Mercury Venus
Earth Mars
Jupiter Saturn
Uranus Neptune
The eight planets of the Solar System
Mercury, Venus, Earth, and Mars
Jupiter and Saturn ( gas giants)
Uranus and Neptune ( ice giants)

Shown in order from the Sun and in true color. Sizes are not to scale.

A planet is an astronomical body orbiting a star or stellar remnant that

The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Several planets in the Solar System can be seen with 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 modern 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 such.

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. At about the same time, by careful analysis of pre-telescopic observation data collected by Tycho Brahe, Johannes Kepler found the planets' orbits were not circular but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, 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 are generally divided into two main types: large low-density giant planets, and smaller rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System. 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.

Several thousands of planets around other stars (" extrasolar planets" or "exoplanets") have been discovered in the Milky Way. As of 1 October 2017, 3,671 known extrasolar planets in 2,751 planetary systems (including 616 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 habitable zone. [3] [4] On December 20, 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 [b] stars is thought to have an Earth-sized [c] planet in its habitable [d] zone.


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, 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. [11] Ancient Greeks called these lights πλάνητες ἀστέρες (planētes asteres, "wandering stars") or simply πλανῆται (planētai, "wanderers"), [12] from which today's word "planet" was derived. [13] [14] [15] In ancient Greece, China, Babylon, and indeed all pre-modern civilizations, [16] [17] 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 [18] and the apparently common-sense perceptions that Earth was solid and stable and that it was not moving but at rest.


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. [19] 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. [20] The Babylonian astrologers also laid the foundations of what would eventually become Western astrology. [21] The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC, [22] comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets. [23] [24] 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. [25]

Greco-Roman astronomy

Ptolemy's 7 planetary spheres

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). [26] 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. [19] [27] 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): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. [15] [27] [28]


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. [29] 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. [30]

In 1500, Nilakantha Somayaji of the Kerala school of astronomy and mathematics, in his Tantrasangraha, revised Aryabhata's model. [31] 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. [31] [32]

Medieval Muslim astronomy

In the 11th century, the transit of Venus was observed by Avicenna, who established that Venus was, at least sometimes, below the Sun. [33] 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. [34] Ibn Bajjah could not have observed a transit of Venus, because none occurred in his lifetime. [35]

European Renaissance

Renaissance planets,
c. 1543 to 1610 and c. 1680 to 1781

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, [36] 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. [37] 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

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. [38]

20th century

Planets 1854–1930, Solar planets 2006–present

In the 20th century, Pluto was discovered. After initial observations led to the belief it was larger than Earth, [39] the object was immediately accepted as the ninth planet. Further monitoring found the body was actually much smaller: in 1936, Raymond Lyttleton suggested that Pluto may be an escaped satellite of Neptune, [40] and Fred Whipple suggested in 1964 that Pluto may be a comet. [41] As it was still larger than all known asteroids and seemingly did not exist within a larger population, [42] it kept its status until 2006.

(Solar) planets 1930–2006

In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar, PSR B1257+12. [43] 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). [44]

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. [45]

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). [46]

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]

  1. 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 [47]) 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.
  2. 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.
  3. 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 has since been widely used by astronomers when publishing discoveries of exoplanets in academic journals. [48] Although temporary, it remains an effective working definition until a more permanent one is formally adopted. It does not address the dispute over the lower mass limit, [49] and so it steered clear of the controversy regarding objects within the Solar System. This definition also makes no comment on the planetary status of objects orbiting brown dwarfs, such as 2M1207b.

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. [50] 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.

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. [51] 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. [52] The Extrasolar Planets Encyclopaedia includes objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around 13 MJ in the observed mass spectrum reinforces the choice to forget this mass limit." [53] 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." [54] The NASA Exoplanet Archive includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses. [55]

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. [56] [57]

2006 IAU definition of planet

Euler diagram showing the types of bodies in the Solar System.

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, 232 members of the 10,000 member assembly, who nevertheless constituted 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. [58] After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets. [59]

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: [60]

"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."

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. 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. [61] [62] This formula produces a value 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. [63]

Objects formerly considered planets

The table below lists Solar System bodies once considered to be planets.

Body Current classification Notes
Sun Star Classified as classical planets (Ancient Greek πλανῆται, wanderers) in classical antiquity and medieval Europe, in accordance with the now-disproved geocentric model. [64]
Moon Natural satellite
Io, Europa, Ganymede, and Callisto Natural satellites 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. [65]
Titan, [e] Iapetus, [f] Rhea, [f] Tethys, [g] and Dione [g] Natural satellites 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. [65]
Pallas, Juno, and Vesta Asteroids Regarded as planets from their discoveries between 1801 and 1807 until they were reclassified as asteroids during the 1850s. [67]

Ceres was subsequently classified as a dwarf planet in 2006.

Ceres Dwarf planet and asteroid
Astraea, Hebe, Iris, Flora, Metis, Hygiea, Parthenope, Victoria, Egeria, Irene, Eunomia Asteroids 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. [68]
Pluto Dwarf planet and Kuiper belt object 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.

