"asteroids". In the absence of any formal definition, aplanet came to be understood as any large body thatorbited the Sun. Because there was a dramatic size gapbetween the asteroids and the planets, and the spate ofnew discoveries seemed to have ended after the discov-ery of Neptune in 1846, there was no apparent need tohave a formal definition.[36]

1.7 20th century

In the 20th century, Pluto was discovered. After initialobservations led to the belief it was larger than Earth,[37]the object was immediately accepted as the ninth planet.Further monitoring found the body was actually muchsmaller: in 1936, Raymond Lyttleton suggested that Plutomay be an escaped satellite of Neptune,[38] and FredWhipple suggested in 1964 that Pluto may be a comet.[39]As it was still larger than all known asteroids and seem-ingly did not exist within a larger population,[40] it keptits status until 2006.In 1992, astronomers Aleksander Wolszczan and DaleFrail announced the discovery of planets around a pulsar,PSR B1257+12.[41] This discovery is generally consid-ered to be the first definitive detection of a planetary sys-tem around another star. Then, on October 6, 1995,Michel Mayor and Didier Queloz of the University ofGeneva announced the first definitive detection of an ex-oplanet orbiting an ordinary main-sequence star (51 Pe-gasi).[42]

The discovery of extrasolar planets led to another am-biguity in defining a planet: the point at which a planetbecomes a star. Many known extrasolar planets aremany times the mass of Jupiter, approaching that of stel-lar objects known as "brown dwarfs".[43] Brown dwarfsare generally considered stars due to their ability to fusedeuterium, a heavier isotope of hydrogen. Although ob-jects more massive than 75 times that of Jupiter fuse hy-drogen, objects of only 13 Jupiter masses can fuse deu-terium. Deuterium is quite rare, and most brown dwarfswould have ceased fusing deuterium long before their dis-covery, making them effectively indistinguishable fromsupermassive planets.[44]

1.8 21st century

With the discovery during the latter half of the 20thcentury of more objects within the Solar System andlarge objects around other stars, disputes arose over whatshould constitute a planet. There were particular dis-agreements over whether an object should be considereda planet if it was part of a distinct population such as abelt, or if it was large enough to generate energy by thethermonuclear fusion of deuterium.A growing number of astronomers argued for Pluto tobe declassified as a planet, because many similar objectsapproaching its size had been found in the same region ofthe Solar System (the Kuiper belt) during the 1990s andearly 2000s. Pluto was found to be just one small body ina population of thousands.Some of them, such as Quaoar, Sedna, and Eris, wereheralded in the popular press as the tenth planet, failing toreceive widespread scientific recognition. The announce-ment of Eris in 2005, an object 27% more massive thanPluto, created the necessity and public desire for an offi-

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4 1 HISTORY

cial definition of a planet.Acknowledging the problem, the IAU set about creat-ing the definition of planet, and produced one in August2006. The number of planets dropped to the eight signifi-cantly larger bodies that had cleared their orbit (Mercury,Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Nep-tune), and a new class of dwarf planets was created, ini-tially containing three objects (Ceres, Pluto and Eris).[45]

1.8.1 Extrasolar planet definition

In 2003, The International Astronomical Union (IAU)Working Group on Extrasolar Planets made a positionstatement on the definition of a planet that incorporatedthe following working definition, mostly focused uponthe boundary between planets and brown dwarfs:[3]

1. Objects with truemasses below the limitingmass forthermonuclear fusion of deuterium (currently calcu-lated to be 13 times the mass of Jupiter for objectswith the same isotopic abundance as the Sun[46]) thatorbit stars or stellar remnants are planets (no mat-ter how they formed). The minimum mass and sizerequired for an extrasolar object to be considered aplanet should be the same as that used in the SolarSystem.

2. Substellar objects with true masses above the lim-iting mass for thermonuclear fusion of deuteriumare "brown dwarfs", no matter how they formed orwhere they are located.

3. Free-floating objects in young star clusters withmasses below the limiting mass for thermonuclearfusion of deuterium are not planets, but are sub-brown dwarfs (or whatever name is most appropri-ate).

This definition has since been widely used by astronomerswhen publishing discoveries of exoplanets in academicjournals.[47] Although temporary, it remains an effectiveworking definition until a more permanent one is for-mally adopted. It does not address the dispute over thelower mass limit,[48] and so it steered clear of the con-troversy regarding objects within the Solar System. Thisdefinition also makes no comment on the planetary statusof objects orbiting brown dwarfs, such as 2M1207b.One definition of a sub-brown dwarf is a planet-massobject that formed through cloud collapse rather thanaccretion. This formation distinction between a sub-brown dwarf and a planet is not universally agreed upon;astronomers are divided into two camps as whether toconsider the formation process of a planet as part of itsdivision in classification.[49] One reason for the dissent isthat often it may not be possible to determine the forma-tion process. For example, a planet formed by accretionaround a star may get ejected from the system to become

free-floating, and likewise a sub-brown dwarf that formedon its own in a star cluster through cloud collapse may getcaptured into orbit around a star.The 13 Jupiter-mass cutoff is a rule of thumb rather thansomething of precise physical significance. The ques-tion arises: what is meant by deuterium burning? Thisquestion arises because large objects will burn most oftheir deuterium and smaller ones will burn only a lit-tle, and the 13 MJ value is somewhere in between. Theamount of deuterium burnt depends not only on mass butalso on the composition of the planet, on the amount ofhelium and deuterium present.[50] The Extrasolar PlanetsEncyclopaedia includes objects up to 25 Jupiter masses,saying, The fact that there is no special feature around13 MJ in the observed mass spectrum reinforces thechoice to forget this mass limit.[51] The Exoplanet DataExplorer includes objects up to 24 Jupiter masses withthe advisory: The 13 Jupiter-mass distinction by theIAU Working Group is physically unmotivated for plan-ets with rocky cores, and observationally problematic dueto the sin i ambiguity.[52] The NASA Exoplanet Archiveincludes objects with a mass (or minimummass) equal toor less than 30 Jupiter masses.[53]

Another criterion for separating planets and browndwarfs, rather than deuterium burning, formation processor location, is whether the core pressure is dominated bycoulomb pressure or electron degeneracy pressure.[54][55]

1.8.2 2006 definition

Main article: IAU definition of planet

The matter of the lower limit was addressed during the2006 meeting of the IAUs General Assembly. Aftermuch debate and one failed proposal, 232 members ofthe 10,000 member assembly, who nevertheless consti-tuted a large majority of those remaining at the meeting,voted to pass a resolution. The 2006 resolution redefinesplanets within the Solar System as:[2]

A celestial body that (a) is in orbitaround the Sun, (b) has sufficient mass forits self-gravity to overcome rigid body forcesso that it assumes a hydrostatic equilibrium(nearly round) shape, and (c) has cleared theneighbourhood around its orbit.

Under this definition, the Solar System is considered tohave eight planets. Bodies that fulfill the first two con-ditions but not the third (such as Ceres, Pluto, and Eris)are classified as dwarf planets, provided they are not alsonatural satellites of other planets. Originally an IAU com-mittee had proposed a definition that would have includeda much larger number of planets as it did not include (c)as a criterion.[56] After much discussion, it was decided

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5

via a vote that those bodies should instead be classified asdwarf planets.[57]

This definition is based in theories of planetary forma-tion, in which planetary embryos initially clear their or-bital neighborhood of other smaller objects. As describedby astronomer Steven Soter:[58]

The end product of secondary disk accre-tion is a small number of relatively large bodies(planets) in either non-intersecting or resonantorbits, which prevent collisions between them.Minor planets and comets, including KBOs[Kuiper belt objects], differ from planets inthat they can collide with each other and withplanets.

