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 consensus definition as to what counts as a planet vs. other objects orbiting the sun has changed several times, previously encompassing asteroids and dwarf planets like Pluto.^[4]

The five classical planets of the Solar System, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, and ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the "fixed stars", which maintained a constant relative position in the sky.^[5] Ancient Greeks called these lights πλάνητες ἀστέρες (planētes asteres, "wandering stars") or simply πλανῆται (planētai, "wanderers"),^[6] from which today's word "planet" was derived.^[7][8][9] In ancient Greece, China, Babylon, and indeed all pre-modern civilizations,^[10][11] 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^[12] and the apparently common-sense perceptions that Earth was solid and stable and that it was not moving but at rest.^[13]

Babylon

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.^[14] 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.^[15] The Babylonian astrologers also laid the foundations of what would eventually become Western astrology.^[16] The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC,^[17] comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.^[18][19] 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.^[20]

Greco-Roman astronomy

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),^[21] though this had long been known by the Babylonians. In the 3rd century BC, Aristarchus of Samos proposed a heliocentric system, according to which Earth and the planets revolved around the Sun. The geocentric system remained dominant until the Scientific Revolution.^[13]

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.^[14][22] To the Greeks and Romans there were seven known planets, each presumed to be circling Earth according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order and using modern names): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.^[9][22][23]

India

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.^[24] 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.^[25]

In 1500, Nilakantha Somayaji of the Kerala school of astronomy and mathematics, in his Tantrasangraha, revised Aryabhata's model.^[26] 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.^[26][27]

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.^[28] 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.^[29] Ibn Bajjah could not have observed a transit of Venus, because none occurred in his lifetime.^[30]

European Renaissance

With the advent of the Scientific Revolution and the heliocentric model of Copernicus, Galileo and Kepler, use of the term "planet" changed from something that moved around the sky relative to the fixed star to a body that orbited the Sun, directly (a primary planet) or indirectly (a secondary or satellite planet). Thus the Earth was added to the roster of planets^[31] and the Sun was removed. The Copernican count of primary planets stood until 1781, when Uranus was discovered.^[32]

When four satellites of Jupiter and five of Saturn were discovered in the 17th century, they were thought of as "satellite planets" or "secondary planets" orbiting the primary planets, though in the following decades they would come to be called simply "satellites" for short, and it's not always clear whether they were still considered to be planets. The last satellites to be explicitly called "planets" in their discovery reports were Uranus' Titania and Oberon in 1787,^[33] though references to "secondary planets" can be found even after that.^[34]

19th century

In the first decade of the 19th century, four new planets were discovered: Ceres (in 1801), Pallas (in 1802), Juno (in 1804), and Vesta (in 1807). However, it soon became apparent that they were rather different from previously known planets: they shared the same general region of space, between Mars and Jupiter (the asteroid belt), with sometimes overlapping orbits, where only one planet had been expected, and they were much much smaller; indeed, it was suspected that they might be shards of a larger planet that had broken up. They were called "asteroid" because even in the largest telescopes they resembled stars, without a resolvable disk.^[35]

The situation was stable for four decades, but in the mid-1840s several additional asteroids were discovered (Astraea in 1845, Hebe in 1847, Iris in 1847, Flora in 1848, Metis in 1848, and Hygeia in 1849), and soon new "planets" were discovered every year. As a result, and although they would continue to be called "planets" into the 21st century, astronomers began tabulating the asteroids (minor planets) separately from the major planets, and assigning them numbers instead of abstract planetary symbols.^[35] It was also believed in the late 19th century that there might be another planet inside Mercury's orbit.^[36]

20th century

Pluto was discovered in 1930. After initial observations led to the belief that it was larger than Earth,^[37] the object was immediately accepted as the ninth major planet. Further monitoring found the body was actually much smaller: in 1936, Ray Lyttleton suggested that Pluto may be an escaped satellite of Neptune,^[38] and Fred Whipple suggested in 1964 that Pluto may be a comet.^[39] As it was still larger than all known asteroids and the population of dwarf planets and other trans-Neptunian objects was not well observed,^[40] it kept its status until 2006.^[41]

In 1992, astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar, PSR B1257+12.^[42] 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).^[43]

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 theoretical ability to fuse deuterium, a heavier isotope of hydrogen. Although objects more massive than 75 times that of Jupiter fuse simple hydrogen, objects of 13 Jupiter masses can fuse deuterium. Deuterium is quite rare, constituting less than 0.0026% of the hydrogen in the galaxy, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.^[44]

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.^[45]

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.^[45]

Some of them, such as Quaoar, Sedna, Eris, and Haumea^[46] were heralded in the popular press as the tenth planet. The announcement of Eris in 2005, an object 27% more massive than Pluto, created the impetus for an official definition of a planet.^[45]

To acknowledge the problem, the IAU set about creating the definition of planet, and produced one in August 2006. Their definition 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).^[47]

