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{{Quote box
| title = The [[International Astronomical Union]]'s<br />definition of a planet in the [[Solar System]]
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Object is in [[orbit]] around the Sun|
Object has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a [[hydrostatic equilibrium]] (nearly round) shape|
Object has [[cleared the neighbourhood]] around its orbit}}

Source: {{cite news|url=http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf|title=IAU 2006 General Assembly: Resolutions 5 and 6|date=August 24, 2006|publisher=IAU|access-date=June 23, 2009}}
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The definition of the term ''[[planet]]'' has changed several times since the word was coined by the [[ancient Greeks]]. [[Greek astronomy|Greek astronomers]] employed the term {{lang|grc|ἀστέρες πλανῆται}} ({{transl|grc|asteres planetai}}), 'wandering stars', for star-like objects which apparently moved over the sky. Over the millennia, the term has included a variety of different [[astronomical object|celestial bodies]], from the [[Sun]] and the [[Moon]] to [[natural satellite|satellites]] and [[asteroid]]s.

In modern astronomy, there are two primary conceptions of a 'planet'. Disregarding the often inconsistent technical details, they are whether an astronomical body ''dynamically dominates'' its region (that is, whether it controls the fate of other smaller bodies in its vicinity) or whether it is in ''hydrostatic equilibrium'' (in practice, this is always taken rather loosely to mean whether it has become gravitationally rounded and compacted). These may be characterized as [[IAU definition of planet|the dynamical dominance definition]] and [[geophysical definition of planet|the geophysical definition]].

The issue of a clear [[definition]] for ''[[planet]]'' came to a head in January 2005 with the discovery of the [[trans-Neptunian object]] [[Eris (dwarf planet)|Eris]], a body more massive than the smallest then-accepted planet, [[Pluto]].<ref name="NYT-20220118">{{cite news |last=Chang |first=Kenneth |title=Quiz - Is Pluto A Planet? - Who doesn't love Pluto? It shares a name with the Roman god of the underworld and a Disney dog. But is it a planet? - Interactive |url=https://www.nytimes.com/interactive/2022/science/is-pluto-a-planet.html  |date=18 January 2022 |work=[[The New York Times]] |accessdate=18 January 2022 }}</ref> In its August 2006 response, the [[International Astronomical Union]] (IAU), recognised by astronomers as the world body responsible for resolving issues of [[nomenclature]], released [[2006 definition of planet|its decision]] on the matter during a meeting in [[Prague]]. This definition, which applies only to the Solar System (though exoplanets had been addressed in 2003), states that a planet is a body that orbits the Sun, is massive enough for its [[hydrostatic equilibrium|own gravity to make it round]], and has "[[Clearing the neighbourhood|cleared its neighbourhood]]" of smaller objects approaching its orbit. Pluto fulfills the first two of these criteria, but not the third, and therefore does not qualify as a planet under this formalized definition. The IAU's decision has not resolved all controversies, and while many astronomers have accepted it, some planetary scientists have rejected it outright, proposing a geophysical or similar definition instead.

== History ==
{{see also|Geocentric model|Heliocentrism|Celestial spheres|Classical planet}}

=== Planets in antiquity ===
[[File:Apparent retrograde motion of Mars in 2003.gif|thumb|The motion of 'lights' moving across the background of stars is the basis of the classical definition of planets: wandering stars.]]
While knowledge of the planets predates history and is common to most civilizations, the word ''planet'' dates back to [[ancient Greece]]. Most Greeks believed the Earth to be stationary and at the center of the universe in accordance with the [[geocentric model]] and that the objects in the sky, and indeed the sky itself, revolved around it (an exception was [[Aristarchus of Samos]], who put forward an early version of [[heliocentrism]]). Greek astronomers employed the term {{lang|grc|ἀστέρες πλανῆται}} ({{transl|grc|asteres planetai}}), 'wandering stars',<ref>{{cite web|url=http://www.m-w.com/dictionary/planet|title=Definition of planet|publisher=Merriam-Webster OnLine|access-date=July 23, 2007}}</ref><ref>{{cite web|url=http://www.wordsources.info/words-mod-planets.html|title=Words For Our Modern Age: Especially words derived from Latin and Greek sources|publisher=Wordsources.info|access-date=July 23, 2007}}</ref> to describe those starlike lights in the heavens that moved over the course of the year, in contrast to the {{lang|grc|ἀστέρες ἀπλανεῖς}} ({{transl|grc|asteres aplaneis}}), the '[[fixed stars]]', which stayed motionless relative to one another. The five bodies currently called "planets" that were known to the Greeks were those visible to the naked eye: [[Mercury (planet)|Mercury]], [[Venus]], [[Mars]], [[Jupiter]], and [[Saturn]].

Graeco-Roman [[Timeline of cosmology|cosmology]] commonly considered seven planets, with the Sun and the Moon counted among them (as is the case in modern [[Planets in astrology|astrology]]); however, there is some ambiguity on that point, as many ancient astronomers distinguished the five star-like planets from the Sun and Moon. As the 19th-century German naturalist [[Alexander von Humboldt]] noted in his work ''[[Cosmos (Humboldt)|Cosmos]]'',

<blockquote>Of the seven cosmical bodies which, by their continually varying relative positions and distances apart, have ever since the remotest antiquity been distinguished from the "unwandering orbs" of the heaven of the "fixed stars", which to all sensible appearance preserve their relative positions and distances unchanged, five only—Mercury, Venus, Mars, Jupiter and Saturn—wear the appearance of stars—"''cinque stellas errantes''"—while the Sun and Moon, from the size of their disks, their importance to man, and the place assigned to them in mythological systems, were classed apart.<ref>{{cite book |title=Cosmos: A Sketch of a Physical Description of the Universe |author=Alexander von Humboldt |year=1849 |work=digitised 2006 |url=https://archive.org/details/cosmosasketchap00humbgoog |page=[https://archive.org/details/cosmosasketchap00humbgoog/page/n337 297] |access-date=2007-07-23 |publisher=H.G. Bohn |isbn=978-0-8018-5503-0}}</ref></blockquote>
In his ''[[Timaeus (dialogue)|Timaeus]]'', written in roughly 360&nbsp;[[BCE]], [[Plato]] mentions, "the Sun and Moon and five other stars, which are called the planets".<ref>{{cite web|title=Timaeus by Plato|work=The Internet Classics|url=http://classics.mit.edu/Plato/timaeus.html|access-date= February 22, 2007}}</ref> His student [[Aristotle]] makes a similar distinction in his ''[[On the Heavens]]'': "The movements of the sun and moon are fewer than those of some of the planets".<ref>{{cite web|title=On the Heavens by Aristotle, Translated by J. L. Stocks, volume II|work=University of Adelaide Library|url=https://ebooks.adelaide.edu.au/a/aristotle/heavens/book2.html|year=2004|access-date=February 24, 2007|archive-date=August 23, 2008|archive-url=https://web.archive.org/web/20080823061709/http://ebooks.adelaide.edu.au/a/aristotle/heavens/book2.html|url-status=dead}}</ref> In his ''Phaenomena'', which set to verse an astronomical treatise written by the philosopher [[Eudoxus of Cnidus|Eudoxus]] in roughly 350&nbsp;BCE,<ref>{{cite web|title=Phaenomena Book I&nbsp;— ARATUS of SOLI|url=http://www.geocities.com/astrologysources/classicalgreece/phaenomena/index.htm|access-date=June 16, 2007|archive-url = https://web.archive.org/web/20050901044449/http://www.geocities.com/astrologysources/classicalgreece/phaenomena/index.htm |archive-date = September 1, 2005|url-status=dead}}</ref> the poet [[Aratus]] describes "those five other orbs, that intermingle with [the constellations] and wheel wandering on every side of the twelve figures of the Zodiac."<ref>{{cite web|author=Aratus |title=Phaemonema|translator=A. W. & G. R. Mair |work=theoi.com|url=http://www.theoi.com/Text/AratusPhaenomena.html|access-date=June 16, 2007}}</ref>