Beyond the scientific community, Pluto still holds culturalsignificance for many in the general public due to its his-torical classification as a planet from 1930 to 2006.[59]

1.9 Objects formerly considered planets

The table below lists Solar System bodies once consideredto be planets.A few astronomers, such as Alan Stern, consider dwarfplanets and the larger moons to be planets, based on apurely geophysical definition of planet.[65]

2 Mythology and naming

See also: Weekday names and Naked-eye planetThe names for the planets in the Western world are de-rived from the naming practices of the Romans, which ul-timately derive from those of the Greeks and the Babylo-nians. In ancient Greece, the two great luminaries the Sunand the Moon were called Helios and Selene; the farthestplanet (Saturn) was called Phainon, the shiner; followedby Phaethon (Jupiter), bright"; the red planet (Mars) wasknown as Pyroeis, the fiery"; the brightest (Venus) wasknown as Phosphoros, the light bringer; and the fleetingfinal planet (Mercury) was called Stilbon, the gleamer.The Greeks also made each planet sacred to one amongtheir pantheon of gods, the Olympians: Helios and Selenewere the names of both planets and gods; Phainon wassacred to Cronus, the Titan who fathered the Olympians;Phaethon was sacred to Zeus, Cronuss son who deposedhim as king; Pyroeis was given to Ares, son of Zeus andgod of war; Phosphoros was ruled by Aphrodite, the god-dess of love; and Hermes, messenger of the gods and godof learning and wit, ruled over Stilbon.[17]

The Greek practice of grafting of their gods names ontothe planets was almost certainly borrowed from the Baby-lonians. The Babylonians named Phosphoros after their

The gods of Olympus, after whom the Solar Systems planets arenamed

goddess of love, Ishtar; Pyroeis after their god of war,Nergal, Stilbon after their god of wisdom Nabu, andPhaethon after their chief god,Marduk.[66] There are toomany concordances between Greek and Babylonian nam-ing conventions for them to have arisen separately.[17]The translation was not perfect. For instance, the Babylo-nian Nergal was a god of war, and thus the Greeks iden-tified him with Ares. Unlike Ares, Nergal was also godof pestilence and the underworld.[67]

Today, most people in the western world know the planetsby names derived from the Olympian pantheon of gods.Although modern Greeks still use their ancient names forthe planets, other European languages, because of theinfluence of the Roman Empire and, later, the CatholicChurch, use the Roman (Latin) names rather than theGreek ones. The Romans, who, like the Greeks, wereIndo-Europeans, shared with them a common pantheonunder different names but lacked the rich narrative tra-ditions that Greek poetic culture had given their gods.During the later period of the Roman Republic, Romanwriters borrowed much of the Greek narratives and ap-plied them to their own pantheon, to the point wherethey became virtually indistinguishable.[68]When the Ro-mans studied Greek astronomy, they gave the planetstheir own gods names: Mercurius (for Hermes), Venus(Aphrodite), Mars (Ares), Iuppiter (Zeus) and Saturnus(Cronus). When subsequent planets were discovered in

https://en.wikipedia.org/wiki/Steven_Soterhttps://en.wikipedia.org/wiki/Solar_Systemhttps://en.wikipedia.org/wiki/Alan_Sternhttps://en.wikipedia.org/wiki/Weekday_nameshttps://en.wikipedia.org/wiki/Naked-eye_planethttps://en.wikipedia.org/wiki/Ancient_Greecehttps://en.wikipedia.org/wiki/Helioshttps://en.wikipedia.org/wiki/Selenehttps://en.wikipedia.org/wiki/Twelve_Olympianshttps://en.wikipedia.org/wiki/Cronushttps://en.wikipedia.org/wiki/Titan_(mythology)https://en.wikipedia.org/wiki/Zeushttps://en.wikipedia.org/wiki/Areshttps://en.wikipedia.org/wiki/Aphroditehttps://en.wikipedia.org/wiki/Hermeshttps://en.wikipedia.org/wiki/Hesperus#Variant_nameshttps://en.wikipedia.org/wiki/Mount_Olympushttps://en.wikipedia.org/wiki/Ishtarhttps://en.wikipedia.org/wiki/Nergalhttps://en.wikipedia.org/wiki/Nabuhttps://en.wikipedia.org/wiki/Mardukhttps://en.wikipedia.org/wiki/Roman_Empirehttps://en.wikipedia.org/wiki/Catholic_Churchhttps://en.wikipedia.org/wiki/Catholic_Churchhttps://en.wikipedia.org/wiki/Indo-European_mythologyhttps://en.wikipedia.org/wiki/Roman_mythologyhttps://en.wikipedia.org/wiki/Greek_mythologyhttps://en.wikipedia.org/wiki/Roman_Republichttps://en.wikipedia.org/wiki/Mercury_(mythology)https://en.wikipedia.org/wiki/Venus_(mythology)https://en.wikipedia.org/wiki/Mars_(mythology)https://en.wikipedia.org/wiki/Jupiter_(mythology)https://en.wikipedia.org/wiki/Saturn_(mythology)

6 3 FORMATION

the 18th and 19th centuries, the naming practice was re-tained withNeptnus (Poseidon). Uranus is unique in thatit is named for a Greek deity rather than his Roman coun-terpart.Some Romans, following a belief possibly originatingin Mesopotamia but developed in Hellenistic Egypt, be-lieved that the seven gods after whom the planets werenamed 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 closestplanet).[69] Therefore, the first day was started by Saturn(1st hour), second day by Sun (25th hour), followed byMoon (49th hour), Mars, Mercury, Jupiter and Venus.Because each day was named by the god that startedit, this is also the order of the days of the week in theRoman calendar after the Nundinal cycle was rejected and still preserved in many modern languages.[70] InEnglish, Saturday, Sunday, and Monday are straightfor-ward translations of these Roman names. The other dayswere renamed after Tiw, (Tuesday)Wden (Wednesday),Thunor (Thursday), and Frge (Friday), the Anglo-Saxongods considered similar or equivalent to Mars, Mercury,Jupiter, and Venus, respectively.Earth is the only planet whose name in English is notderived from Greco-Roman mythology. Because it wasonly generally accepted as a planet in the 17th century,[34]there is no tradition of naming it after a god. (The sameis true, in English at least, of the Sun and the Moon,though they are no longer generally considered planets.)The name originates from the 8th century Anglo-Saxonword erda, which means ground or soil and was first usedin writing as the name of the sphere of Earth perhapsaround 1300.[71][72] As with its equivalents in the otherGermanic languages, it derives ultimately from the Proto-Germanic word ertho, ground,[72] as can be seen in theEnglish earth, the German Erde, the Dutch aarde, andthe Scandinavian jord. Many of the Romance languagesretain the old Roman word terra (or some variation of it)that was used with the meaning of dry land as opposedto sea.[73] The non-Romance languages use their ownnative words. The Greeks retain their original name, (Ge).Non-European cultures use other planetary-naming sys-tems. India uses a system based on the Navagraha,which incorporates the seven traditional planets (Suryafor the Sun, Chandra for the Moon, and Budha, Shukra,Mangala, Bhaspati and Shani for Mercury, Venus, Mars,Jupiter and Saturn) and the ascending and descendinglunar nodes Rahu and Ketu. China and the countries ofeastern Asia historically subject to Chinese cultural in-fluence (such as Japan, Korea and Vietnam) use a nam-ing system based on the five Chinese elements: water(Mercury), metal (Venus), fire (Mars), wood (Jupiter)and earth (Saturn).[70]

3 Formation

Main article: Nebular hypothesisIt is not known with certainty how planets are formed.

An artists impression of protoplanetary disk

The prevailing theory is that they are formed during thecollapse of a nebula into a thin disk of gas and dust.A protostar forms at the core, surrounded by a rotat-ing protoplanetary disk. Through accretion (a processof sticky collision) dust particles in the disk steadily ac-cumulate mass to form ever-larger bodies. Local con-centrations of mass known as planetesimals form, andthese accelerate the accretion process by drawing in ad-ditional material by their gravitational attraction. Theseconcentrations become ever denser until they collapseinward under gravity to form protoplanets.[74] After aplanet reaches a diameter larger than the Moon, it be-gins to accumulate an extended atmosphere, greatly in-creasing the capture rate of the planetesimals by meansof atmospheric drag.[75]

Asteroid collision - building planets (artist concept).