This definition has not been universally accepted. Dwarf planets had been proposed as a category of small planet (as opposed to planetoids as sub-planetary objects), and planetary geologists continue to treat them as planets despite the IAU definition.^[48] The number of dwarf planets even among known objects is not certain, but there is general consensus on Ceres in the asteroid belt and on at least eight trans-Neptunians: Quaoar, Sedna, Orcus, Pluto, Haumea, Eris, Makemake, and Gonggong.^[49] Planetary geologists also typically include the nineteen known planetary-mass moons as "satellite planets", including Earth's Moon, like the early modern astronomers.^[50] Some go even further and include relatively large, geologically evolved bodies that are nonetheless not very round today, such as Pallas and Vesta, though not all planetary geologists do so.^[3]

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^[51]) 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]
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.^[2]
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).^[2]

This working definition was amended by the IAU's Commission F2: Exoplanets and the Solar System in August 2018.^[52] The official working definition of an exoplanet is now as follows:

• Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the L4/L5 instability (M/M_central < 2/(25+√621) are "planets" (no matter how they formed).
• The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.^[52]

The IAU noted that this definition could be expected to evolve as knowledge improves.^[52]

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.^[53] 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.^[54]

One study suggests that objects above 10 M_Jup formed through gravitational instability and should not be thought of as planets.^[55]

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 M_J 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 M_J.^[56] 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.^[57] As of 2011 the Extrasolar Planets Encyclopaedia included objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around 13 M_Jup in the observed mass spectrum reinforces the choice to forget this mass limit".^[58] As of 2016 this limit was increased to 60 Jupiter masses^[59] based on a study of mass–density relationships.^[60] 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."^[61] The NASA Exoplanet Archive includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses.^[62]

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.^[63][64]

In close binary star systems one of the stars can lose mass to a heavier companion. Accretion-powered pulsars may drive mass loss. The shrinking star can then become a planetary-mass object. An example is a Jupiter-mass object orbiting the pulsar PSR J1719-1438.^[65] These shrunken white dwarfs may become a helium planet or carbon planet.^[66]

A 2016 study suggests that when classified by mass and radius, brown dwarfs are indistinguishable from high-mass planets, and that a change happens only with the onset of hydrogen burning at about 0.080 ± 0.008 M_☉, when the object becomes a red dwarf.^[67]

Several computer simulations of stellar and planetary system formation have suggested that some objects of planetary mass would be ejected into interstellar space.^[68] Such objects are typically called rogue planets or free-floating planets.^[69] Sub-brown dwarfs might be considered as rogue planets, or they might be considered as planetary-mass brown dwarfs.^[70] Rogue planets in stellar clusters have similar velocities to the stars and so can be recaptured. They are typically captured into wide orbits between 100 and 10⁵ AU. The capture efficiency decreases with increasing cluster volume, and for a given cluster size it increases with the host/primary mass. It is almost independent of the planetary mass. Single and multiple planets could be captured into arbitrary unaligned orbits, non-coplanar with each other or with the stellar host spin, or pre-existing planetary system.^[71]

2006 IAU definition of planet

The matter of the lower limit was addressed during the 2006 meeting of the IAU's General Assembly. After much debate and one failed proposal, a large majority of those remaining at the meeting voted to pass a resolution. The 2006 resolution defines planets within the Solar System as follows:^[1]

A "planet" [1] is a celestial body inside the Solar System 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.^[72] After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.^[73]

This definition is based in theories of planetary formation, in which planetary embryos initially clear their orbital neighborhood of other smaller objects. As described by astronomer Steven Soter:

The end product of secondary disk accretion is a small number of relatively large bodies (planets) in either non-intersecting or resonant orbits, which prevent collisions between them. Minor planets and comets, including KBOs [Kuiper belt objects], differ from planets in that they can collide with each other and with planets.^[74]

The 2006 IAU definition presents some challenges for exoplanets because the language is specific to the Solar System and the criteria of roundness and orbital zone clearance are not presently observable for exoplanets.^[75]

Margot's criterion

Astronomer Jean-Luc Margot proposed a mathematical criterion that determines whether an object can clear its orbit during the lifetime of its host star, based on the mass of the planet, its semimajor axis, and the mass of its host star.^[76][77] The formula produces a value^[a] called π that is greater than 1 for planets. The eight known planets and all known exoplanets have π values above 100, while Ceres, Pluto, and Eris have π values of 0.1, or less. Objects with π values of 1 or more are also expected to be approximately spherical, so that objects that fulfill the orbital zone clearance requirement automatically fulfill the roundness requirement.^[78]

Geophysical definitions

The IAU definition is not fully accepted by all astronomers and planetary scientists. Planetary scientists are often interested in planetary geology rather than dynamics: a celestial body may have a dynamic (planetary) geology at approximately the mass required for its mantle to become plastic under its own weight (hydrostatic equilibrium), which results in the body acquiring a round shape. This is adopted as the hallmark of planethood by geophysical definitions, for example:^[79]

a substellar-mass body that has never undergone nuclear fusion and has enough gravitation to be round due to hydrostatic equilibrium, regardless of its orbital parameters.^[80]