In his ''[[Almagest]]'' written in the 2nd century, [[Claudius Ptolemy|Ptolemy]] refers to "the Sun, Moon and five planets."<ref>{{cite book|title=The Almagest |author=Ptolemy|translator=R. Gatesby Taliaterro|publisher= University of Chicago Press|year= 1952|page=270}}</ref> [[Gaius Julius Hyginus|Hyginus]] explicitly mentions "the five stars which many have called wandering, and which the Greeks call Planeta."<ref name=planeta>{{cite web|title=Astra Planeta|author=theoi.com|url=http://www.theoi.com/Titan/AstraPlaneta.html|access-date=February 25, 2007}}</ref> [[Marcus Manilius]], a Latin writer who lived during the time of [[Caesar Augustus]] and whose poem ''Astronomica'' is considered one of the principal texts for modern [[astrology]], says, "Now the [[dodecatemory]] is divided into five parts, for so many are the stars called wanderers which with passing brightness shine in heaven."<ref>{{cite book|author=Marcus Manilius |translator=G. P. Goold |title=Astronomica|publisher=Harvard University Press|year=1977|page=141}}</ref>

The single view of the seven planets is found in [[Cicero]]'s ''[[Dream of Scipio]]'', written sometime around 53&nbsp;BCE, where the spirit of [[Scipio Africanus]] proclaims, "Seven of these spheres contain the planets, one planet in each sphere, which all move contrary to the movement of heaven."<ref>{{cite web|title=The Dream of Scipio|author=Cicero|work=Roman Philosophy |translator=Richard Hooker|url=http://www.wsu.edu/~dee/ROME/SCIPIO.HTM|year=1996|access-date=June 16, 2007|archive-url=https://web.archive.org/web/20070703203835/http://www.wsu.edu/~dee/ROME/SCIPIO.HTM |archive-date=July 3, 2007}}</ref> In his ''[[Natural History (Pliny)|Natural History]]'', written in 77&nbsp;CE, [[Pliny the Elder]] refers to "the seven stars, which owing to their motion we call planets, though no stars wander less than they do."<ref>{{cite book|title=Natural History vol 1|author=IH Rackham|publisher=William Heinemann Ltd.|year=1938|page= 177, viii}}</ref> [[Nonnus]], the 5th century Greek poet, says in his ''[[Dionysiaca]]'', "I have oracles of history on seven tablets, and the tablets bear the names of the seven planets."<ref name=planeta />

=== Planets in the Middle Ages ===
[[File:Copernican heliocentrism diagram-2.jpg|thumb|Sketch of the heliocentric model of the Solar System by Copernicus]]
Medieval and Renaissance writers generally accepted the idea of seven planets. The standard medieval introduction to astronomy, [[Sacrobosco]]'s ''[[De sphaera mundi|De Sphaera]]'', includes the Sun and Moon among the planets,<ref>Sacrobosco, "On the Sphere", in Edward Grant, ed. ''A Source Book in Medieval Science,'' (Cambridge: Harvard University Press, 1974), p. 450.  "every planet except the sun has an epicycle."</ref> the more advanced ''Theorica planetarum'' presents the "theory of the seven planets,"<ref>Anonymous, "The Theory of the Planets," in Edward Grant, ed. ''A Source Book in Medieval Science,'' (Cambridge: Harvard University Press, 1974), p. 452.</ref>  while the instructions to the ''[[Alfonsine Tables]]'' show how "to find by means of tables the mean ''motuses'' of the sun, moon, and the rest of the planets."<ref>[[John of Saxony (astronomer)|John of Saxony]], "Extracts from the Alfonsine Tables and Rules for their use", in Edward Grant, ed. ''A Source Book in Medieval Science,'' (Cambridge: Harvard University Press, 1974), p. 466.</ref>  In his ''[[Confessio Amantis]]'', 14th-century poet [[John Gower]], referring to the planets' connection with the craft of [[Classical planets in Western alchemy|alchemy]], writes, "Of the planetes ben begonne/The gold is tilted to the Sonne/The Mone of Selver hath his part...", indicating that the Sun and the Moon were planets.<ref>{{cite journal|author=P. Heather|title=The Seven Planets|journal=Folklore|year=1943|pages=338–361|doi=10.1080/0015587x.1943.9717687|volume=54|issue=3}}</ref> Even [[Nicolaus Copernicus]], who rejected the geocentric model, was ambivalent concerning whether the Sun and Moon were planets. In his ''[[De Revolutionibus]]'', Copernicus clearly separates "the sun, moon, planets and stars";<ref name=koper>{{cite web|title=The text of Nicholas Copernicus' De Revolutionibus (On the Revolutions), 1543 C.E.|author=Edward Rosen (trans.)|work=Calendars Through the Ages|url=http://webexhibits.org/calendars/year-text-Copernicus.html|access-date=February 28, 2007}}</ref> however, in his Dedication of the work to Pope Paul III, Copernicus refers to, "the motion of the sun and the moon... and of the five other planets."<ref>{{cite web|title=Dedication of the Revolutions of the Heavenly Bodies to Pope Paul III|author=Nicolaus Copernicus|work=The Harvard Classics. 1909–14|url=http://www.bartleby.com/39/12.html|access-date=February 23, 2007}}</ref>

=== Earth ===
Eventually, when Copernicus's [[heliocentric model]] was accepted over the [[geocentric]], Earth was placed among the planets and the Sun and Moon were reclassified, necessitating a conceptual revolution in the understanding of planets. As the [[historian of science]] [[Thomas Kuhn]] noted in his book, ''[[The Structure of Scientific Revolutions]]'':<ref>Thomas S. Kuhn, (1962) ''The Structure of Scientific Revolutions'', 1st. ed., (Chicago: University of Chicago Press), pp. 115, 128–9.</ref>

<blockquote>The Copernicans who denied its traditional title 'planet' to the sun ... were changing the meaning of 'planet' so that it would continue to make useful distinctions in a world where all celestial bodies ... were seen differently from the way they had been seen before... Looking at the moon, the convert to Copernicanism ... says, 'I once took the moon to be (or saw the moon as) a planet, but I was mistaken.'</blockquote>

Copernicus obliquely refers to Earth as a planet in ''De Revolutionibus'' when he says, "Having thus assumed the motions which I ascribe to the Earth later on in the volume, by long and intense study I finally found that if the motions of the other planets are correlated with the orbiting of the earth..."<ref name=koper/> [[Galileo Galilei|Galileo]] also asserts that Earth is a planet in the ''[[Dialogue Concerning the Two Chief World Systems]]'': "[T]he Earth, no less than the moon or any other planet, is to be numbered among the natural bodies that move circularly."<ref name=gal>{{cite web|title=Dialogue Concerning the Two Chief World Systems|work=Calendars Through the Ages|url=http://www.webexhibits.org/calendars/year-text-Galileo.html|access-date=June 14, 2008}}</ref>

=== Modern planets ===
[[File:Uranuslauf November Dezember2022.gif|thumb|Motion of Uranus across the sky as seen by two images taken 25 days apart]]
In 1781, the astronomer [[William Herschel]] was searching the sky for elusive [[stellar parallax]]es, when he observed what he termed a [[comet]] in the constellation of [[Taurus (constellation)|Taurus]]. Unlike stars, which remained mere points of light even under high magnification, this object's size increased in proportion to the power used. That this strange object might have been a planet simply did not occur to Herschel; the five planets beyond Earth had been part of humanity's conception of the universe since antiquity. As the asteroids had yet to be discovered, comets were the only moving objects one expected to find in a telescope.<ref name=ken/> However, unlike a comet, this object's orbit was nearly circular and within the ecliptic plane. Before Herschel announced his discovery of his "comet", his colleague, British [[Astronomer Royal]] [[Nevil Maskelyne]], wrote to him, saying, "I don't know what to call it. It is as likely to be a regular planet moving in an orbit nearly circular to the sun as a Comet moving in a very eccentric ellipsis. I have not yet seen any [[Coma (cometary)|coma]] or tail to it."<ref>{{cite book|title=William Herschel: Astronomer and Musician of 19 New King Street, Bath|author=Patrick Moore|publisher=PME Erwood|year=1981|page=8|isbn=978-0-907322-06-1}}</ref> The "comet" was also very far away, too far away for a mere comet to resolve itself. Eventually it was recognised as the seventh planet and named [[Uranus]] after the father of Saturn.