When the protostar has grown such that it ignites to forma star, the surviving disk is removed from the insideoutward by photoevaporation, the solar wind, PoyntingRobertson drag and other effects.[76][77] Thereafter therestill may be many protoplanets orbiting the star or eachother, but over time many will collide, either to form asingle larger planet or release material for other largerprotoplanets or planets to absorb.[78] Those objects thathave become massive enough will capture most matter intheir orbital neighbourhoods to become planets. Proto-planets that have avoided collisions may become naturalsatellites of planets through a process of gravitational cap-

https://en.wikipedia.org/wiki/Neptune_(mythology)https://en.wikipedia.org/wiki/Poseidonhttps://en.wikipedia.org/wiki/Uranus_(mythology)https://en.wikipedia.org/wiki/Caelushttps://en.wikipedia.org/wiki/Caelushttps://en.wikipedia.org/wiki/Ancient_Romehttps://en.wikipedia.org/wiki/Mesopotamiahttps://en.wikipedia.org/wiki/Hellenistic_Egypthttps://en.wikipedia.org/wiki/Week-day_nameshttps://en.wikipedia.org/wiki/Roman_calendarhttps://en.wikipedia.org/wiki/Roman_Calendar#Nundinal_cyclehttps://en.wikipedia.org/wiki/T%C3%BDrhttps://en.wikipedia.org/wiki/Wodenhttps://en.wikipedia.org/wiki/Thorhttps://en.wikipedia.org/wiki/Frigehttps://en.wikipedia.org/wiki/Anglo-Saxon_godshttps://en.wikipedia.org/wiki/Anglo-Saxon_godshttps://en.wikipedia.org/wiki/Old_English_languagehttps://en.wikipedia.org/wiki/Germanic_languageshttps://en.wikipedia.org/wiki/Proto-Germanichttps://en.wikipedia.org/wiki/Proto-Germanichttps://en.wikipedia.org/wiki/Romance_languageshttps://en.wikipedia.org/wiki/Terra_(mythology)https://en.wikipedia.org/wiki/Gaia_(mythology)https://en.wikipedia.org/wiki/Indiahttps://en.wikipedia.org/wiki/Navagrahahttps://en.wikipedia.org/wiki/Suryahttps://en.wikipedia.org/wiki/Chandrahttps://en.wikipedia.org/wiki/Budhahttps://en.wikipedia.org/wiki/Shukrahttps://en.wikipedia.org/wiki/Mangalahttps://en.wikipedia.org/wiki/B%E1%B9%9Bhaspatihttps://en.wikipedia.org/wiki/Shanihttps://en.wikipedia.org/wiki/Lunar_nodehttps://en.wikipedia.org/wiki/Rahuhttps://en.wikipedia.org/wiki/Ketu_(mythology)https://en.wikipedia.org/wiki/Chinese_cultural_spherehttps://en.wikipedia.org/wiki/Chinese_cultural_spherehttps://en.wikipedia.org/wiki/Koreahttps://en.wikipedia.org/wiki/Vietnamhttps://en.wikipedia.org/wiki/Wu_Xinghttps://en.wikipedia.org/wiki/Water_(classical_element)https://en.wikipedia.org/wiki/Metal_(classical_element)https://en.wikipedia.org/wiki/Fire_(classical_element)https://en.wikipedia.org/wiki/Wood_(classical_element)https://en.wikipedia.org/wiki/Earth_(classical_element)https://en.wikipedia.org/wiki/Nebular_hypothesishttps://en.wikipedia.org/wiki/Nebulahttps://en.wikipedia.org/wiki/Protostarhttps://en.wikipedia.org/wiki/Protoplanetary_diskhttps://en.wikipedia.org/wiki/Accretion_(astrophysics)https://en.wikipedia.org/wiki/Planetesimalhttps://en.wikipedia.org/wiki/Protoplanethttps://en.wikipedia.org/wiki/Drag_(physics)https://en.wikipedia.org/wiki/Starhttps://en.wikipedia.org/wiki/Photoevaporationhttps://en.wikipedia.org/wiki/Solar_windhttps://en.wikipedia.org/wiki/Poynting%E2%80%93Robertson_effecthttps://en.wikipedia.org/wiki/Poynting%E2%80%93Robertson_effecthttps://en.wikipedia.org/wiki/Natural_satellitehttps://en.wikipedia.org/wiki/Natural_satellite

7

ture, or remain in belts of other objects to become eitherdwarf planets or small bodies.The energetic impacts of the smaller planetesimals (aswell as radioactive decay) will heat up the growing planet,causing it to at least partially melt. The interior ofthe planet begins to differentiate by mass, developing adenser core.[79] Smaller terrestrial planets lose most oftheir atmospheres because of this accretion, but the lostgases can be replaced by outgassing from the mantle andfrom the subsequent impact of comets.[80] (Smaller plan-ets will lose any atmosphere they gain through variousescape mechanisms.)With the discovery and observation of planetary systemsaround stars other than the Sun, it is becoming possibleto elaborate, revise or even replace this account. Thelevel of metallicityan astronomical term describing theabundance of chemical elements with an atomic numbergreater than 2 (helium)is now believed to determinethe likelihood that a star will have planets.[81] Hence, it isthought that a metal-rich population I star will likely havea more substantial planetary system than a metal-poor,population II star.

Supernova remnant ejecta producing planet-formingmaterial.

4 Solar System

Planets of the Solar System (Sizes to scale; distances andillumination not to scale)

The inner planets. From left to right: Mercury, Venus,Earth and Mars in true-color. (Sizes to scale; distancesnot to scale)

https://en.wikipedia.org/wiki/Small_Solar_System_bodyhttps://en.wikipedia.org/wiki/Radioactive_decayhttps://en.wikipedia.org/wiki/Comethttps://en.wikipedia.org/wiki/Atmospheric_escapehttps://en.wikipedia.org/wiki/Planetary_systemhttps://en.wikipedia.org/wiki/Metallicityhttps://en.wikipedia.org/wiki/Chemical_elementhttps://en.wikipedia.org/wiki/Atomic_numberhttps://en.wikipedia.org/wiki/Heliumhttps://en.wikipedia.org/wiki/Population_I_starhttps://en.wikipedia.org/wiki/Population_II_starhttps://en.wikipedia.org/wiki/Supernova_remnanthttps://en.wikipedia.org/wiki/Mercury_(planet)https://en.wikipedia.org/wiki/Venushttps://en.wikipedia.org/wiki/Earthhttps://en.wikipedia.org/wiki/Mars

8 5 EXOPLANETS

The four gas giants against the Sun: Jupiter, Saturn,Uranus, Neptune (Sizes to scale; distances not to scale)Main article: Solar SystemSee also: List of gravitationally rounded objects of theSolar System

According to the IAU, there are eight planets in the SolarSystem. In increasing distance from the Sun, the planetsare:

1. Mercury

2. Venus

3. Earth

4. Mars

5. Jupiter

6. Saturn

7. Uranus

8. Neptune

Jupiter is the largest, at 318 Earth masses, whereas Mer-cury is the smallest, at 0.055 Earth masses.The planets of the Solar System can be divided into cat-egories based on their composition:

Terrestrials: Planets that are similar to Earth, withbodies largely composed of rock: Mercury, Venus,Earth and Mars. At 0.055 Earth masses, Mercury isthe smallest terrestrial planet (and smallest planet)in the Solar System. Earth is the largest terrestrialplanet.

Giant planets (Jovians): Massive planets signif-icantly more massive than the terrestrials: Jupiter,Saturn, Uranus, Neptune.

Gas giants, Jupiter and Saturn, are giant plan-ets primarily composed of hydrogen and he-lium and are the most massive planets in theSolar System. Jupiter, at 318 Earth masses, isthe largest planet in the Solar System, and Sat-urn is one third as massive, at 95 Earth masses.

Ice giants, Uranus and Neptune, are primarilycomposed of low-boiling-point materials suchas water, methane, and ammonia, with thickatmospheres of hydrogen and helium. Theyhave a significantly lower mass than the gas gi-ants (only 14 and 17 Earth masses).

4.1 Planetary attributes

5 Exoplanets

Main article: ExoplanetAn exoplanet (extrasolar planet) is a planet outside the

Exoplanets, by year of discovery, through September 2014.