In the Solar System, this mass is generally less than the mass required for a body to clear its orbit, and thus some objects that are considered "planets" under geophysical definitions are not considered as such under the IAU definition, such as Ceres and Pluto.^[3] Proponents of such definitions often argue that location should not matter and that planethood should be defined by the intrinsic properties of an object.^[3]

Geophysical definitions also often do not require planets to orbit stars, so that round satellites such as our moon or Jupiter's Galilean moons are also considered planets.^[3] They are then sometimes called "satellite planets".^[81]

Some other words have been used for the bodies meeting geophysical definitions of "planet", such as "planetary-mass object", "planemo",^[82] or "world".^[83]

The names for the planets in the Western world are derived from the naming practices of the Romans, which ultimately derive from those of the Greeks and the Babylonians. In ancient Greece, the two great luminaries the Sun and the Moon were called Helios and Selene, two ancient Titanic deities; the slowest planet (Saturn) was called Phainon, the shiner; followed by Phaethon (Jupiter), "bright"; the red planet (Mars) was known as Pyroeis, the "fiery"; the brightest (Venus) was known as Phosphoros, the light bringer; and the fleeting final planet (Mercury) was called Stilbon, the gleamer. The Greeks also assigned each planet to one among their pantheon of gods, the Olympians and the earlier Titans:^[14]

• Helios and Selene were the names of both planets and gods, both of them Titans (later supplanted by Olympians Apollo and Artemis);
• Phainon was sacred to Cronus, the Titan who fathered the Olympians;
• Phaethon was sacred to Zeus, Cronus's son who deposed him as king;
• Pyroeis was given to Ares, son of Zeus and god of war;
• Phosphoros was ruled by Aphrodite, the goddess of love; and
• Stilbon with its speedy motion, was ruled over by Hermes, messenger of the gods and god of learning and wit.^[14]

The Greek practice of grafting their gods' names onto the planets was almost certainly borrowed from the Babylonians. The Babylonians named Phosphoros [Venus] after their goddess of love, Ishtar; Pyroeis [Mars] after their god of war, Nergal, Stilbon [Saturn] after their god of wisdom Nabu, and Phaethon [Jupiter] after their chief god, Marduk.^[84] There are too many concordances between Greek and Babylonian naming conventions for them to have arisen separately.^[14] The translation was not perfect. For instance, the Babylonian Nergal was a god of war, and thus the Greeks identified him with Ares. Unlike Ares, Nergal was also god of pestilence and the underworld.^[85]

Today, most people in the western world know the planets by names derived from the Olympian pantheon of gods. Although modern Greeks still use their ancient names for the planets, other European languages, because of the influence of the Roman Empire and, later, the Catholic Church, use the Roman (Latin) names rather than the Greek ones. The Romans, who, like the Greeks, were Indo-Europeans, shared with them a common pantheon under different names but lacked the rich narrative traditions that Greek poetic culture had given their gods. During the later period of the Roman Republic, Roman writers borrowed much of the Greek narratives and applied them to their own pantheon, to the point where they became virtually indistinguishable.^[86] When the Romans studied Greek astronomy, they gave the planets their own gods' names: Mercurius (for Hermes), Venus (Aphrodite), Mars (Ares), Iuppiter (Zeus) and Saturnus (Cronus). When subsequent planets were discovered in the 18th and 19th centuries, Uranus was named for a Greek deity and Neptune for a Roman one (the counterpart of Poseidon).

Ceres, Orcus, Pluto, and Eris continued the Roman and Greek scheme; however, the other consensus dwarf planets are named after gods and goddesses from other cultures (e.g. Quaoar is named after a Tongva god). Objects beyond Neptune follow various naming conventions depending on their orbits: those in the 2:3 resonance with Neptune (the plutinos) are given names from underworld myths, while others are given names from creation myths.^[87][88]

The moons (including the planetary-mass ones) are generally given names with some association with their parent planet. The planetary-mass moons of Jupiter are named after four of Zeus' lovers (or other sexual partners); those of Saturn are named after Cronus' brothers and sisters, the Titans; those of Uranus are named after characters from Shakespeare and Pope (originally specifically from fairy mythology, befitting Uranus as god of the sky and air, but that ended with the naming of Miranda). Neptune's planetary-mass moon Triton is named after the god's son; Pluto's planetary-mass moon Charon is named after the ferryman of the dead, who carries the souls of the newly deceased to the underworld (Pluto's domain).^[89]

Some Romans, following a belief possibly originating in Mesopotamia but developed in Hellenistic Egypt, believed that the seven gods after whom the planets were named took hourly shifts in looking after affairs on Earth. The order of shifts went Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon (from the farthest to the closest planet).^[90] Therefore, the first day was started by Saturn (1st hour), second day by Sun (25th hour), followed by Moon (49th hour), Mars, Mercury, Jupiter and Venus. Because each day was named by the god that started it, this is also the order of the days of the week in the Roman calendar after the Nundinal cycle was rejected – and still preserved in many modern languages.^[91] In English, Saturday, Sunday, and Monday are straightforward translations of these Roman names. The other days were renamed after Tīw (Tuesday), Wōden (Wednesday), Þunor (Thursday), and Frīġ (Friday), the Anglo-Saxon gods considered similar or equivalent to Mars, Mercury, Jupiter, and Venus, respectively.^[92]