Gravitationally induced irregularities in Uranus's observed orbit led eventually to the discovery of [[Neptune]] in 1846, and presumed irregularities in Neptune's orbit subsequently led to a search which did not find the perturbing object (it was later found to be a mathematical artefact caused by an overestimation of Neptune's mass) but did find [[Pluto]] in 1930. Initially believed to be roughly the mass of the Earth, observation gradually shrank Pluto's estimated mass until it was revealed to be a mere five hundredth as large; far too small to have influenced Neptune's orbit at all.<ref name=ken>
{{cite book | author=Croswell, Ken | year=1999 | title=Planet Quest: The Epic Discovery of Alien Solar Systems| publisher= Oxford University Press|pages=48, 66 |isbn=978-0-19-288083-3}}</ref> In 1989, [[Voyager 2]] determined the irregularities to be due to an overestimation of Neptune's mass.<ref>{{cite web|title=Hopes Fade in hunt for Planet X|author=Ken Croswell|year=1993|url=http://kencroswell.com/HopesFadeInHuntForPlanetX.html |access-date=November 4, 2007}}</ref>

=== Satellites ===
{{see also|Satellite planet}}
[[File:Jupiter and the Galilean moons animation.gif|thumb|Motion of the Galilean moons seen from a telescope]]
When Copernicus placed Earth among the planets, he also placed the Moon in orbit around Earth, making the Moon the first [[natural satellite]] to be identified. When [[Galileo Galilei|Galileo]] discovered his four [[Galilean moons|satellites]] of Jupiter in 1610, they lent weight to Copernicus's argument, because if other planets could have satellites, then Earth could too. However, there remained some confusion as to whether these objects were "planets"; Galileo referred to them as "four planets flying around the star of Jupiter at unequal intervals and periods with wonderful swiftness."<ref>{{cite book|title=Siderius Nuncius|author=Galileo Galilei|publisher=University of Chicago Press|year=1989|page=26|others=Albert van Helden}}</ref> Similarly, [[Christiaan Huygens]], upon discovering Saturn's largest moon [[Titan (moon)|Titan]] in 1655, employed many terms to describe it, including "planeta" (planet), "stella" (star), "luna" (moon), and "satellite" (attendant), a word coined by [[Johannes Kepler]].<ref>{{cite web|title=Johannes Kepler: His Life, His Laws and Times|date=September 24, 2016|publisher=NASA|url=https://www.nasa.gov/kepler/education/johannes|accessdate=2022-09-22|archive-date=June 24, 2021|archive-url=https://web.archive.org/web/20210624003856/https://www.nasa.gov/kepler/education/johannes/|url-status=dead}}</ref><ref>{{cite book|title=Systema Saturnium: Sive de Causis Miradorum Saturni Phaenomenon, et comite ejus Planeta Novo|author= Christiani Hugenii (Christiaan Huygens)|publisher=Adriani Vlacq|year= 1659|pages= 1–50}}</ref> [[Giovanni Cassini]], in announcing his discovery of Saturn's moons [[Iapetus (moon)|Iapetus]] and [[Rhea (moon)|Rhea]] in 1671 and 1672, described them as ''Nouvelles Planetes autour de Saturne'' ("New planets around Saturn").<ref>{{cite book|title=Decouverte de deux Nouvelles Planetes autour de Saturne|author=Giovanni Cassini|publisher=Sabastien Mabre-Craniusy|year= 1673| pages=6–14}}</ref> However, when the "Journal de Scavans" reported Cassini's discovery of two new Saturnian moons ([[Dione (moon)|Dione]] and [[Tethys (moon)|Tethys]]) in 1686, it referred to them strictly as "satellites", though sometimes Saturn as the "primary planet".<ref name="Cassini1686–1692">{{cite journal| doi = 10.1098/rstl.1686.0013| last = Cassini | first = G. D.| author-link = Giovanni Domenico Cassini| year = 1686–1692| title = An Extract of the Journal Des Scavans. Of April 22 st. N. 1686. Giving an Account of Two New Satellites of Saturn, Discovered Lately by Mr. Cassini at the Royal Observatory at Paris| journal = Philosophical Transactions of the Royal Society of London| volume = 16| issue = 179–191| pages = 79–85| jstor = 101844| bibcode = 1686RSPT...16...79C| doi-access = free}}<!-- This journal became the Philosophical Transactions of the Royal Society of London in 1775. Are there any earlier publications? Two years seems a long time... --></ref> When William Herschel announced his discovery of two objects in orbit around Uranus in 1787 ([[Titania (moon)|Titania]] and [[Oberon (moon)|Oberon]]), he referred to them as "satellites" and "secondary planets".<ref>{{cite book|title=An Account of the Discovery of Two Satellites Around the Georgian Planet. Read at the Royal Society|author=William Herschel|publisher=J. Nichols|year=1787|pages=1–4}}</ref> All subsequent reports of natural satellite discoveries used the term "satellite" exclusively,<ref>See primary citations in [[Timeline of discovery of Solar System planets and their moons#References|Timeline of discovery of Solar System planets and their moons]]</ref> though the 1868 book "Smith's Illustrated Astronomy" referred to satellites as "secondary planets".<ref name="smith">{{cite book
 |first = Asa | last = Smith
 |year=1868
 |title=Smith's Illustrated Astronomy
 |publisher=Nichols & Hall
 |url=https://archive.org/details/smithsillustrat00smitgoog |page = [https://archive.org/details/smithsillustrat00smitgoog/page/n27 23] |quote = secondary planet Herschel. }}</ref>

=== Minor planets ===
[[File:AnimatedOrbitOf1Ceres.gif|thumb|Orbit of Ceres, which fits with the description of a seemingly "missing" planet between Mars and Jupiter, as predicted by [[Bode's law]].]]
One of the unexpected results of [[William Herschel]]'s discovery of Uranus was that it appeared to validate [[Bode's law]], a mathematical function which generates the size of the [[semimajor axis]] of planetary [[orbit]]s. Astronomers had considered the "law" a meaningless coincidence, but Uranus fell at very nearly the exact distance it predicted. Since Bode's law also predicted a body between Mars and Jupiter that at that point had not been observed, astronomers turned their attention to that region in the hope that it might be vindicated again. Finally, in 1801, astronomer [[Giuseppe Piazzi]] found a miniature new world, [[Ceres (dwarf planet)|Ceres]], lying at just the correct point in space. The object was hailed as a new planet.<ref name=Hilton>{{cite web | author=Hilton, James L. | title=When did asteroids become minor planets? | work=U.S. Naval Observatory | url=http://sd-www.jhuapl.edu/weaver_projects/GPD/Contributed_Talks/hilton_gpd_poster.pdf | access-date=May 25, 2006}}</ref>

Then in 1802, [[Heinrich Wilhelm Matthäus Olbers|Heinrich Olbers]] discovered [[2 Pallas|Pallas]], a second "planet" at roughly the same distance from the Sun as Ceres. The fact that two planets could occupy the same orbit was an affront to centuries of thinking.<ref>{{cite book|title=King Henry the Fourth Part One in The Globe Illustrated Shakespeare: The Complete Works Annotated|author=William Shakespeare|publisher=Granercy Books|year= 1979|page= 559}}</ref> In 1804, another world, [[3 Juno|Juno]], was discovered in a similar orbit.<ref name=Hilton/> In 1807, Olbers discovered a fourth object, [[4 Vesta|Vesta]], at a similar orbital distance.

