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.
classical planets, being visible to the naked eye, have been known since ancient times and have had a significant impact on
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.
 Ancient Greeks called these lights
ἀστέρες (planētes asteres, "wandering stars") or simply
πλανῆται (planētai, "wanderers"),
 from which today's word "planet" was derived.
Babylon, and indeed all pre-modern civilizations,
 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
 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.
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.
Babylonian astrologers also laid the foundations of what would eventually become
Enuma anu enlil, written during the
Neo-Assyrian period in the 7th century BC,
 comprises a list of
omens and their relationships with various celestial phenomena including the motions of the planets.
Mercury, and the outer planets
Saturn were all identified by
Babylonian astronomers. These would remain the only known planets until the invention of the
telescope in early modern times.
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.
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
 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
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.
 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.
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
 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.
Nilakantha Somayaji of the
Kerala school of astronomy and mathematics, in his
Tantrasangraha, revised Aryabhata's model.
 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.
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.
 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
Qotb al-Din Shirazi in the 13th century.
 Ibn Bajjah could not have observed a transit of Venus, because none occurred in his lifetime.
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
Kepler gained sway.
Thus, Earth became included in the list of planets,
 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.
 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.
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
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.
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,
 the object was immediately accepted as the ninth planet. Further monitoring found the body was actually much smaller: in 1936,
Ray Lyttleton suggested that Pluto may be an escaped satellite of
Fred Whipple suggested in 1964 that Pluto may be a comet.
 As it was still larger than all known asteroids and seemingly did not exist within a larger population,
 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
 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 (
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.
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
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
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 (
Pluto and Eris).
There is no official definition of
. 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:
- 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
) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass and size required for an extrasolar object to be considered a planet should be the same as that used in the Solar System.
- Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "
brown dwarfs", no matter how they formed or where they are located.
- Free-floating objects in young
star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).
This working definition has since been widely used by astronomers when publishing discoveries of exoplanets in
 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,
 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
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.
 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.
 The amount of deuterium fused depends not only on mass but also on the composition of the object, on the amount of
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."
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."
NASA Exoplanet Archive includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses.
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.
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, 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:
A "planet"  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.
 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.
 After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.
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
- "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.
 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.
Objects formerly considered planets
The table below lists
Solar System bodies once considered to be planets.
classical planets (Ancient Greek πλανῆται, wanderers) in
classical antiquity and
medieval Europe, in accordance with the now-disproved
||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
Medici family. They were known as
Saturn's larger moons, discovered by
Christiaan Huygens and
Giovanni Domenico Cassini. As with Jupiter's major moons, they were known as secondary planets.
||Regarded as planets from their discoveries between 1801 and 1807 until they were reclassified as asteroids during the 1850s.
Ceres was subsequently classified as a
dwarf planet in 2006.
||Dwarf planet and asteroid
||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.
||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.
 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.