Earth is the only planet whose name in English is not derived from Greco-Roman mythology. Because it was only generally accepted as a planet in the 17th century,^[31] there is no tradition of naming it after a god. (The same is true, in English at least, of the Sun and the Moon, though they are no longer generally considered planets.) The name originates from the Old English word eorþe, which was the word for "ground" and "dirt" as well as the Earth itself.^[93] As with its equivalents in the other Germanic languages, it derives ultimately from the Proto-Germanic word erþō, as can be seen in the English earth, the German Erde, the Dutch aarde, and the Scandinavian jord. Many of the Romance languages retain the old Roman word terra (or some variation of it) that was used with the meaning of "dry land" as opposed to "sea".^[94] The non-Romance languages use their own native words. The Greeks retain their original name, Γή (Ge).^[95]

Non-European cultures use other planetary-naming systems. India uses a system based on the Navagraha, which incorporates the seven traditional planets (Surya for the Sun, Chandra for the Moon, Budha for Mercury, Shukra for Venus, Mangala for Mars, Bṛhaspati for Jupiter, and Shani for Saturn) and the ascending and descending lunar nodes Rahu and Ketu.^[96]

China and the countries of eastern Asia historically subject to Chinese cultural influence (such as Japan, Korea and Vietnam) use a naming system based on the five Chinese elements: water (Mercury), metal (Venus), fire (Mars), wood (Jupiter) and earth (Saturn).^[91]

In traditional Hebrew astronomy, the seven traditional planets have (for the most part) descriptive names – the Sun is חמה Ḥammah or "the hot one," the Moon is לבנה Levanah or "the white one," Venus is כוכב נוגה Kokhav Nogah or "the bright planet," Mercury is כוכב Kokhav or "the planet" (given its lack of distinguishing features), Mars is מאדים Ma'adim or "the red one," and Saturn is שבתאי Shabbatai or "the resting one" (in reference to its slow movement compared to the other visible planets).^[97] The odd one out is Jupiter, called צדק Tzedeq or "justice". Steiglitz suggests that this may be a euphemism for the original name of כוכב בעל Kokhav Ba'al or "Baal's planet", seen as idolatrous and euphemized in a similar manner to Ishbosheth from II Samuel.^[97]

In Arabic, Mercury is عُطَارِد (ʿUṭārid, cognate with Ishtar / Astarte), Venus is الزهرة (az-Zuhara, "the bright one",^[98] an epithet of the goddess Al-'Uzzá^[99]), Earth is الأرض (al-ʾArḍ, from the same root as eretz), Mars is اَلْمِرِّيخ (al-Mirrīkh, meaning "featherless arrow" due to its retrograde motion^[100]), Jupiter is المشتري (al-Muštarī, "the reliable one", from Akkadian^[101]) and Saturn is زُحَل (Zuḥal, "withdrawer"^[102]).^[103][104]

It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk. Through accretion (a process of sticky collision) dust particles in the disk steadily accumulate mass to form ever-larger bodies. Local concentrations of mass known as planetesimals form, and these accelerate the accretion process by drawing in additional material by their gravitational attraction. These concentrations become ever denser until they collapse inward under gravity to form protoplanets.^[105] After a planet reaches a mass somewhat larger than Mars' mass, it begins to accumulate an extended atmosphere,^[106] greatly increasing the capture rate of the planetesimals by means of atmospheric drag.^[107][108] Depending on the accretion history of solids and gas, a giant planet, an ice giant, or a terrestrial planet may result.^{[109][110][111]} It is thought that the regular satellites of Jupiter, Saturn, and Uranus formed in a similar way;^[112][113] however, Triton was likely captured by Neptune,^[114] and Earth's Moon^[115] and Pluto's Charon might have formed in collisions.^[116]

When the protostar has grown such that it ignites to form a star, the surviving disk is removed from the inside outward by photoevaporation, the solar wind, Poynting–Robertson drag and other effects.^[117][118] Thereafter there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb.^[119] Those objects that have become massive enough will capture most matter in their orbital neighbourhoods to become planets. Protoplanets that have avoided collisions may become natural satellites of planets through a process of gravitational capture, or remain in belts of other objects to become either dwarf planets or small bodies.^[120][121]

The energetic impacts of the smaller planetesimals (as well as radioactive decay) will heat up the growing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, developing a denser core.^[122] Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by outgassing from the mantle and from the subsequent impact of comets.^[123] (Smaller planets will lose any atmosphere they gain through various escape mechanisms.)^[124]

With the discovery and observation of planetary systems around stars other than the Sun, it is becoming possible to elaborate, revise or even replace this account. The level of metallicity—an astronomical term describing the abundance of chemical elements with an atomic number greater than 2 (helium)—is now thought to determine the likelihood that a star will have planets.^[125] Hence, it is thought that a metal-rich population I star will likely have a more substantial planetary system than a metal-poor, population II star.^[126]