Herschel suggested that these four worlds be given their own separate classification, [[asteroid]]s (meaning "starlike" since they were too small for their disks to resolve and thus resembled [[star]]s), though most astronomers preferred to refer to them as planets.<ref name=Hilton/> This conception was entrenched by the fact that, due to the difficulty of distinguishing asteroids from yet-uncharted stars, those four remained the only asteroids known until 1845.<ref name=18planets>{{cite web
  |title=The Planet Hygea
  |year=1849
  |work=spaceweather.com
  |url=http://spaceweather.com/swpod2006/13sep06/Pollock1.jpg
  |access-date=June 24, 2008 }}</ref><ref name=police>{{cite journal|title=Call the Police! The story behind the discovery of the asteroids|journal=Astronomy Now|first=Keith|last= Cooper|pages=60–61|volume=21|issue=6|date=June 2007}}</ref> Science textbooks in 1828, after Herschel's death, still numbered the asteroids among the planets.<ref name=Hilton/> With the arrival of more refined star charts, the search for asteroids resumed, and a fifth and sixth were discovered by [[Karl Ludwig Hencke]] in 1845 and 1847.<ref name=police /> By 1851 the number of asteroids had increased to 15, and a new method of classifying them, by affixing a number before their names in order of discovery, was adopted, inadvertently placing them in their own distinct category. Ceres became "(1) Ceres", Pallas became "(2) Pallas", and so on. By the 1860s, the number of known asteroids had increased to over a hundred, and observatories in Europe and the United States began referring to them collectively as "[[minor planet]]s", or "small planets", though it took the first four asteroids longer to be grouped as such.<ref name=Hilton/> To this day, "minor planet" remains the official designation for all small bodies in orbit around the Sun, and each new discovery is numbered accordingly in the IAU's [[Minor Planet Catalogue]].<ref>{{cite web|title=The MPC Orbit (MPCORB) Database|url=http://www.minorplanetcenter.org/iau/MPCORB.html|access-date=October 15, 2007}}</ref>

=== Pluto ===
{{main|Planets beyond Neptune}}
[[File:EightTNOs.png|thumb|300x300px|To-scale comparison of the largest objects beyond Neptune's orbit, against the Earth and Moon. For a long time, only Pluto was discovered, so it was considered as a planet.]]
The long road from planethood to reconsideration undergone by [[Ceres (dwarf planet)|Ceres]] is mirrored in the story of [[Pluto]], which was named a planet soon after its discovery by [[Clyde Tombaugh]] in 1930. Uranus and Neptune had been declared planets based on their circular orbits, large masses and proximity to the ecliptic plane. None of these applied to Pluto, a tiny and icy world in a region of [[gas giant]]s with an orbit that carried it high above the [[ecliptic]] and even inside that of Neptune. In 1978, astronomers discovered Pluto's largest moon, [[Charon (moon)|Charon]], which allowed them to determine its mass. Pluto was found to be much tinier than anyone had expected: only one-sixth the mass of Earth's Moon.  However, as far as anyone could yet tell, it was unique. Then, beginning in 1992, astronomers began to detect large numbers of icy bodies beyond the orbit of Neptune that were similar to Pluto in composition, size, and orbital characteristics. They concluded that they had discovered the long-hypothesised [[Kuiper belt]] (sometimes called the Edgeworth–Kuiper belt), a band of icy debris that is the source for "short-period" comets—those with orbital periods of up to 200 years.<ref>{{cite journal | author=Weissman, Paul R. | title=The Kuiper Belt|  bibcode=1995ARA&A..33..327W|doi = 10.1146/annurev.aa.33.090195.001551 | year=1995 | journal=Annual Review of Astronomy and Astrophysics | volume=33 | pages=327–357 }}</ref>

Pluto's orbit lay within this band and thus its planetary status was thrown into question.  Many scientists concluded that tiny Pluto should be reclassified as a minor planet, just as Ceres had been a century earlier. [[Michael E. Brown|Mike Brown]] of the [[California Institute of Technology]] suggested that a "planet" should be redefined as "any body in the Solar System that is more massive than the total mass of all of the other bodies in a similar orbit."<ref>{{cite web | author=Brown, Mike. | title=A World on the Edge | work=[[NASA]] Solar System Exploration | url=http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=105 | access-date=May 25, 2006 | url-status=dead | archive-url=https://web.archive.org/web/20060427091759/http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=105 | archive-date=April 27, 2006 }}</ref> Those objects under that mass limit would become minor planets. In 1999, [[Brian G. Marsden]] of [[Harvard University]]'s [[Minor Planet Center]] suggested that Pluto be given the [[minor planet number]] 10000 while still retaining its official position as a planet.<ref name=comet>{{cite web
  |title=Is Pluto a giant comet?
  |publisher=Central Bureau for Astronomical Telegrams
  |url=http://www.icq.eps.harvard.edu/ICQPluto.html
  |access-date=July 3, 2011}}</ref><ref>{{cite news|title=Xena becomes Eris&nbsp;– Pluto reduced to a number|author=Kenneth Chang|work= New York Times|url=http://www.sfgate.com/cgi-bin/article.cgi?file=/c/a/2006/09/15/MNGS8L67LJ1.DTL|access-date=June 18, 2008|date=September 15, 2006}}</ref> The prospect of Pluto's "demotion" created a public outcry, and in response the [[International Astronomical Union]] clarified that it was not at that time proposing to remove Pluto from the planet list.<ref>{{cite web |year=1999 |title=The Status of Pluto:A clarification |work=[[International Astronomical Union]], Press release |url=http://www.bio.aps.anl.gov/~dgore/fun/PSL/Pluto19990203.txt |access-date=May 25, 2006 |url-status=dead |archive-url=https://web.archive.org/web/20060923202926/http://www.bio.aps.anl.gov/~dgore/fun/PSL/Pluto19990203.txt |archive-date=September 23, 2006 }} [http://www.bio.aps.anl.gov/ Copy kept] {{Webarchive|url=https://web.archive.org/web/20081005210115/http://www.bio.aps.anl.gov/ |date=October 5, 2008 }} at the [[Argonne National Laboratory]].</ref><ref>{{cite web|year=1999 |author=Witzgall, Bonnie B. |title=Saving Planet Pluto |work=Amateur Astronomer article |url=http://www.asterism.org/newsletter/l9904-3.htm |access-date=May 25, 2006 |url-status=dead |archive-url=https://web.archive.org/web/20061016080949/http://www.asterism.org/newsletter/l9904-3.htm |archive-date=October 16, 2006 }}</ref>

The discovery of several other [[trans-Neptunian object]]s, such as [[50000 Quaoar|Quaoar]] and [[90377 Sedna|Sedna]], continued to erode arguments that Pluto was exceptional from the rest of the trans-Neptunian population. On July 29, 2005, Mike Brown and his team announced the discovery of a trans-Neptunian object confirmed to be more massive than Pluto,<ref>{{cite web | year=2006 | author=Brown, Mike | title=The discovery of 2003 UB313, the 10th planet.| work= California Institute of Technology| url=http://www.gps.caltech.edu/~mbrown/planetlila/ | access-date=May 25, 2006}}</ref> named [[136199 Eris|Eris]].<ref>{{cite web | year=2005 |author1=M. E. Brown |author2=C. A. Trujillo |author3=D. L. Rabinowitz | title=DISCOVERY OF A PLANETARY-SIZED OBJECT IN THE SCATTERED KUIPER BELT| work= The American Astronomical Society.| url=http://www.gps.caltech.edu/%7Embrown/papers/ps/xena.pdf | access-date=August 15, 2006}}</ref>

In the immediate aftermath of the object's discovery, there was much discussion as to whether it could be termed a "[[tenth planet]]". NASA even put out a press release describing it as such.<ref>{{cite web|title=NASA-Funded Scientists Discover Tenth Planet|work=Jet Propulsion Laboratory|url=http://www.nasa.gov/vision/universe/solarsystem/newplanet-072905.html|year=2005|access-date=February 22, 2007|archive-date=March 19, 2015|archive-url=https://web.archive.org/web/20150319203306/http://www.nasa.gov/vision/universe/solarsystem/newplanet-072905.html|url-status=dead}}</ref> However, acceptance of Eris as the tenth planet implicitly demanded a definition of planet that set Pluto as an arbitrary minimum size. Many astronomers, claiming that the definition of planet was of little scientific importance, preferred to recognise Pluto's historical identity as a planet by "[[grandfathering]]" it into the planet list.<ref>{{cite web|title=Topic&nbsp;— First Mission to Pluto and the Kuiper Belt; "From Darkness to Light: The Exploration of the Planet Pluto"|author= Bonnie Buratti|work=Jet Propulsion Laboratory|url=http://www.jpl.nasa.gov/events/lectures/nov05.cfm|year=2005|access-date=February 22, 2007}}</ref>