Solar System. Around 1800 such planets have beendiscovered[83][84][85] (1932 planets in 1222 planetary sys-tems including 484 multiple planetary systems as of 10July 2015).[4]

In early 1992, radio astronomers Aleksander Wolszczanand Dale Frail announced the discovery of two plan-ets orbiting the pulsar PSR 1257+12.[41] This discoverywas confirmed, and is generally considered to be the firstdefinitive detection of exoplanets. These pulsar planetsare believed to have formed from the unusual remnantsof the supernova that produced the pulsar, in a secondround of planet formation, or else to be the remainingrocky cores of giant planets that survived the supernovaand then decayed into their current orbits.The first confirmed discovery of an extrasolar planet or-biting an ordinary main-sequence star occurred on 6 Oc-tober 1995, when Michel Mayor and Didier Queloz ofthe University of Geneva announced the detection of anexoplanet around 51 Pegasi. From then until the Keplermission most known extrasolar planets were gas giantscomparable in mass to Jupiter or larger as they were moreeasily detected. The catalog of Kepler candidate plan-ets consists mostly of planets the size of Neptune andsmaller, down to smaller than Mercury.There are types of planets that do not exist in the SolarSystem: super-Earths and mini-Neptunes, which couldbe rocky like Earth or a mixture of volatiles and gas likeNeptunea radius of 1.75 times that of Earth is a pos-

https://en.wikipedia.org/wiki/Solar_Systemhttps://en.wikipedia.org/wiki/List_of_gravitationally_rounded_objects_of_the_Solar_Systemhttps://en.wikipedia.org/wiki/List_of_gravitationally_rounded_objects_of_the_Solar_Systemhttps://en.wikipedia.org/wiki/International_Astronomical_Unionhttps://en.wikipedia.org/wiki/Solar_Systemhttps://en.wikipedia.org/wiki/Solar_Systemhttps://en.wikipedia.org/wiki/Sunhttps://en.wikipedia.org/wiki/Mercury_(planet)https://en.wikipedia.org/wiki/Venushttps://en.wikipedia.org/wiki/Earthhttps://en.wikipedia.org/wiki/Marshttps://en.wikipedia.org/wiki/Jupiterhttps://en.wikipedia.org/wiki/Saturnhttps://en.wikipedia.org/wiki/Uranushttps://en.wikipedia.org/wiki/Neptunehttps://en.wikipedia.org/wiki/Terrestrial_planethttps://en.wikipedia.org/wiki/Rock_(geology)https://en.wikipedia.org/wiki/Giant_planethttps://en.wikipedia.org/wiki/Gas_gianthttps://en.wikipedia.org/wiki/Ice_gianthttps://en.wikipedia.org/wiki/Exoplanethttps://en.wikipedia.org/wiki/Planetary_systemhttps://en.wikipedia.org/wiki/Planetary_systemhttps://en.wikipedia.org/wiki/List_of_multiplanetary_systemshttps://en.wikipedia.org/wiki/Aleksander_Wolszczanhttps://en.wikipedia.org/wiki/Dale_Frailhttps://en.wikipedia.org/wiki/Pulsarhttps://en.wikipedia.org/wiki/PSR_1257+12https://en.wikipedia.org/wiki/Supernovahttps://en.wikipedia.org/wiki/Giant_planethttps://en.wikipedia.org/wiki/Michel_Mayorhttps://en.wikipedia.org/wiki/Didier_Quelozhttps://en.wikipedia.org/wiki/University_of_Genevahttps://en.wikipedia.org/wiki/51_Pegasihttps://en.wikipedia.org/wiki/Kepler_(spacecraft)https://en.wikipedia.org/wiki/Kepler_(spacecraft)https://en.wikipedia.org/wiki/Super-Earthhttps://en.wikipedia.org/wiki/Mini-Neptune

9

Sizes of Kepler Planet Candidates based on 2,740 candidatesorbiting 2,036 stars as of 4 November 2013 (NASA).

sible dividing line between the two types of planet.[86]There are hot Jupiters that orbit very close to their starand may evaporate to become chthonian planets, whichare the leftover cores. Another possible type of planetis carbon planets, which form in systems with a higherproportion 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 ev-ery star in the Milky Way.[10]

On December 20, 2011, the Kepler Space Telescopeteam reported the discovery of the first Earth-sizeexoplanets, Kepler-20e[5] and Kepler-20f,[6] orbiting aSun-like star, Kepler-20.[7][8][9]

Around 1 in 5 Sun-like[lower-alpha 2] stars have an Earth-sized[lower-alpha 3] planet in the habitable[lower-alpha 4] zone,so the nearest would be expected to be within 12 light-years distance from Earth.[87][88] The frequency of occur-rence of such terrestrial planets is one of the variablesin the Drake equation, which estimates the number ofintelligent, communicating civilizations that exist in theMilky Way.[89]

There are exoplanets that are much closer to their parentstar than any planet in the Solar System is to the Sun, andthere are also exoplanets that are much further from theirstar. Mercury, the closest planet to the Sun at 0.4AU,takes 88-days for an orbit, but the shortest known orbitsfor exoplanets take only a few hours, e.g. Kepler-70b.The Kepler-11 system has five of its planets in shorter or-bits than Mercury. Neptune is 30AU from the Sun andtakes 165 years to orbit, but there are exoplanets that arehundreds of AU from their star and takemore than a thou-sand years to orbit, e.g. 1RXS1609 b.The next few space telescopes to study exoplanets are ex-pected to be Gaia launched in December 2013, CHEOPSin 2017, TESS in 2017, and the James Webb Space Tele-scope in 2018.

6 Planetary-mass objects

See also: List of gravitationally rounded objects of theSolar System

A planetary-mass object (PMO), planemo/plnmo/, or planetary body is a celestial ob-ject with a mass that falls within the range of thedefinition of a planet: massive enough to achieve hydro-static equilibrium (to be rounded under its own gravity),but not enough to sustain core fusion like a star.[90] Bydefinition, all planets are planetary-mass objects, butthe purpose of this term is to refer to objects that donot conform to typical expectations for a planet. Theseinclude dwarf planets, the larger moons, and free-floatingplanemos, which may have been ejected from a system(rogue planets) or formed through cloud-collapse ratherthan accretion (sometimes called sub-brown dwarfs).

6.1 Rogue planets

Main article: Rogue planet

Several computer simulations of stellar and planetary sys-tem formation have suggested that some objects of plan-etary mass would be ejected into interstellar space.[91]Some scientists have argued that such objects foundroaming in deep space should be classed as planets, al-though others have suggested that they should be calledlow-mass brown dwarfs.[92][93]

6.2 Sub-brown dwarfs

Main article: Sub-brown dwarf

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 sometimescalled sub-brown dwarfs. Sub-brown dwarfs may befree-floating such as Cha 110913-773444[92] and OTS44,[94] or orbiting a larger object such as 2MASSJ04414489+2301513.For a brief time in 2006, astronomers believed they hadfound a binary system of such objects, Oph 162225-240515, which the discoverers described as planemos,or planetary-mass objects. Recent analysis of the ob-jects has determined that their masses are probably eachgreater than 13 Jupiter-masses, making the pair browndwarfs.[95][96][97]