According to the IAU definition, there are eight planets in the Solar System, which are in increasing distance from the Sun,^[1] Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Jupiter is the largest, at 318 Earth masses, whereas Mercury is the smallest, at 0.055 Earth masses.^[127] The planets of the Solar System can be divided into categories based on their composition. Terrestrials are similar to Earth, with bodies largely composed of rock and metal: Mercury, Venus, Earth, and Mars. Earth is the largest terrestrial planet.^[128] Giant planets are significantly more massive than the terrestrials: Jupiter, Saturn, Uranus, and Neptune.^[128] They also differ from the terrestrial planets in composition. The gas giants, Jupiter and Saturn, are primarily composed of hydrogen and helium and are the most massive planets in the Solar System. Saturn is one third as massive as Jupiter, at 95 Earth masses.^[129] The Ice giants, Uranus and Neptune, are primarily composed of low-boiling-point materials such as water, methane, and ammonia, with thick atmospheres of hydrogen and helium. They have a significantly lower mass than the gas giants (only 14 and 17 Earth masses).^[129]

The number of geophysical planets in the Solar System is unknown – previously considered to be potentially in the hundreds, but now only estimated at only the low double digits.^[130] These include the eight classical planets, as well as two more populations. Nine objects are generally agreed to be dwarf planets, with some others being disputed candidates. Dwarf planets are gravitationally rounded, but do not clear their orbits. In increasing order of average distance from the Sun, they are Ceres, Orcus, Pluto, Haumea, Quaoar, Makemake, Gonggong, Eris and Sedna.^[48]

Ceres is the largest object in the asteroid belt, between the orbits of Mars and Jupiter. The other eight all orbit beyond Neptune. Orcus, Pluto, Haumea, Quaoar, and Makemake orbit in the Kuiper belt, which is a second belt of small Solar System bodies beyond the orbit of Neptune. Gonggong and Eris orbit in the scattered disc, which is somewhat further out and, unlike the Kuiper belt, is unstable towards interactions with Neptune. Sedna is the largest known detached object, a population that never comes close enough to the Sun to interact with any of the classical planets; the origins of their orbits are still being debated. All nine are similar to terrestrial planets in having a solid surface, but they are made of ice and rock, rather than rock and metal. Moreover, all of them are smaller than Mercury, with Pluto being the largest known dwarf planet, and Eris being the most massive known.^[131][132]

There are also at least nineteen planetary-mass moons or satellite planets, i.e. moons large enough to take on ellipsoidal shapes. The nineteen generally agreed are:^[3]

• One satellite of Earth – the Moon
• Four satellites of Jupiter – Io, Europa, Ganymede, and Callisto
• Seven satellites of Saturn – Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Iapetus
• Five satellites of Uranus – Miranda, Ariel, Umbriel, Titania, and Oberon
• One satellite of Neptune – Triton
• One satellite of Pluto – Charon

The Moon, Io, and Europa have compositions similar to the terrestrial planets; the others are made of ice and rock like the dwarf planets, with Tethys being made of almost pure ice. (Europa is often considered an icy planet, though, because its surface ice layer makes it difficult to study its interior.)^[3][133] Ganymede and Titan are larger than Mercury by radius, and Callisto almost equals it, but all three are much less massive. Mimas is the smallest object generally agreed to be a geophysical planet, at about six millionths of Earth's mass, though there are many larger bodies that may not be geophysical planets (e.g. Salacia).^[48]

Planetary attributes

An exoplanet (extrasolar planet) is a planet outside the Solar System. As of 1 May 2022, there are 5,017 confirmed exoplanets in 3,694 planetary systems, with 822 systems having more than one planet.^{[134][135][136][137]} Known exoplanets range in size from gas giants about twice as large as Jupiter down to just over the size of the Moon. More than 100 of these planets are approximately the same size as Earth, nine of which orbit in the habitable zone of their star.^[138][139] In 2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets orbiting a Sun-like star, Kepler-20e^[140] and Kepler-20f.^{[141][142][143][144]} A 2012 study, analyzing gravitational microlensing data, estimates a minimum of 1.6 bound planets on average for every star in the Milky Way.^[145] As of 2013, one in five Sun-like^[c] stars is thought to have an Earth-sized^[d] planet in its habitable^[e] zone.^[146][147]

In early 1992, radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of two planets orbiting the pulsar PSR 1257+12.^[42] This discovery was confirmed, and is generally considered to be the first definitive detection of exoplanets. Because the three pulsar planets are coplanar and neutron stars are born with a kick, researchers suspect they formed from an unusual disk remnant of the supernova that produced the pulsar in a second round of planet formation, rather than being planets that formed when the progenitor stars formed or perhaps the remaining rocky cores of giant planets that survived the supernova and then decayed into their current orbits.^[148]

The first confirmed discovery of an extrasolar planet orbiting an ordinary main-sequence star occurred on 6 October 1995, when Michel Mayor and Didier Queloz of the University of Geneva announced the detection of an exoplanet around 51 Pegasi. From then until the Kepler mission most known extrasolar planets were gas giants comparable in mass to Jupiter or larger as they were more easily detected. The catalog of Kepler candidate planets consists mostly of planets the size of Neptune and smaller, down to smaller than Mercury.^[149]

There are types of planets that do not exist in the Solar System: super-Earths and mini-Neptunes, which could be rocky like Earth or a mixture of volatiles and gas like Neptune—the dividing line between the two is currently thought to occur at about twice the mass of Earth.^[67] There are hot Jupiters that orbit very close to their star and may evaporate to become chthonian planets, which are the leftover cores.