== IAU definition ==
{{main|IAU definition of planet}}The discovery of [[Eris (dwarf planet)|Eris]] forced the [[International Astronomical Union|IAU]] to act on a definition. In October 2005, a group of 19 IAU members, which had already been working on a definition since the discovery of [[90377 Sedna|Sedna]] in 2003, narrowed their choices to a shortlist of three, using [[approval voting]]. The definitions were:

* A planet is any object in [[orbit]] around the Sun with a diameter greater than 2,000&nbsp;km. (eleven votes in favour)
* A planet is any object in orbit around the Sun whose shape is stable due to its own gravity. (eight votes in favour)
* A planet is any object in orbit around the Sun that is dominant in its immediate neighbourhood. (six votes in favour)<ref>{{cite web | author=McKee, Maggie| year=2006 | title=Xena reignites a planet-sized debate | work=NewScientistSpace | url=http://www.newscientistspace.com/article.ns?id=dn8681| access-date=May 25, 2006}}</ref><ref>{{cite web | author=Croswell, Ken| year=2006 | title=The Tenth Planet's First Anniversary| url=http://kencroswell.com/TenthPlanetFirstAnniversary.html| access-date=May 25, 2006}}</ref>

Since no consensus could be reached, the committee decided to put these three definitions to a wider vote at the IAU General Assembly meeting in [[Prague]] in August 2006,<ref>{{cite web|year=2006 |title=Planet Definition |work=IAU |url=http://www.astronomy2006.com/planet-definition.php |archive-url=https://web.archive.org/web/20060826193101/http://www.astronomy2006.com/planet-definition.php |url-status=dead |archive-date=August 26, 2006 |access-date=August 14, 2006 }}</ref> and on August 24, the IAU put a final draft to a vote, which combined elements from two of the three proposals. It essentially created a medial classification between ''planet'' and ''rock'' (or, in the new parlance, ''[[small Solar System body]]''), called ''[[dwarf planet]]'' and placed [[Pluto]] in it, along with Ceres and Eris.<ref>{{cite web |date=August 24, 2006 |title=IAU General Assembly Newspaper |url=http://astro.cas.cz/nuncius/nsiii_09.pdf |access-date=March 3, 2007}}</ref><ref name="IAU0602">{{cite web |date=August 24, 2006 |title=The Final IAU Resolution on the Definition of "Planet" Ready for Voting |url=http://www.iau.org/public_press/news/release/iau0602/ |access-date=March 2, 2007 |publisher=IAU (News Release&nbsp;— IAU0602)}}</ref>

{{quote|The IAU therefore resolves that planets and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:

# A "planet"{{efn|name=fn1}} is a celestial body that: {{Ordered list
|is in orbit around the Sun,
|has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a [[hydrostatic equilibrium]] (nearly round) shape, and
|has [[cleared the neighbourhood]] around its orbit.
|list_style_type=lower-alpha}}
# A "dwarf planet" is a celestial body that: {{Ordered list
|is in orbit around the Sun,
|has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape,{{efn|name=fn2}}
|has {{em|not}} cleared the neighbourhood around its orbit, and
|is not a satellite.
|list_style_type=lower-alpha}}
# All other objects,{{efn|name=fn3}} except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

{{notelist|refs=
{{efn|name=fn1|The planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.}}
{{efn|name=fn2|An IAU process will be established to assign borderline objects into either "dwarf planet" and other categories.}}
{{efn|name=fn3|These currently include most of the Solar System asteroids, most [[trans-Neptunian object]]s (TNOs), comets, and other small bodies.}}
}}

----
The IAU further resolves:
* [[Pluto]] is a "dwarf planet" by the above definition and is recognised as the prototype of a new category of trans-Neptunian objects.|multiline=true}}

The vote was passed, with 424 astronomers taking part in the ballot.<ref name="britt" /><ref>{{cite news|url=http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf|title=IAU 2006 General Assembly: Resolutions 5 and 6|date=August 24, 2006|publisher=IAU|access-date=June 23, 2009}}</ref><ref name="IAU0603">{{cite press release|date=August 24, 2006 |publisher=International Astronomical Union (News Release&nbsp;— IAU0603) |title=IAU 2006 General Assembly: Result of the IAU Resolution votes |url=http://www.iau.org/public_press/news/release/iau0603/ |access-date=December 31, 2007 }} ([http://www.iau.org/iau0603.414.0.html orig link] {{webarchive|url=https://web.archive.org/web/20070103145836/http://www.iau.org/iau0603.414.0.html |date=January 3, 2007 }})</ref> The IAU also resolved that "''planets'' and ''dwarf planets'' are two distinct classes of objects", meaning that dwarf planets, despite their name, would not be considered planets.<ref name=IAU0603/>

On September 13, 2006, the IAU placed Eris, its moon Dysnomia, and Pluto into their [[Minor Planet Catalogue]], giving them the official minor planet designations [[Pluto|(134340) Pluto]], [[Eris (dwarf planet)|(136199) Eris]], and [[Dysnomia (moon)|(136199) Eris I Dysnomia]].<ref>{{cite web|title=Circular No. 8747 |author=Central Bureau for Astronomical Telegrams, International Astronomical Union |url=http://www.cbat.eps.harvard.edu/iauc/08700/08747.html |year=2006 |access-date=July 3, 2011 }} [https://web.archive.org/web/20070205035336/http://www.cfa.harvard.edu/iau/special/08747.pdf web.archive]</ref> Other [[possible dwarf planets]], such as [[Haumea (dwarf planet)|2003 EL<sub>61</sub>]], [[Makemake (dwarf planet)|2005 FY<sub>9</sub>]], [[90377 Sedna|Sedna]] and [[50000 Quaoar|Quaoar]], were left in temporary limbo until a formal decision could be reached regarding their status.

On June 11, 2008, the IAU executive committee announced the establishment of a subclass of dwarf planets comprising the aforementioned "new category of trans-Neptunian objects" to which Pluto is a prototype. This new class of objects, termed [[plutoid]]s, would include Pluto, Eris and any other trans-Neptunian dwarf planets, but excluded Ceres. The IAU decided that those TNOs with an [[absolute magnitude]] brighter than +1 would be named by a joint commissions of the planetary and minor-planet naming committees, under the assumption that they were likely to be dwarf planets. To date, only two other TNOs, 2003&nbsp;EL<sub>61</sub> and 2005&nbsp;FY<sub>9</sub>, have met the absolute magnitude requirement, while other possible dwarf planets, such as Sedna, Orcus and Quaoar, were named by the minor-planet committee alone.<ref>{{cite web
 |date=June 11, 2008 |location=Paris 
 |title=Plutoid chosen as name for Solar System objects like Pluto 
 |publisher=[[International Astronomical Union]] (News Release&nbsp;— IAU0804) 
 |url=http://www.iau.org/public_press/news/release/iau0804 
 |access-date=June 11, 2008 |url-status=dead 
 |archive-url=https://web.archive.org/web/20080613121232/http://www.iau.org/public_press/news/release/iau0804/ 
 |archive-date=June 13, 2008 }}</ref> On July 11, 2008, the Working Group on Planetary Nomenclature named 2005&nbsp;FY<sub>9</sub> ''[[Makemake (dwarf planet)|Makemake]]'',<ref name="name">{{cite web  |date=July 11, 2008 <!-- 11:42:58 -->|title=Dwarf Planets and their Systems  |publisher= Working Group for Planetary System Nomenclature (WGPSN)  |url=http://planetarynames.wr.usgs.gov/append7.html#DwarfPlanets|access-date=July 13, 2008}}</ref> and on September 17, 2008, they named 2003&nbsp;EL<sub>61</sub> ''[[Haumea (dwarf planet)|Haumea]]''.<ref name=usgs>{{cite news| title =USGS Gazetteer of Planetary Nomenclature| url=http://planetarynames.wr.usgs.gov/append7.html| access-date=September 17, 2008}}</ref>

== Acceptance of the IAU definition ==
[[File:Outersolarsystem objectpositions labels comp.png|thumb|Plot of the positions of all known [[Kuiper belt objects]], set against the outer planets, as of 2000.]]

Among the most vocal proponents of the IAU's decided definition are [[Michael E. Brown|Mike Brown]], the discoverer of Eris; [[Steven Soter]], professor of astrophysics at the [[American Museum of Natural History]]; and [[Neil deGrasse Tyson]], director of the [[Hayden Planetarium]].