6.3 Former stars

In close binary star systems one of the stars can lose massto a heavier companion. Accretion-powered pulsars may

https://en.wikipedia.org/wiki/Hot_Jupiterhttps://en.wikipedia.org/wiki/Chthonian_planethttps://en.wikipedia.org/wiki/Carbon_planethttps://en.wikipedia.org/wiki/Gravitational_microlensinghttps://en.wikipedia.org/wiki/Arithmetic_meanhttps://en.wikipedia.org/wiki/Kepler_(spacecraft)https://en.wikipedia.org/wiki/Terrestrial_planethttps://en.wikipedia.org/wiki/Exoplanethttps://en.wikipedia.org/wiki/Kepler-20ehttps://en.wikipedia.org/wiki/Kepler-20fhttps://en.wikipedia.org/wiki/Solar_analoghttps://en.wikipedia.org/wiki/Kepler-20https://en.wikipedia.org/wiki/Drake_equationhttps://en.wikipedia.org/wiki/Extraterrestrial_lifehttps://en.wikipedia.org/wiki/Milky_Wayhttps://en.wikipedia.org/wiki/Mercury_(planet)https://en.wikipedia.org/wiki/Astronomical_unithttps://en.wikipedia.org/wiki/Kepler-70bhttps://en.wikipedia.org/wiki/Kepler-11https://en.wikipedia.org/wiki/Neptunehttps://en.wikipedia.org/wiki/Astronomical_unithttps://en.wikipedia.org/wiki/1RXS1609_bhttps://en.wikipedia.org/wiki/Space_telescopehttps://en.wikipedia.org/wiki/Gaia_(spacecraft)https://en.wikipedia.org/wiki/CHEOPS_(spacecraft)https://en.wikipedia.org/wiki/Transiting_Exoplanet_Survey_Satellitehttps://en.wikipedia.org/wiki/James_Webb_Space_Telescopehttps://en.wikipedia.org/wiki/James_Webb_Space_Telescopehttps://en.wikipedia.org/wiki/List_of_gravitationally_rounded_objects_of_the_Solar_Systemhttps://en.wikipedia.org/wiki/List_of_gravitationally_rounded_objects_of_the_Solar_Systemhttps://en.wikipedia.org/wiki/Help:IPA_for_Englishhttps://en.wikipedia.org/wiki/Dwarf_planethttps://en.wikipedia.org/wiki/Natural_satellitehttps://en.wikipedia.org/wiki/Rogue_planethttps://en.wikipedia.org/wiki/Sub-brown_dwarfhttps://en.wikipedia.org/wiki/Rogue_planethttps://en.wikipedia.org/wiki/Computer_simulationhttps://en.wikipedia.org/wiki/Spacehttps://en.wikipedia.org/wiki/Sub-brown_dwarfhttps://en.wikipedia.org/wiki/Cha_110913-773444https://en.wikipedia.org/wiki/OTS_44https://en.wikipedia.org/wiki/OTS_44https://en.wikipedia.org/wiki/2MASS_J04414489+2301513https://en.wikipedia.org/wiki/2MASS_J04414489+2301513https://en.wikipedia.org/wiki/Oph_162225-240515https://en.wikipedia.org/wiki/Oph_162225-240515https://en.wikipedia.org/wiki/Brown_dwarfhttps://en.wikipedia.org/wiki/Brown_dwarfhttps://en.wikipedia.org/wiki/Binary_starhttps://en.wikipedia.org/wiki/Accretion-powered_pulsars

10 7 ATTRIBUTES

drive mass loss. The shrinking star can then becomea planetary-mass object. An example is a Jupiter-massobject orbiting the pulsar PSR J1719-1438.[98] Theseshrunken white dwarfs may become a helium planet ordiamond planet.

6.4 Satellite planets and belt planets

Some large satellites are of similar size or larger than theplanet Mercury, e.g. Jupiters Galilean moons and Titan.Alan Stern has argued that location should not matterand that only geophysical attributes should be taken intoaccount in the definition of a planet, and proposes theterm satellite planet for a planet-sized satellite. Likewise,dwarf planets in the asteroid belt and Kuiper belt shouldbe considered planets according to Stern.[65]

6.5 Captured planets

Free-floating planets in stellar clusters have similar veloc-ities to the stars and so can be recaptured. They are typi-cally captured into wide orbits between 100 and 105 AU.The capture efficiency decreases with increasing clustersize, and for a given cluster size it increases with thehost/primary mass. It is almost independent of the plane-tary mass. Single and multiple planets could be capturedinto arbitrary unaligned orbits, non-coplanar with eachother or with the stellar host spin, or pre-existing plane-tary system.[99]

7 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 naturalsatellites, have only as yet been observed in planets inthe Solar System, whereas others are also commonly ob-served in extrasolar planets.

7.1 Dynamic characteristics

7.1.1 Orbit

Main articles: Orbit and Orbital elementsSee also: Keplers laws of planetary motionAccording to current definitions, all planets must revolvearound stars; thus, any potential "rogue planets" are ex-cluded. In the Solar System, all the planets orbit theSun in the same direction as the Sun rotates (counter-clockwise as seen from above the Suns north pole). Atleast one extrasolar planet, WASP-17b, has been foundto orbit in the opposite direction to its stars rotation.[100]The period of one revolution of a planets orbit is knownas its sidereal period or year.[101] A planets year depends

Pluto

Q

q Neptune

Q q

The orbit of the planet Neptune compared to that of Pluto.Note the elongation of Plutos orbit in relation to Neptunes(eccentricity), as well as its large angle to the ecliptic (inclination).

on its distance from its star; the farther a planet is fromits star, not only the longer the distance it must travel,but also the slower its speed, because it is less affectedby its stars gravity. No planets orbit is perfectly circu-lar, and hence the distance of each varies over the courseof its year. The closest approach to its star is calledits periastron (perihelion in the Solar System), whereasits farthest separation from the star is called its apastron(aphelion). As a planet approaches periastron, its speedincreases as it trades gravitational potential energy for ki-netic energy, just as a falling object on Earth acceleratesas it falls; as the planet reaches apastron, its speed de-creases, just as an object thrown upwards on Earth slowsdown as it reaches the apex of its trajectory.[102]

Each planets orbit is delineated by a set of elements:

The eccentricity of an orbit describes how elongateda planets orbit is. Planets with low eccentricitieshave more circular orbits, whereas planets with higheccentricities have more elliptical orbits. The plan-ets in the Solar System have very low eccentrici-ties, and thus nearly circular orbits.[101] Comets andKuiper belt objects (as well as several extrasolarplanets) have very high eccentricities, and thus ex-ceedingly elliptical orbits.[103][104]

Illustration of the semi-major axis

The semi-major axis is the distance from a planet tothe half-way point along the longest diameter of itselliptical orbit (see image). This distance is not thesame as its apastron, because no planets orbit has itsstar at its exact centre.[101]

https://en.wikipedia.org/wiki/Planetary-mass_objecthttps://en.wikipedia.org/wiki/Pulsarhttps://en.wikipedia.org/wiki/PSR_J1719-1438https://en.wikipedia.org/wiki/Helium_planethttps://en.wikipedia.org/wiki/Diamond_planethttps://en.wikipedia.org/wiki/Mercury_(planet)https://en.wikipedia.org/wiki/Galilean_moonshttps://en.wikipedia.org/wiki/Titan_(moon)https://en.wikipedia.org/wiki/Alan_Sternhttps://en.wikipedia.org/wiki/Asteroid_belthttps://en.wikipedia.org/wiki/Kuiper_belthttps://en.wikipedia.org/wiki/Rogue_planethttps://en.wikipedia.org/wiki/Open_clusterhttps://en.wikipedia.org/wiki/Orbithttps://en.wikipedia.org/wiki/Orbital_elementshttps://en.wikipedia.org/wiki/Kepler%2527s_laws_of_planetary_motionhttps://en.wikipedia.org/wiki/Rogue_planethttps://en.wikipedia.org/wiki/WASP-17bhttps://en.wikipedia.org/wiki/Sidereal_periodhttps://en.wikipedia.org/wiki/Plutohttps://en.wikipedia.org/wiki/Orbital_eccentricityhttps://en.wikipedia.org/wiki/Inclinationhttps://en.wikipedia.org/wiki/Gravityhttps://en.wikipedia.org/wiki/Periastronhttps://en.wikipedia.org/wiki/Perihelionhttps://en.wikipedia.org/wiki/Apastronhttps://en.wikipedia.org/wiki/Aphelionhttps://en.wikipedia.org/wiki/Orbital_eccentricityhttps://en.wikipedia.org/wiki/Semi-major_axis

7.1 Dynamic characteristics 11

The inclination of a planet tells how far above or be-low an established reference plane its orbit lies. Inthe Solar System, the reference plane is the plane ofEarths orbit, called the ecliptic. For extrasolar plan-ets, the plane, known as the sky plane or plane ofthe sky, is the plane perpendicular to the observersline of sight from Earth.[105] The eight planets ofthe Solar System all lie very close to the ecliptic;comets and Kuiper belt objects like Pluto are at farmore extreme angles to it.[106] The points at whicha planet crosses above and below its reference planeare called its ascending and descending nodes.[101]The longitude of the ascending node is the anglebetween the reference planes 0 longitude and theplanets ascending node. The argument of periapsis(or perihelion in the Solar System) is the angle be-tween a planets ascending node and its closest ap-proach to its star.[101]