Beyond the scientific community, Pluto still holds cultural significance for many in the general public due to its historical classification as a planet from 1930 to 2006. [69] A few astronomers, such as Alan Stern, consider dwarf planets and the larger moons to be planets, based on a purely geophysical definition of planet. [70]

Other Languages
Afrikaans: Planeet
Alemannisch: Planet
አማርኛ: ፕላኔት
العربية: كوكب
aragonés: Planeta
ܐܪܡܝܐ: ܦܠܝܛܐ
armãneashti: Planetâ
arpetan: Planèta
asturianu: Planeta
Avañe'ẽ: Mbyjajere
azərbaycanca: Planet
تۆرکجه: گزه‌گن
বাংলা: গ্রহ
Bahasa Banjar: Palanit
Bân-lâm-gú: He̍k-chheⁿ
Basa Banyumasan: Planet
башҡортса: Планета
беларуская: Планета
беларуская (тарашкевіца)‎: Плянэта
भोजपुरी: ग्रह
български: Планета
Boarisch: Planet
བོད་ཡིག: གཡོ་སྐར།
bosanski: Planeta
brezhoneg: Planedenn
буряад: Гараг
català: Planeta
Чӑвашла: Планета
čeština: Planeta
chiShona: Chindeya
Cymraeg: Planed
dansk: Planet
Deutsch: Planet
ދިވެހިބަސް: ގުރަހަ
eesti: Planeet
Ελληνικά: Πλανήτης
emiliàn e rumagnòl: Pianaid
эрзянь: Чарытеште
español: Planeta
Esperanto: Planedo
estremeñu: Praneta
euskara: Planeta
فارسی: سیاره
Fiji Hindi: Grah
føroyskt: Gongustjørnur
français: Planète
Frysk: Planeet
furlan: Planet
Gaeilge: Pláinéad
Gaelg: Planaid
Gàidhlig: Planaid
galego: Planeta
ગુજરાતી: ગ્રહ
客家語/Hak-kâ-ngî: Hàng-sên
한국어: 행성
Hausa: Duniyoyi
Hawaiʻi: Hōkū hele
Հայերեն: Մոլորակ
हिन्दी: ग्रह
hrvatski: Planet
Ido: Planeto
Ilokano: Planeta
Bahasa Indonesia: Planet
interlingua: Planeta
Ирон: Планетæ
íslenska: Reikistjarna
italiano: Pianeta
עברית: כוכב לכת
Basa Jawa: Planèt
ಕನ್ನಡ: ಗ್ರಹ
Kapampangan: Planeta
ქართული: პლანეტა
қазақша: Ғаламшар
kernowek: Planet
Kiswahili: Sayari
коми: Планет
Kongo: Mweta
Kreyòl ayisyen: Planèt
Kurdî: Gerstêrk
Кыргызча: Планета
лезги: Планета
Latina: Planeta
latviešu: Planēta
Lëtzebuergesch: Planéit
lietuvių: Planeta
Limburgs: Planeet
la .lojban.: plini
lumbaart: Pianeta
magyar: Bolygó
македонски: Планета
Malagasy: Fajiry
മലയാളം: ഗ്രഹം
Malti: Pjaneta
मराठी: ग्रह
მარგალური: პლანეტა
مصرى: كوكب
Bahasa Melayu: Planet
Baso Minangkabau: Planet
Mìng-dĕ̤ng-ngṳ̄: Giàng-sĭng
Mirandés: Planeta
монгол: Гариг
မြန်မာဘာသာ: ဂြိုဟ်
Nederlands: Planeet
Nedersaksies: Planeet
नेपाली: ग्रह
नेपाल भाषा: ग्रह
日本語: 惑星
Napulitano: Chianeta
нохчийн: Планета
Nordfriisk: Planeete
Norfuk / Pitkern: Plaanet
norsk: Planet
norsk nynorsk: Planet
Nouormand: Plianète
Novial: Planete
occitan: Planeta
олык марий: Планете
oʻzbekcha/ўзбекча: Sayyora
ਪੰਜਾਬੀ: ਗ੍ਰਹਿ
Pälzisch: Planed
پنجابی: پاندھی
پښتو: سياره
Patois: Planit
ភាសាខ្មែរ: ភព
Picard: Planète
Piemontèis: Pianeta
Tok Pisin: Planet
Plattdüütsch: Planet
polski: Planeta
português: Planeta
Qaraqalpaqsha: Planeta
qırımtatarca: Seyyare
română: Planetă
Romani: Shiyaron
rumantsch: Planet
Runa Simi: Pachahina
русиньскый: Планета
русский: Планета
саха тыла: Планета
davvisámegiella: Planehta
sardu: Praneta
Scots: Planet
Seeltersk: Planet
shqip: Planeti
sicilianu: Pianeta
සිංහල: ග්‍රහලෝක
Simple English: Planet
slovenčina: Planéta
slovenščina: Planet
ślůnski: Planeta
Soomaaliga: Meere
کوردی: هەسارە
српски / srpski: Планета
srpskohrvatski / српскохрватски: Planeta
Basa Sunda: Planét
suomi: Planeetta
svenska: Planet
Tagalog: Planeta
தமிழ்: கோள்
татарча/tatarça: Планета
తెలుగు: గ్రహం
тоҷикӣ: Сайёра
ತುಳು: ಗ್ರಹ
Türkçe: Gezegen
українська: Планета
اردو: سیارہ
vèneto: Pianeta
Tiếng Việt: Hành tinh
Võro: Hod'otäht
文言: 行星
Winaray: Planeta
吴语: 行星
ייִדיש: פלאנעט
Yorùbá: Plánẹ̀tì
粵語: 行星
Zazaki: Geyrenık
Zeêuws: Planete
žemaitėška: Planeta
中文: 行星
डोटेली: ग्रह