In the early 2000s, when the Hayden Planetarium was undergoing a $100&nbsp;million renovation, Tyson refused to refer to Pluto as the ninth planet at the planetarium.<ref>{{Cite web|author1=Space com Staff|date=2001-02-02|title=Astronomer Responds to Pluto-Not-a-Planet Claim|url=https://www.space.com/1925-astronomer-responds-pluto-planet-claim.html|access-date=2023-03-29|website=Space.com|language=en}}</ref> He explained that he would rather group planets according to their commonalities rather than counting them. This decision resulted in Tyson receiving large amounts of hate mail, primarily from children.<ref>''[[The Colbert Report]]'', August 17, 2006</ref> In 2009, Tyson [[The Pluto Files|wrote a book]] detailing the demotion of Pluto.

In an article in the January 2007 issue of ''[[Scientific American]]'', Soter cited the definition's incorporation of current theories of the [[formation and evolution of the Solar System]]; that as the earliest [[protoplanet]]s emerged from the swirling dust of the [[protoplanetary disc]], some bodies "won" the initial competition for limited material and, as they grew, their increased gravity meant that they accumulated more material, and thus grew larger, eventually outstripping the other bodies in the Solar System by a very wide margin. The asteroid belt, disturbed by the gravitational tug of nearby Jupiter, and the Kuiper belt, too widely spaced for its constituent objects to collect together before the end of the initial formation period, both failed to win the accretion competition.

When the numbers for the winning objects are compared to those of the losers, the contrast is striking; if Soter's concept that each planet occupies an "orbital zone"{{Ref label|B|b|none}} is accepted, then the least orbitally dominant planet, Mars, is larger than all other collected material in its orbital zone by a factor of 5100. Ceres, the largest object in the asteroid belt, only accounts for one third of the material in its orbit; Pluto's ratio is even lower, at around 7 percent.<ref name=Soter>{{Cite journal|title=What is a Planet?|author= Steven Soter|journal=The Astronomical Journal |volume= 132|issue= 6|pages= 2513–2519|date=August 16, 2006|arxiv= astro-ph/0608359|bibcode = 2006AJ....132.2513S |doi = 10.1086/508861 |s2cid= 14676169}}</ref> Mike Brown asserts that this massive difference in orbital dominance leaves "absolutely no room for doubt about which objects do and do not belong."<ref name=Brown>{{cite web|title=The Eight Planets|author=Michael E. Brown|work=Caltech|url=http://www.gps.caltech.edu/~mbrown/eightplanets/|year=2006| access-date=February 21, 2007}}</ref>

== Ongoing controversies ==
Despite the IAU's declaration, a number of critics remain unconvinced. The definition is seen by some as arbitrary and confusing. A number of [[Pluto]]-as-planet proponents, in particular [[Alan Stern]], head of [[NASA]]'s ''[[New Horizons]]'' mission to [[Pluto]], have circulated a petition among astronomers to alter the definition. Stern's claim is that, since less than 5 percent of astronomers voted for it, the decision was not representative of the entire astronomical community.<ref name=britt>{{cite web | year=2006 | author=Robert Roy Britt| title=Pluto demoted in highly controversial definition| work= Space.com| url=http://www.space.com/scienceastronomy/060824_planet_definition.html| access-date=August 24, 2006}}</ref><ref>{{cite web | year=2006 | author=Robert Roy Britt| title=Pluto: Down But Maybe Not Out| work= Space.com| url=http://www.space.com/scienceastronomy/060831_planet_definition.html| access-date=August 24, 2006}}</ref> Even with this controversy excluded, however, there remain several ambiguities in the definition.

=== Clearing the neighbourhood ===
{{main|Clearing the neighbourhood}}
One of the main points at issue is the precise meaning of "cleared the neighbourhood around its [[orbit]]". [[Alan Stern]] objects that "it is impossible and contrived to put a dividing line between dwarf planets and planets",<ref>{{cite news|author=Paul Rincon|url=http://news.bbc.co.uk/2/hi/science/nature/5283956.stm| title=Pluto vote 'hijacked' in revolt|work=BBC News|access-date=February 28, 2007 | date=August 25, 2006}}</ref> and that since neither Earth, Mars, Jupiter, nor Neptune have entirely cleared their regions of debris, none could properly be considered planets under the [[International Astronomical Union|IAU]] definition.{{Ref label|C|c|none}}

[[File:InnerSolarSystem-en.png|thumb|The asteroids of the inner Solar System; note the [[Trojan asteroid]]s (green), trapped into Jupiter's orbit by its gravity]]
Mike Brown responds to these claims by saying that, far from not having cleared their orbits, the major planets completely control the orbits of the other bodies within their orbital zone. Jupiter may coexist with a large number of small bodies in its orbit (the [[Trojan asteroid]]s), but these bodies only exist in Jupiter's orbit because they are in the sway of the planet's huge gravity. Similarly, Pluto may cross the orbit of Neptune, but Neptune long ago locked Pluto and its attendant Kuiper belt objects, called [[plutino]]s, into a 3:2 resonance, i.e., they orbit the Sun twice for every three Neptune orbits. The orbits of these objects are entirely dictated by Neptune's gravity, and thus, Neptune is gravitationally dominant.<ref name=Brown />

In October 2015, astronomer [[Jean-Luc Margot]] of the [[University of California Los Angeles]] proposed a metric for orbital zone clearance derived from whether an object can clear an orbital zone of extent 2{{radic|3}} of its [[Hill radius]] in a specific time scale. This metric places a clear dividing line between the dwarf planets and the planets of the solar system.<ref name=Margot>{{Cite journal|date=2015|title=A Quantitative Criterion For Defining Planets|journal=The Astronomical Journal|volume=150|issue=6|pages=185|author=Jean-Luc Margot |arxiv=1507.06300|doi=10.1088/0004-6256/150/6/185|bibcode=2015AJ....150..185M|s2cid=51684830}}</ref> The calculation is based on the mass of the host star, the mass of the body, and the orbital period of the body. An Earth-mass body orbiting a solar-mass star clears its orbit at distances of up to 400 [[astronomical unit]]s from the star. A Mars-mass body at the orbit of Pluto clears its orbit. This metric, which leaves Pluto as a dwarf planet, applies to both the Solar System and to extrasolar systems.<ref name=Margot/>

Some opponents of the definition have claimed that "clearing the neighbourhood" is an ambiguous concept. Mark Sykes, director of the Planetary Science Institute in Tucson, Arizona, and organiser of the petition, expressed this opinion to [[National Public Radio]]. He believes that the definition does not categorise a planet by composition or formation, but, effectively, by its location. He believes that a Mars-sized or larger object beyond the orbit of Pluto would not be considered a planet, because he believes that it would not have time to clear its orbit.<ref>{{cite news| url=https://www.npr.org/templates/story/story.php?storyId=5788798| format=RealPlayer| title= Astronomers Prepare to Fight Pluto Demotion| first=Sykes| last=Mark| newspaper=NPR.org| date=September 8, 2006| access-date=October 4, 2006}}</ref>

Brown notes, however, that were the "clearing the neighbourhood" criterion to be abandoned, the number of planets in the Solar System could rise from eight to [[List of possible dwarf planets|more than 50]], with hundreds more potentially to be discovered.<ref name=browndwarf/>

=== Hydrostatic equilibrium ===
[[File:Proteus (Voyager 2).jpg|thumb|left|[[Proteus (moon)|Proteus]], a moon of [[Neptune]], is irregular, despite being larger than the spheroidal [[Mimas (moon)|Mimas]].]]
The [[International Astronomical Union|IAU's]] definition mandates that planets be large enough for their own [[gravity]] to form them into a state of [[hydrostatic equilibrium]]; this means that they will reach a round, [[ellipsoid]]al shape. Up to a certain mass, an object can be irregular in shape, but beyond that point gravity begins to pull an object towards its own [[centre of mass]] until the object collapses into an ellipsoid. (None of the large objects of the Solar System are truly spherical. Many are [[spheroid]]s, and several, such as the larger moons of Saturn and the dwarf planet {{dp|Haumea}}, have been further distorted into ellipsoids by rapid rotation or [[tidal force]]s, but still in hydrostatic equilibrium.<ref>{{cite web| author= Brown, Michael E.| title=2003EL61| work= California Institute of Technology| url=http://www.gps.caltech.edu/~mbrown/2003EL61| access-date=May 25, 2006}}</ref>)