7.1.2 Axial tilt

Main article: Axial tiltPlanets also have varying degrees of axial tilt; they lie

Earths axial tilt is about 23.

at an angle to the plane of their stars equators. Thiscauses the amount of light received by each hemisphere tovary over the course of its year; when the northern hemi-sphere points away from its star, the southern hemispherepoints towards it, and vice versa. Each planet thereforehas seasons; changes to the climate over the course ofits year. The time at which each hemisphere points far-thest or nearest from its star is known as its solstice. Eachplanet has two in the course of its orbit; when one hemi-sphere has its summer solstice, when its day is longest,the other has its winter solstice, when its day is short-est. The varying amount of light and heat received byeach hemisphere creates annual changes in weather pat-terns for each half of the planet. Jupiters axial tilt is verysmall, so its seasonal variation is minimal; Uranus, on theother hand, has an axial tilt so extreme it is virtually onits side, which means that its hemispheres are either per-

petually in sunlight or perpetually in darkness around thetime of its solstices.[107] Among extrasolar planets, axialtilts are not known for certain, though most hot Jupitersare believed to have negligible to no axial tilt as a resultof their proximity to their stars.[108]

7.1.3 Rotation

The planets rotate around invisible axes through their cen-tres. A planets rotation period is known as a stellar day.Most of the planets in the Solar System rotate in thesame direction as they orbit the Sun, which is counter-clockwise as seen from above the Suns north pole, theexceptions being Venus[109] and Uranus,[110] which ro-tate clockwise, though Uranuss extreme axial tilt meansthere are differing conventions on which of its poles isnorth, and therefore whether it is rotating clockwiseor anti-clockwise.[111] Regardless of which convention isused, Uranus has a retrograde rotation relative to its orbit.The rotation of a planet can be induced by several fac-tors during formation. A net angular momentum can beinduced by the individual angular momentum contribu-tions of accreted objects. The accretion of gas by thegiant planets can also contribute to the angular momen-tum. Finally, during the last stages of planet building,a stochastic process of protoplanetary accretion can ran-domly alter the spin axis of the planet.[112] There is greatvariation in the length of day between the planets, withVenus taking 243 days to rotate, and the giant planetsonly a few hours.[113] The rotational periods of extrasolarplanets are not known, but their proximity to their starsmeans that hot Jupiters are tidally locked (their orbits arein sync with their rotations). This means they only evershow one face to their stars, with one side in perpetualday, the other in perpetual night.[114]

7.1.4 Orbital clearing

Main article: Clearing the neighbourhood

The defining dynamic characteristic of a planet is that ithas cleared its neighborhood. A planet that has clearedits neighborhood has accumulated enough mass to gatherup or sweep away all the planetesimals in its orbit. In ef-fect, it orbits its star in isolation, as opposed to sharingits orbit with a multitude of similar-sized objects. Thischaracteristic was mandated as part of the IAU's offi-cial definition of a planet in August, 2006. This criterionexcludes such planetary bodies as Pluto, Eris and Ceresfrom full-fledged planethood, making them instead dwarfplanets.[2] Although to date this criterion only applies tothe Solar System, a number of young extrasolar systemshave been found in which evidence suggests orbital clear-ing is taking place within their circumstellar discs.[115]

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12 7 ATTRIBUTES

7.2 Physical characteristics

7.2.1 Mass

Main article: Planetary mass

A planets defining physical characteristic is that it is mas-sive enough for the force of its own gravity to dominateover the electromagnetic forces binding its physical struc-ture, leading to a state of hydrostatic equilibrium. This ef-fectivelymeans 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 thechemical makeup of the object, gravity begins to pull anobject towards its own centre of mass until the object col-lapses into a sphere.[116]

Mass is also the prime attribute by which planets are dis-tinguished from stars. The upper mass limit for planet-hood is roughly 13 times Jupiters mass for objects withsolar-type isotopic abundance, beyond which it achievesconditions suitable for nuclear fusion. Other than the Sun,no objects of such mass exist in the Solar System; butthere are exoplanets of this size. The 13-Jupiter-masslimit is not universally agreed upon and the ExtrasolarPlanets Encyclopaedia includes objects up to 20 Jupitermasses,[117] and the Exoplanet Data Explorer up to 24Jupiter masses.[118]

The smallest known planet is PSR B1257+12A, one ofthe first extrasolar planets discovered, which was found in1992 in orbit around a pulsar. Its mass is roughly half thatof the planet Mercury.[4] The smallest known planet or-biting a main-sequence star other than the Sun is Kepler-37b, with a mass (and radius) slightly higher than that ofthe Moon.

7.2.2 Internal differentiation

Main article: Planetary differentiationEvery planet began its existence in an entirely fluid state;in early formation, the denser, heavier materials sank tothe centre, leaving the lighter materials near the surface.Each therefore has a differentiated interior consisting ofa dense planetary core surrounded by a mantle that eitheris or was a fluid. The terrestrial planets are sealed withinhard crusts,[119] but in the giant planets the mantle simplyblends into the upper cloud layers. The terrestrial plan-ets have cores of elements such as iron and nickel, andmantles of silicates. Jupiter and Saturn are believed tohave cores of rock and metal surrounded by mantles ofmetallic hydrogen.[120] Uranus and Neptune, which aresmaller, have rocky cores surrounded bymantles of water,ammonia, methane and other ices.[121] The fluid actionwithin these planets cores creates a geodynamo that gen-erates a magnetic field.[119]

Illustration of the interior of Jupiter, with a rocky core overlaidby a deep layer of metallic hydrogen

7.2.3 Atmosphere

Main articles: Atmosphere and Extraterrestrial atmo-spheresSee also: Extraterrestrial skiesAll of the Solar System planets except Mercury[122] have

Earths atmosphere

substantial atmospheres because their gravity is strongenough to keep gases close to the surface. The larger gi-ant planets are massive enough to keep large amounts ofthe light gases hydrogen and helium, whereas the smallerplanets lose these gases into space.[123] The compositionof Earths atmosphere is different from the other planetsbecause the various life processes that have transpired onthe planet have introduced free molecular oxygen.[124]

Planetary atmospheres are affected by the varyinginsolation or internal energy, leading to the formationof dynamic weather systems such as hurricanes, (onEarth), planet-wide dust storms (onMars), an Earth-sizedanticyclone on Jupiter (called the Great Red Spot), andholes in the atmosphere (on Neptune).[107] At least oneextrasolar planet, HD 189733 b, has been claimed to havesuch a weather system, similar to the Great Red Spot but

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7.3 Secondary characteristics 13

twice as large.[125]

Hot Jupiters, due to their extreme proximities to theirhost stars, have been shown to be losing their atmospheresinto space due to stellar radiation, much like the tails ofcomets.[126][127] These planets may have vast differencesin temperature between their day and night sides that pro-duce supersonic winds,[128] although the day and nightsides of HD 189733 b appear to have very similar temper-atures, indicating that that planets atmosphere effectivelyredistributes the stars energy around the planet.[125]

7.2.4 Magnetosphere

Main article: MagnetosphereOne important characteristic of the planets is their in-

Magnetosheath

Neutral sheet

Plasma sheet

Magnetotail

Earth's atmosphere0 - 100 km

Polar cusp

Bow shock

Solar wind

Van Allen radiation belt

Incoming solar wind particles

Deflected solar wind particles

Schematic of Earths magnetosphere

trinsic magnetic moments, which in turn give rise to mag-netospheres. The presence of a magnetic field indicatesthat the planet is still geologically alive. In other words,magnetized planets have flows of electrically conduct-ing material in their interiors, which generate their mag-netic fields. These fields significantly change the interac-tion of the planet and solar wind. A magnetized planetcreates a cavity in the solar wind around itself calledmagnetosphere, which the wind cannot penetrate. Themagnetosphere can be much larger than the planet it-self. In contrast, non-magnetized planets have only smallmagnetospheres induced by interaction of the ionospherewith the solar wind, which cannot effectively protect theplanet.[129]