A 2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way.^[145]

On 20 December 2011, the Kepler Space Telescope team reported the discovery of the first Earth-size exoplanets, Kepler-20e^[140] and Kepler-20f,^[141] orbiting a Sun-like star, Kepler-20.^{[142][143][144]}

Around 1 in 5 Sun-like stars have an "Earth-sized"^[d] planet in the habitable^[e] zone, so the nearest would be expected to be within 12 light-years distance from Earth.^[146][150] The frequency of occurrence of such terrestrial planets is one of the variables in the Drake equation, which estimates the number of intelligent, communicating civilizations that exist in the Milky Way.^[151]

There are exoplanets that are much closer to their parent star than any planet in the Solar System is to the Sun, and there are also exoplanets that are much farther from their star. Mercury, the closest planet to the Sun at 0.4 AU, takes 88 days for an orbit, but the shortest known orbits for exoplanets take only a few hours, see Ultra-short period planet. The Kepler-11 system has five of its planets in shorter orbits than Mercury's, all of them much more massive than Mercury. Neptune is 30 AU from the Sun and takes 165 years to orbit, but there are exoplanets that are thousands of AU from their star and take more than a million years to orbit. e.g. COCONUTS-2b.

Although each planet has unique physical characteristics, a number of broad commonalities do exist among them. Some of these characteristics, such as rings or natural satellites, have only as yet been observed in planets in the Solar System, whereas others are also commonly observed in extrasolar planets.^[152]

Dynamic characteristics

Orbit

In the Solar System, all the planets orbit the Sun in the same direction as the Sun rotates (counter-clockwise as seen from above the Sun's north pole). At least one extrasolar planet, WASP-17b, has been found to orbit in the opposite direction to its star's rotation.^[153] The period of one revolution of a planet's orbit is known as its sidereal period or year.^[154] A planet's year depends on its distance from its star; the farther a planet is from its star, not only the longer the distance it must travel, but also the slower its speed, because it is less affected by its star's gravity. No planet's orbit is perfectly circular, and hence the distance of each varies over the course of its year. The closest approach to its star is called its periastron (perihelion in the Solar System), whereas its farthest separation from the star is called its apastron (aphelion). As a planet approaches periastron, its speed increases as it trades gravitational potential energy for kinetic energy, just as a falling object on Earth accelerates as it falls; as the planet reaches apastron, its speed decreases, just as an object thrown upwards on Earth slows down as it reaches the apex of its trajectory.^[155]

Each planet's orbit is delineated by a set of elements:

• The eccentricity of an orbit describes how elongated a planet's orbit is. Planets with low eccentricities have more circular orbits, whereas planets with high eccentricities have more elliptical orbits. The planets in the Solar System have very low eccentricities, and thus nearly circular orbits.^[154] Comets and Kuiper belt objects (as well as several extrasolar planets) have very high eccentricities, and thus exceedingly elliptical orbits.^[156][157]
•  

  Illustration of the semi-major axis
  The semi-major axis is the distance from a planet to the half-way point along the longest diameter of its elliptical orbit (see image). This distance is not the same as its apastron, because no planet's orbit has its star at its exact centre.^[154]
• The inclination of a planet tells how far above or below an established reference plane its orbit lies. In the Solar System, the reference plane is the plane of Earth's orbit, called the ecliptic. For extrasolar planets, the plane, known as the sky plane or plane of the sky, is the plane perpendicular to the observer's line of sight from Earth.^[158] The eight planets of the Solar System all lie very close to the ecliptic; comets and Kuiper belt objects like Pluto are at far more extreme angles to it.^[159] The points at which a planet crosses above and below its reference plane are called its ascending and descending nodes.^[154] The longitude of the ascending node is the angle between the reference plane's 0 longitude and the planet's ascending node. The argument of periapsis (or perihelion in the Solar System) is the angle between a planet's ascending node and its closest approach to its star.^[154]