However, there is no precise point at which an object can be said to have reached hydrostatic equilibrium. As Soter noted in his article, "how are we to quantify the degree of roundness that distinguishes a planet? Does gravity dominate such a body if its shape deviates from a spheroid by 10 percent or by 1 percent? Nature provides no unoccupied gap between round and nonround shapes, so any boundary would be an arbitrary choice."<ref name=Soter /> Furthermore, the point at which an object's mass compresses it into an ellipsoid varies depending on the chemical makeup of the object. Objects made of ices,{{Ref label|D|d|none}} such as Enceladus and Miranda, assume that state more easily than those made of rock, such as Vesta and Pallas.<ref name=browndwarf>{{cite web|title=The Dwarf Planets|author=Mike Brown|url=http://www.gps.caltech.edu/~mbrown/dwarfplanets/|access-date=August 4, 2007}}</ref> Heat energy, from [[gravitational collapse]], [[impact event|impacts]], tidal forces such as orbital resonances, or [[radioactive decay]], also factors into whether an object will be ellipsoidal or not; Saturn's icy moon Mimas is ellipsoidal (though no longer in hydrostatic equilibrium), but Neptune's larger moon [[Proteus (moon)|Proteus]], which is similarly composed but colder because of its greater distance from the Sun, is irregular. In addition, the much larger Iapetus is ellipsoidal but does not have the dimensions expected for its current speed of rotation, indicating that it was once in hydrostatic equilibrium but no longer is,<ref name="Thomas2010">{{cite journal| doi = 10.1016/j.icarus.2010.01.025| last1 = Thomas| first1 = P. C.| date = July 2010| title = Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission| journal = Icarus| volume = 208| issue = 1| pages = 395–401| url = http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| bibcode = 2010Icar..208..395T| access-date = September 4, 2015| archive-date = December 23, 2018| archive-url = https://web.archive.org/web/20181223003125/http://www.ciclops.org/media/sp/2011/6794_16344_0.pdf| url-status = dead}}</ref> and the same is true for Earth's moon.<ref>Garrick-Bethell et al. (2014) "The tidal-rotational shape of the Moon and evidence for polar wander", ''Nature'' 512, 181–184.</ref><ref name=Bursa>{{Cite journal|last=Bursa|first=M.|date=1984-10-01|title=Secular Love Numbers and Hydrostatic Equilibrium of Planets|url=https://ui.adsabs.harvard.edu/abs/1984EM&P...31..135B|journal=Earth, Moon, and Planets|volume=31|issue=2 |pages=135–140|doi=10.1007/BF00055525|bibcode=1984EM&P...31..135B |s2cid=119815730 |issn=0167-9295}}</ref> Even Mercury, universally regarded as a planet, is not in hydrostatic equilibrium.<ref name="Mercury">Sean Solomon, Larry Nittler & Brian Anderson, eds. (2018) ''Mercury: The View after MESSENGER''. Cambridge Planetary Science series no. 21, Cambridge University Press, pp. 72–73.</ref> Thus the IAU definition is not taken literally even by the IAU, as it includes Mercury as a planet; its requirement for hydrostatic equilibrium is in practice ignored in favour of a requirement for roundedness.<ref>{{Cite tweet |user=plutokiller |last=Brown |first=Mike |number=1624127764969459713 |title=The real answer here is to not get too hung up on definitions, which I admit is hard when the IAU tries to make them sound official and clear, but, really, we all understand the intent of the hydrostatic equilibrium point, and the intent is clearly to include Merucry & the moon}}</ref>

=== Double planets and moons ===
{{main|Double planet}}
[[File:Pluto and Charon from Hubble.jpg|thumb|A [[telescope|telescopic]] image of [[Pluto]] and [[Charon (moon)|Charon]]]]

The definition specifically excludes [[natural satellite|satellites]] from the category of dwarf planet, though it does not directly define the term "satellite".<ref name=IAU0603/> In the original draft proposal, an exception was made for [[Pluto]] and its largest satellite, [[Charon (moon)|Charon]], which possess a [[barycenter]] outside the volume of either body. The initial proposal classified Pluto–Charon as a double planet, with the two objects orbiting the Sun in tandem. However, the final draft made clear that, even though they are similar in relative size, only Pluto would currently be classified as a dwarf planet.<ref name=IAU0603/>

[[File:Moon trajectory1.png|thumb|A diagram illustrating the [[Moon]]'s co-orbit with the Earth]]

However, some have suggested that the Moon nonetheless deserves to be called a planet. In 1975, [[Isaac Asimov]] noted that the timing of the Moon's orbit is in tandem with the Earth's own orbit around the Sun—looking down on the [[ecliptic]], the Moon never actually loops back on itself, and in essence it orbits the Sun in its own right.<ref>Asimov, Isaac (1975). ''Just Mooning Around'', In: Of time and space, and other things. Avon.</ref>

Also many moons, even those that do not orbit the Sun directly, often exhibit features in common with true planets. There are 20 moons in the Solar System that are massive enough to have achieved hydrostatic equilibrium (the so-called [[planetary-mass moon]]s); they would be considered planets if only the physical parameters are considered. Both Jupiter's moon [[Ganymede (moon)|Ganymede]] and Saturn's moon [[Titan (moon)|Titan]] are larger than Mercury, and Titan even has a substantial atmosphere, thicker than the Earth's. Moons such as [[Io (moon)|Io]] and [[Triton (moon)|Triton]] demonstrate obvious and ongoing geological activity, and Ganymede has a [[magnetic field]]. Just as [[star]]s in orbit around other stars are still referred to as stars, some astronomers argue that objects in orbit around planets that share all their characteristics could also be called planets.<ref name=Buie2005>{{cite web
 |date=March 2005
 |title=Definition of a Planet
 |publisher=Southwest Research Institute
 |author=Marc W. Buie
 |url=http://www.boulder.swri.edu/~buie/pluto/planetdefn.html
 |access-date=July 7, 2008|author-link=Marc W. Buie
 }}</ref><ref name=stern2008>{{cite web
  |date=June 15, 2008
|title=IAU Snobbery
  |publisher=NASA Watch (not a NASA Website)
  |url=http://nasawatch.com/archives/2008/06/iau-snobbery.html
  |access-date=July 5, 2008}}</ref><ref>{{cite book|author=Serge Brunier|title=Solar System Voyage|publisher=Cambridge University Press|year=2000|pages=160–165|isbn=978-0-521-80724-1}}</ref> Indeed, Mike Brown makes just such a claim in his dissection of the issue, saying:<ref name=Brown />

<blockquote>It is hard to make a consistent argument that a 400&nbsp;km iceball should count as a planet because it might have interesting geology, while a 5000&nbsp;km satellite with a massive atmosphere, methane lakes, and dramatic storms [Titan] shouldn't be put into the same category, whatever you call it.</blockquote>

However, he goes on to say that, "For most people, considering round satellites (including our Moon) 'planets' violates the idea of what a planet is."<ref name=Brown />

[[Alan Stern]] has argued that location should not matter and that only geophysical attributes should be taken into account in the definition of a planet, and proposes the term ''satellite planet'' for [[planetary-mass moon]]s.<ref name="satelliteplanet">{{cite web |url=http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html#post-a-comment |title=Should Large Moons Be Called 'Satellite Planets'? |publisher=News.discovery.com |date=May 14, 2010 |access-date=November 4, 2011 |archive-date=May 5, 2012 |archive-url=https://web.archive.org/web/20120505221146/http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html#post-a-comment |url-status=dead }}</ref>