Of the eight planets in the Solar System, only Venusand Mars lack such a magnetic field.[129] In addition, themoon of Jupiter Ganymede also has one. Of the magne-tized planets themagnetic field ofMercury is the weakest,and is barely able to deflect the solar wind. Ganymedesmagnetic field is several times larger, and Jupiters is thestrongest in the Solar System (so strong in fact that itposes a serious health risk to future manned missions toits moons). The magnetic fields of the other giant planetsare roughly similar in strength to that of Earth, but theirmagnetic moments are significantly larger. The magnetic

fields of Uranus and Neptune are strongly tilted relativethe rotational axis and displaced from the centre of theplanet.[129]

In 2004, a team of astronomers in Hawaii observed anextrasolar planet around the star HD 179949, which ap-peared to be creating a sunspot on the surface of its par-ent star. The team hypothesized that the planets magne-tosphere was transferring energy onto the stars surface,increasing its already high 7,760 C temperature by anadditional 400 C.[130]

7.3 Secondary characteristics

Main articles: Natural satellite and Planetary ringSeveral planets or dwarf planets in the Solar System

The rings of Saturn

(such as Neptune and Pluto) have orbital periods thatare in resonance with each other or with smaller bod-ies (this is also common in satellite systems). All exceptMercury and Venus have natural satellites, often calledmoons. Earth has one, Mars has two, and the giantplanets have numerous moons in complex planetary-typesystems. Many moons of the giant planets have featuressimilar to those on the terrestrial planets and dwarf plan-ets, and some have been studied as possible abodes of life(especially Europa).[131][132][133]

The four giant planets are also orbited by planetary ringsof varying size and complexity. The rings are composedprimarily of dust or particulate matter, but can host tiny'moonlets' whose gravity shapes andmaintains their struc-ture. Although the origins of planetary rings is not pre-cisely known, they are believed to be the result of naturalsatellites that fell below their parent planets Roche limitand were torn apart by tidal forces.[134][135]

No secondary characteristics have been observed around

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14 10 REFERENCES

extrasolar planets. The sub-brown dwarf Cha 110913-773444, which has been described as a rogue planet, isbelieved to be orbited by a tiny protoplanetary disc[92] andthe sub-brown dwarf OTS 44 was shown to be surroundedby a substantial protoplanetary disk of at least 10 Earthmasses.[94]

8 See also Double planet Two planetarymass objects orbitingeach other

List of hypothetical Solar System objects

Landings on other planets

Mesoplanet Planets smaller than Mercury butlarger than Ceres

Minor planet A celestial body smaller than a planet

Planetary mnemonic A phrase used to rememberthe names of the planets

Planetary habitability The measure of a planetsability to sustain life

Planetary science The scientific study of planets

Planets in astrology

Planets in science fiction

Theoretical planetology

9 Notes[1] This definition is drawn from two separate IAU declara-

tions; a formal definition agreed by the IAU in 2006, andan informal working definition established by the IAUin 2001/2003 for objects outside of the Solar System.The official 2006 definition applies only to the Solar Sys-tem, whereas the 2003 definition applies to planets aroundother stars. The extrasolar planet issue was deemed toocomplex to resolve at the 2006 IAU conference.

[2] For the purpose of this 1 in 5 statistic, Sun-like meansG-type star. Data for Sun-like stars wasn't available so thisstatistic is an extrapolation from data about K-type stars

[3] For the purpose of this 1 in 5 statistic, Earth-sized means12 Earth radii

[4] For the purpose of this 1 in 5 statistic, habitable zonemeans the region with 0.25 to 4 times Earths stellar flux(corresponding to 0.52 AU for the Sun).

[5] Referred to by Huygens as a Planetes novus (new planet)in his Systema Saturnium

[6] Both labelled nouvelles plantes (new planets) by Cassiniin hisDcouverte de deux nouvelles planetes autour de Sat-urne[62]

[7] Both once referred to as planets by Cassini in his AnExtract of the Journal Des Scavans.... The term satellitehad already begun to be used to distinguish such bodiesfrom those around which they orbited (primary planets).

[8] Measured relative to Earth.

[9] Jupiter has the most verified satellites (67) in the SolarSystem.[82]

[10] See Earth article for absolute values.

10 References[1] "Planet Etymology. dictionary.com. Retrieved 29 June

2015.

[2] IAU 2006 General Assembly: Result of the IAU Reso-lution votes. International Astronomical Union. 2006.Retrieved 2009-12-30.

[3] Working Group on Extrasolar Planets (WGESP) of theInternational Astronomical Union. IAU. 2001. Retrieved2008-08-23.

[4] Schneider, Jean (16 January 2013). Interactive Extra-solar Planets Catalog. The Extrasolar Planets Ency-clopaedia. Retrieved 2013-01-15.

[5] NASA Staff (20 December 2011). Kepler: A Search ForHabitable Planets Kepler-20e. NASA. Retrieved 2011-12-23.

[6] NASA Staff (20 December 2011). Kepler: A Search ForHabitable Planets Kepler-20f. NASA. Retrieved 2011-12-23.

[7] Johnson, Michele (20 December 2011). NASA Discov-ers First Earth-size Planets Beyond Our Solar System.NASA. Retrieved 2011-12-20.

[8] Hand, Eric (20 December 2011). Kepler dis-covers first Earth-sized exoplanets. Nature.doi:10.1038/nature.2011.9688.

[9] Overbye, Dennis (20 December 2011). Two Earth-SizePlanets Are Discovered. New York Times. Retrieved2011-12-21.

[10] Cassan, Arnaud; D. Kubas; J.-P. Beaulieu; M. Do-minik et al. (12 January 2012). One or morebound planets per Milky Way star from microlens-ing observations. Nature 481 (7380): 167169.arXiv:1202.0903. Bibcode:2012Natur.481..167C.doi:10.1038/nature10684. PMID 22237108. Retrieved11 January 2012.

[11] , H. G. Liddell and R. Scott, A GreekEnglishLexicon, ninth edition, (Oxford: Clarendon Press, 1940).

[12] Definition of planet. Merriam-Webster OnLine. Re-trieved 2007-07-23.

[13] planet, n. Oxford English Dictionary. 2007. Retrieved2008-02-07. Note: select the Etymology tab