Axial tilt

Planets also have varying degrees of axial tilt; they lie at an angle to the plane of their stars' equators. This causes the amount of light received by each hemisphere to vary over the course of its year; when the northern hemisphere points away from its star, the southern hemisphere points towards it, and vice versa. Each planet therefore has seasons, changes to the climate over the course of its year. The time at which each hemisphere points farthest or nearest from its star is known as its solstice. Each planet has two in the course of its orbit; when one hemisphere has its summer solstice, when its day is longest, the other has its winter solstice, when its day is shortest. The varying amount of light and heat received by each hemisphere creates annual changes in weather patterns for each half of the planet. Jupiter's axial tilt is very small, so its seasonal variation is minimal; Uranus, on the other hand, has an axial tilt so extreme it is virtually on its side, which means that its hemispheres are either perpetually in sunlight or perpetually in darkness around the time of its solstices.^[160] Among extrasolar planets, axial tilts are not known for certain, though most hot Jupiters are believed to have negligible to no axial tilt as a result of their proximity to their stars.^[161]

Rotation

The planets rotate around invisible axes through their centres. A planet's rotation period is known as a stellar day. Most of the planets in the Solar System rotate in the same direction as they orbit the Sun, which is counter-clockwise as seen from above the Sun's north pole, the exceptions being Venus^[162] and Uranus,^[163] which rotate clockwise, though Uranus's extreme axial tilt means there are differing conventions on which of its poles is "north", and therefore whether it is rotating clockwise or anti-clockwise.^[164] Regardless of which convention is used, Uranus has a retrograde rotation relative to its orbit.^[163]

The rotation of a planet can be induced by several factors during formation. A net angular momentum can be induced by the individual angular momentum contributions of accreted objects. The accretion of gas by the giant planets can also contribute to the angular momentum. Finally, during the last stages of planet building, a stochastic process of protoplanetary accretion can randomly alter the spin axis of the planet.^[165] There is great variation in the length of day between the planets, with Venus taking 243 days to rotate, and the giant planets only a few hours.^[166] The rotational periods of extrasolar planets are not known. However, for "hot" Jupiters, their proximity to their stars means that they are tidally locked (i.e., their orbits are in sync with their rotations). This means, they always show one face to their stars, with one side in perpetual day, the other in perpetual night.^[167]

Orbital clearing

The defining dynamic characteristic of a planet, according to the IAU definition, is that it has cleared its neighborhood. A planet that has cleared its neighborhood has accumulated enough mass to gather up or sweep away all the planetesimals in its orbit. In effect, it orbits its star in isolation, as opposed to sharing its orbit with a multitude of similar-sized objects. This characteristic was mandated as part of the IAU's official definition of a planet in August 2006. This criterion excludes such planetary bodies as Pluto, Eris and Ceres from full-fledged planethood, making them instead dwarf planets.^[1] Although to date this criterion only applies to the Solar System, a number of young extrasolar systems have been found in which evidence suggests orbital clearing is taking place within their circumstellar discs.^[168]

Physical characteristics

Size and shape

A planet's size is defined at least by an average radius (e.g., Earth radius, Jupiter radius, etc.); polar and equatorial radii of a spheroid or more general triaxial ellipsoidal shapes are often estimated (e.g., reference ellipsoid). Derived quantities include the flattening, surface area, and volume. Knowing further the rotation rate and mass, allows the calculation of normal gravity.^[169]

Mass

A planet's defining physical characteristic is that it is massive enough for the force of its own gravity to dominate over the electromagnetic forces binding its physical structure, leading to a state of hydrostatic equilibrium. This effectively means that all planets are spherical or spheroidal. Up to a certain mass, an object can be irregular in shape, but beyond that point, which varies depending on the chemical makeup of the object, gravity begins to pull an object towards its own centre of mass until the object collapses into a sphere.^[170]

Mass is also the prime attribute by which planets are distinguished from stars. While the lower stellar mass limit is estimated to be around 75 times that of Jupiter (M_J), the upper planetary mass limit for planethood is only roughly 13 M_J for objects with solar-type isotopic abundance, beyond which it achieves conditions suitable for nuclear fusion. Other than the Sun, no objects of such mass exist in the Solar System; but there are exoplanets of this size. The 13 M_J limit is not universally agreed upon and the Extrasolar Planets Encyclopaedia includes objects up to 60 M_J,^[59] and the Exoplanet Data Explorer up to 24 M_J.^[171]

The smallest known exoplanet with an accurately known mass is PSR B1257+12A, one of the first extrasolar planets discovered, which was found in 1992 in orbit around a pulsar. Its mass is roughly half that of the planet Mercury.^[139] Even smaller is WD 1145+017 b, orbiting a white dwarf; its mass is roughly that of the dwarf planet Haumea. That said, this object might not qualify as a planet under all definitions. The smallest known planet orbiting a main-sequence star other than the Sun is Kepler-37b, with a mass (and radius) that is probably slightly higher than that of the Moon.^[149]

Internal differentiation

Every planet began its existence in an entirely fluid state; in early formation, the denser, heavier materials sank to the centre, leaving the lighter materials near the surface. Each therefore has a differentiated interior consisting of a dense planetary core surrounded by a mantle that either is or was a fluid. The terrestrial planets are sealed within hard crusts,^[172] but in the giant planets the mantle simply blends into the upper cloud layers. The terrestrial planets have cores of elements such as iron and nickel, and mantles of silicates. Jupiter and Saturn are believed to have cores of rock and metal surrounded by mantles of metallic hydrogen.^[173] Uranus and Neptune, which are smaller, have rocky cores surrounded by mantles of water, ammonia, methane and other ices.^[174] The fluid action within these planets' cores creates a geodynamo that generates a magnetic field.^[172]

Atmosphere

All of the Solar System planets except Mercury^[175] have substantial atmospheres because their gravity is strong enough to keep gases close to the surface. The larger giant planets are massive enough to keep large amounts of the light gases hydrogen and helium, whereas the smaller planets lose these gases into space.^[176] The composition of Earth's atmosphere is different from the other planets because the various life processes that have transpired on the planet have introduced free molecular oxygen.^[177]

Planetary atmospheres are affected by the varying insolation or internal energy, leading to the formation of dynamic weather systems such as hurricanes (on Earth), planet-wide dust storms (on Mars), a greater-than-Earth-sized anticyclone on Jupiter (called the Great Red Spot), and holes in the atmosphere (on Neptune).^[160] At least one extrasolar planet, HD 189733 b, has been claimed to have such a weather system, similar to the Great Red Spot but twice as large.^[178]

Hot Jupiters, due to their extreme proximities to their host stars, have been shown to be losing their atmospheres into space due to stellar radiation, much like the tails of comets.^[179][180] These planets may have vast differences in temperature between their day and night sides that produce supersonic winds,^[181] although the day and night sides of HD 189733 b appear to have very similar temperatures, indicating that that planet's atmosphere effectively redistributes the star's energy around the planet.^[178]

Magnetosphere

One important characteristic of the planets is their intrinsic magnetic moments, which in turn give rise to magnetospheres. The presence of a magnetic field indicates that the planet is still geologically alive. In other words, magnetized planets have flows of electrically conducting material in their interiors, which generate their magnetic fields. These fields significantly change the interaction of the planet and solar wind. A magnetized planet creates a cavity in the solar wind around itself called the magnetosphere, which the wind cannot penetrate. The magnetosphere can be much larger than the planet itself. In contrast, non-magnetized planets have only small magnetospheres induced by interaction of the ionosphere with the solar wind, which cannot effectively protect the planet.^[182]

Of the eight planets in the Solar System, only Venus and Mars lack such a magnetic field.^[182] In addition, the moon of Jupiter Ganymede also has one. Of the magnetized planets the magnetic field of Mercury is the weakest, and is barely able to deflect the solar wind. Ganymede's magnetic field is several times larger, and Jupiter's is the strongest in the Solar System (so strong in fact that it poses a serious health risk to future crewed missions to all its moons inward of Callisto). The magnetic fields of the other giant planets are roughly similar in strength to that of Earth, but their magnetic moments are significantly larger. The magnetic fields of Uranus and Neptune are strongly tilted relative the rotational axis and displaced from the centre of the planet.^[182]

In 2004, a team of astronomers in Hawaii observed an extrasolar planet around the star HD 179949, which appeared to be creating a sunspot on the surface of its parent star. The team hypothesized that the planet's magnetosphere was transferring energy onto the star's surface, increasing its already high 7,760 °C temperature by an additional 400 °C.^[183]

Secondary characterisitcs

Several planets or dwarf planets in the Solar System (such as Neptune and Pluto) have orbital periods that are in resonance with each other or with smaller bodies. This is also common in satellite systems (e.g. the resonance between Io, Europa, and Ganymede around Jupiter, or between Enceladus and Dione around Saturn). All except Mercury and Venus have natural satellites, often called "moons". Earth has one, Mars has two, and the giant planets have numerous moons in complex planetary-type systems. Many moons of the giant planets have features similar to those on the terrestrial planets and dwarf planets, and some have been studied as possible abodes of life (especially Europa).^{[184][185][186]}

The four giant planets are also orbited by planetary rings of varying size and complexity. The rings are composed primarily of dust or particulate matter, but can host tiny 'moonlets' whose gravity shapes and maintains their structure. Although the origins of planetary rings is not precisely known, they are believed to be the result of natural satellites that fell below their parent planet's Roche limit and were torn apart by tidal forces.^[187][188]

No secondary characteristics have been observed around extrasolar planets. The sub-brown dwarf Cha 110913-773444, which has been described as a rogue planet, is believed to be orbited by a tiny protoplanetary disc^[189] and the sub-brown dwarf OTS 44 was shown to be surrounded by a substantial protoplanetary disk of at least 10 Earth masses.^[190]

• International Astronomical Union website
• Photojournal NASA
• NASA Planet Quest – Exoplanet Exploration
• Illustration comparing the sizes of the planets with each other, the Sun, and other stars
• "IAU Press Releases since 1999 "The status of Pluto: A Clarification"". Archived from the original on 2007-12-14.
• Planetary Science Research Discoveries (educational site with illustrated articles)
• The Planets, BBC Radio 4 discussion with Paul Murdin, Hugh Jones & Carolin Crawford (In Our Time, May 27, 2004)