https://en.wikipedia.org/wiki/Sub-brown_dwarfhttps://en.wikipedia.org/wiki/Cha_110913-773444https://en.wikipedia.org/wiki/Cha_110913-773444https://en.wikipedia.org/wiki/Rogue_planethttps://en.wikipedia.org/wiki/Protoplanetary_dischttps://en.wikipedia.org/wiki/OTS_44https://en.wikipedia.org/wiki/Double_planethttps://en.wikipedia.org/wiki/List_of_hypothetical_Solar_System_objectshttps://en.wikipedia.org/wiki/Landings_on_other_planetshttps://en.wikipedia.org/wiki/Mesoplanethttps://en.wikipedia.org/wiki/Minor_planethttps://en.wikipedia.org/wiki/Planetary_mnemonichttps://en.wikipedia.org/wiki/Planetary_habitabilityhttps://en.wikipedia.org/wiki/Planetary_sciencehttps://en.wikipedia.org/wiki/Planets_in_astrologyhttps://en.wikipedia.org/wiki/Planets_in_science_fictionhttps://en.wikipedia.org/wiki/Theoretical_planetologyhttps://en.wikipedia.org/wiki/Definition_of_planethttps://en.wikipedia.org/wiki/International_Astronomical_Unionhttps://en.wikipedia.org/wiki/2006_definition_of_planethttps://en.wikipedia.org/wiki/G-type_starhttps://en.wikipedia.org/wiki/K-type_starhttp://www.sil.si.edu/DigitalCollections/HST/Huygens/huygens-text.htmhttp://links.jstor.org/sici?sici=0260-7085%25281686%252F1692%252916%253C79%253AAEOTJD%253E2.0.CO%253B2-Jhttp://links.jstor.org/sici?sici=0260-7085%25281686%252F1692%252916%253C79%253AAEOTJD%253E2.0.CO%253B2-Jhttps://en.wikipedia.org/wiki/Earthhttp://dictionary.reference.com/browse/planethttps://en.wikipedia.org/wiki/Dictionary.comhttp://www.iau.org/news/pressreleases/detail/iau0603/http://www.iau.org/news/pressreleases/detail/iau0603/http://www.dtm.ciw.edu/boss/definition.htmlhttp://www.dtm.ciw.edu/boss/definition.htmlhttp://exoplanet.eu/catalog.phphttp://exoplanet.eu/catalog.phphttps://en.wikipedia.org/wiki/The_Extrasolar_Planets_Encyclopaediahttps://en.wikipedia.org/wiki/The_Extrasolar_Planets_Encyclopaediahttps://en.wikipedia.org/wiki/NASAhttp://kepler.nasa.gov/Mission/discoveries/kepler20e/http://kepler.nasa.gov/Mission/discoveries/kepler20e/https://en.wikipedia.org/wiki/NASAhttps://en.wikipedia.org/wiki/NASAhttp://kepler.nasa.gov/Mission/discoveries/kepler20f/http://kepler.nasa.gov/Mission/discoveries/kepler20f/https://en.wikipedia.org/wiki/NASAhttp://www.nasa.gov/mission_pages/kepler/news/kepler-20-system.htmlhttp://www.nasa.gov/mission_pages/kepler/news/kepler-20-system.htmlhttps://en.wikipedia.org/wiki/NASAhttps://en.wikipedia.org/wiki/Nature_(journal)https://en.wikipedia.org/wiki/Digital_object_identifierhttps://dx.doi.org/10.1038%252Fnature.2011.9688http://www.nytimes.com/2011/12/21/science/space/nasas-kepler-spacecraft-discovers-2-earth-size-planets.htmlhttp://www.nytimes.com/2011/12/21/science/space/nasas-kepler-spacecraft-discovers-2-earth-size-planets.htmlhttp://www.nature.com/nature/journal/v481/n7380/full/nature10684.htmlhttp://www.nature.com/nature/journal/v481/n7380/full/nature10684.htmlhttp://www.nature.com/nature/journal/v481/n7380/full/nature10684.htmlhttps://en.wikipedia.org/wiki/ArXivhttps://arxiv.org/abs/1202.0903https://en.wikipedia.org/wiki/Bibcodehttp://adsabs.harvard.edu/abs/2012Natur.481..167Chttps://en.wikipedia.org/wiki/Digital_object_identifierhttps://dx.doi.org/10.1038%252Fnature10684https://en.wikipedia.org/wiki/PubMed_Identifierhttps://www.ncbi.nlm.nih.gov/pubmed/22237108http://www.perseus.tufts.edu/hopper/text?doc=Perseus%253Atext%253A1999.04.0058%253Aentry%253Dplanh%252Fthshttp://www.merriam-webster.com/dictionary/planethttp://dictionary.oed.com/cgi/entry/50180718?query_type=word&queryword=planet

15

[14] Neugebauer, Otto E. (1945). The History of Ancient As-tronomy Problems and Methods. Journal of Near East-ern Studies 4 (1): 138. doi:10.1086/370729.

[15] Ronan, Colin. Astronomy Before the Telescope. As-tronomy in China, Korea and Japan (Walker ed.). pp.264265.

[16] Kuhn, Thomas S. (1957). The Copernican Revolution.Harvard University Press. pp. 520. ISBN 0-674-17103-9.

[17] Evans, James (1998). The History and Practice of AncientAstronomy. Oxford University Press. pp. 2967. ISBN978-0-19-509539-5. Retrieved 2008-02-04.

[18] Francesca Rochberg (2000). Astronomy and Calendarsin Ancient Mesopotamia. In Jack Sasson. Civilizationsof the Ancient Near East III. p. 1930.

[19] Holden, James Herschel (1996). A History of HoroscopicAstrology. AFA. p. 1. ISBN 978-0-86690-463-6.

[20] Hermann Hunger, ed. (1992). Astrological reports to As-syrian kings. State Archives of Assyria 8. Helsinki Uni-versity Press. ISBN 951-570-130-9.

[21] Lambert, W. G.; Reiner, Erica (1987). Babylonian Plan-etary Omens. Part One. EnumaAnu Enlil, Tablet 63: TheVenus Tablet of Ammisaduqa.. Journal of the AmericanOriental Society 107 (1): 9396. doi:10.2307/602955.JSTOR 602955.

[22] Kasak, Enn; Veede, Raul (2001). Mare Kiva;Andres Kuperjanov, eds. Understanding Planetsin Ancient Mesopotamia (PDF). Electronic Journalof Folklore (Estonian Literary Museum) 16: 735.doi:10.7592/fejf2001.16.planets. Retrieved 2008-02-06.

[23] A. Sachs (May 2, 1974). Babylonian Observa-tional Astronomy. Philosophical Transactions of theRoyal Society (Royal Society of London) 276 (1257):4350 [45 & 489]. Bibcode:1974RSPTA.276...43S.doi:10.1098/rsta.1974.0008. JSTOR 74273.

[24] Burnet, John (1950). Greek philosophy: Thales to Plato.Macmillan and Co. pp. 711. ISBN 978-1-4067-6601-1.Retrieved 2008-02-07.

[25] Goldstein, Bernard R. (1997). Saving the phenomena:the background to Ptolemys planetary theory. Journalfor the History of Astronomy (Cambridge (UK)) 28 (1):112. Bibcode:1997JHA....28....1G.

[26] Ptolemy; Toomer, G. J. (1998). Ptolemys Almagest.Princeton University Press. ISBN 978-0-691-00260-6.

[27] J. J. O'Connor and E. F. Robertson, Aryabhata the Elder,MacTutor History of Mathematics archive

[28] Sarma, K. V. (1997) Astronomy in India in Selin,Helaine (editor) Encyclopaedia of the History of Sci-ence, Technology, and Medicine in Non-Western Cultures,Kluwer Academic Publishers, ISBN 0-7923-4066-3, p.116

[29] Ramasubramanian, K. (1998). Model of planetary mo-tion in the works of Kerala astronomers. Bulletin ofthe Astronomical Society of India 26: 1131 [234].Bibcode:1998BASI...26...11R.

[30] Ramasubramanian etc. (1994)

[31] Sally P. Ragep (2007). Ibn Sn: Ab Alalusayn ibn Abdallh ibn Sn". In ThomasHockey. The Biographical Encyclopedia of As-tronomers. Springer Science+Business Media.pp. 570572. Bibcode:2000eaa..bookE3736..doi:10.1888/0333750888/3736. ISBN 0-333-75088-8.

[32] S. M. Razaullah Ansari (2002). History of oriental as-tronomy: proceedings of the joint discussion-17 at the23rd General Assembly of the International AstronomicalUnion, organised by the Commission 41 (History of As-tronomy), held in Kyoto, August 2526, 1997. Springer.p. 137. ISBN 1-4020-0657-8.

[33] Fred Espenak. Six millennium catalog of Venus tran-sits: 2000 BCE to 4000 CE. NASA/GSFC. Retrieved11 February 2012.

[34] Van Helden, Al (1995). Copernican System. TheGalileo Project. Retrieved 2008-01-28.

[35] See primary citations in Timeline of discovery of SolarSystem planets and their moons

[36] Hilton, James L. (2001-09-17). When Did the Aster-oids Become Minor Planets?". U.S. Naval Observatory.Archived from the original on 2007-09-21. Retrieved2007-04-08.

[37] Croswell, K. (1997). Planet Quest: The Epic Discovery ofAlien Solar Systems. The Free Press. p. 57. ISBN 978-0-684-83252-4.

[38] Lyttleton, Raymond A. (1936). On the possible re-sults of an encounter of Pluto with the Neptuniansystem. Monthly Notices of the Royal Astronomi-cal Society 97: 108. Bibcode:1936MNRAS..97..108L.doi:10.1093/mnras/97.2.108.

[39] Whipple, Fred (1964). The History of the So-lar System. Proceedings of the National Academyof Sciences of the United States of America 52(2): 56