=== Extrasolar planets and brown dwarfs ===
{{main|Extrasolar planet|Brown dwarf}}
The discovery since 1992 of [[extrasolar planet]]s, or planet-sized objects around other stars ({{Extrasolar planet counts|planet_count}} such planets in {{Extrasolar planet counts|system_count}} [[planetary system]]s including {{Extrasolar planet counts|multiplanetsystem_count}} [[List of multiplanetary systems|multiple planetary systems]] as of {{Extrasolar planet counts|asof}}),<ref name="Encyclopaedia">
{{cite encyclopedia
|last1=Schneider |first1=J.
|date=September 10, 2011
|title=Interactive Extra-solar Planets Catalog
|url=https://exoplanet.eu/catalog/
|encyclopedia=[[Extrasolar Planets Encyclopaedia]] |access-date=July 13, 2012
}}</ref> has widened the debate on the nature of planethood in unexpected ways. Many of these planets are of considerable size, approaching the mass of small stars, while many newly discovered brown dwarfs are, conversely, small enough to be considered planets.<ref>{{cite web
 |year=2006 
 |title=IAU General Assembly: Definition of Planet debate 
|url=http://astro2006.meta.mediastream.cz/Astro2006-060822-01.asx
 |archive-url=https://archive.today/20120713105841/http://astro2006.meta.mediastream.cz/Astro2006-060822-01.asx
 |url-status=dead
 |archive-date=July 13, 2012
|access-date=September 24, 2006 }}</ref> The material difference between a low-mass star and a large [[gas giant]] is not clear-cut; apart from size and relative temperature, there is little to separate a gas giant like Jupiter from its host star. Both have similar overall compositions: hydrogen and [[helium]], with trace levels of heavier [[chemical element|elements]] in their [[atmosphere]]s. The generally accepted difference is one of formation; stars are said to have formed from the "top down", out of the gases in a nebula as they underwent gravitational collapse, and thus would be composed almost entirely of hydrogen and helium, while planets are said to have formed from the "bottom up", from the accretion of dust and gas in orbit around the young star, and thus should have cores of [[silicate]]s or ices.<ref>{{cite journal |year= 2004| author=G. Wuchterl| title=Giant planet formation| journal=Institut für Astronomie der Universität Wien| volume=67| issue=1–3| pages=51–65| doi=10.1007/BF00613290|bibcode = 1994EM&P...67...51W | s2cid=119772190}}</ref> As yet it is uncertain whether gas giants possess such cores, though the [[Juno mission|''Juno'' mission]] to Jupiter could resolve the issue. If it is indeed possible that a gas giant could form as a star does, then it raises the question of whether such an object should be considered an orbiting low-mass star rather than a planet.

[[File:Brown Dwarf Gliese 229B.jpg|thumb|left|The brown dwarf [[Gliese 229B]] in orbit around its star]]
Traditionally, the defining characteristic for starhood has been an object's ability to [[Nuclear fusion|fuse]] [[hydrogen]] in its core. However, stars such as brown dwarfs have always challenged that distinction. Too small to commence sustained hydrogen-1<!--deuterium is also hydrogen--> fusion, they have been granted star status on their ability to fuse [[deuterium]]. However, due to the relative rarity of that [[isotope]], this process lasts only a tiny fraction of the star's lifetime, and hence most brown dwarfs would have ceased fusion long before their discovery.<ref>{{Cite journal | last=Basri | first=Gibor | title= Observations of Brown Dwarfs | journal=Annual Review of Astronomy and Astrophysics | year=2000 | volume=38 | pages=485–519 | doi=10.1146/annurev.astro.38.1.485 | bibcode=2000ARA&A..38..485B}}</ref> [[Binary star]]s and other multiple-star formations are common, and many brown dwarfs orbit other stars. Therefore, since they do not produce energy through fusion, they could be described as planets. Indeed, astronomer [[Adam Burrows (astronomer)|Adam Burrows]] of the [[University of Arizona]] claims that "from the theoretical perspective, however different their modes of formation, extrasolar giant planets and brown dwarfs are essentially the same".<ref>{{cite journal |author1=Burrows, Adam |author2=Hubbard, William B. |author3=Lunine, Jonathan I. |author4=Leibert, James | year=2001 | title=The Theory of Brown Dwarfs and Extrasolar Giant Planets | doi=10.1103/RevModPhys.73.719 | journal=Reviews of Modern Physics | volume=73 | issue=3 | pages=719–765 | arxiv=astro-ph/0103383|bibcode = 2001RvMP...73..719B |s2cid=204927572 }}</ref> Burrows also claims that such stellar remnants as [[white dwarfs]] should not be considered stars,<ref>Croswell p. 119</ref> a stance which would mean that an orbiting [[white dwarf]], such as [[Sirius B]], could be considered a planet. However, the current convention among astronomers is that any object massive enough to have possessed the capability to sustain atomic fusion during its lifetime and that is not a black hole should be considered a star.<ref>{{cite book | author=Croswell, Ken | year=1999 | title=Planet Quest: The Epic Discovery of Alien Solar Systems| publisher= Oxford University Press|page=119|isbn=978-0-19-288083-3}}</ref>

The confusion does not end with brown dwarfs. María Rosa Zapatero Osorio et al. have discovered many objects in young [[star cluster]]s of masses below that required to sustain fusion of any sort (currently calculated to be roughly 13 Jupiter masses).<ref>{{cite journal |year= 2000|author1=Zapatero M. R. Osorio |author2=V. J. S. Béjar |author3=E. L. Martín |author4=R. Rebolo |author5=D. Barrado y Navascués |author6=C. A. L. Bailer-Jones |author7=R. Mundt | title=Discovery of Young, Isolated Planetary Mass Objects in the Sigma Orionis Star Cluster| journal= Division of Geological and Planetary Sciences, California Institute of Technology| doi=10.1126/science.290.5489.103 | volume=290 |issue=5489 | pages=103–107|pmid=11021788 |bibcode = 2000Sci...290..103Z }}</ref> These have been described as "[[rogue planet|free floating planets]]" because current theories of Solar System formation suggest that planets may be ejected from their [[star system]]s altogether if their orbits become unstable.<ref>{{cite journal| last=Lissauer| first= J. J.| title= Timescales for Planetary Accretion and the Structure of the Protoplanetary disk| journal= Icarus| volume= 69| pages=249–265| year=1987| doi=10.1016/0019-1035(87)90104-7| bibcode=1987Icar...69..249L| issue=2| hdl= 2060/19870013947| hdl-access=free}}</ref> However, it is also possible that these "free floating planets" could have formed in the same manner as stars.<ref>{{cite news | title=Rogue planet find makes astronomers ponder theory| agency=Reuters| url=http://edition.cnn.com/2000/TECH/space/10/06/space.planets.reut/index.html| access-date=May 25, 2006 | date=October 6, 2000}}</ref>

[[File:Sol Cha-110913-773444 Jupiter.jpg|thumb|The solitary [[Cha 110913-773444]] (middle), a possible [[sub-brown dwarf]], set to scale against the Sun (left) and the planet [[Jupiter]] (right)]]
In 2003, a working group of the IAU released a position statement<ref>{{cite web|year=2001 |title=Working Group on Extrasolar Planets (WGESP) of the International Astronomical Union |work=[[IAU]] |url=http://www.dtm.ciw.edu/boss/definition.html |access-date=May 25, 2006 |url-status=dead |archive-url=https://web.archive.org/web/20060916161707/http://www.dtm.ciw.edu/boss/definition.html |archive-date=September 16, 2006 }}</ref> to establish a working definition as to what constitutes an extrasolar planet and what constitutes a brown dwarf. To date, it remains the only guidance offered by the IAU on this issue. The 2006 planet definition committee did not attempt to challenge it, or to incorporate it into their definition, claiming that the issue of defining a planet was already difficult to resolve without also considering extrasolar planets.<ref>{{cite web|title=General Sessions & Public Talks|url=http://www.astronomy2006.com/media-stream-archive.php|year=2006|publisher=International Astronomical Union|access-date=November 28, 2008|url-status=dead|archive-url=https://web.archive.org/web/20081208140503/http://www.astronomy2006.com/media-stream-archive.php|archive-date=December 8, 2008}}</ref> This working definition was amended by the IAU's Commission F2: Exoplanets and the Solar System in August 2018.<ref>{{cite web |title=Official Working Definition of an Exoplanet |url=https://www.iau.org/science/scientific_bodies/commissions/F2/info/documents/ |work=IAU position statement |access-date=November 29, 2020 }}</ref> The official working definition of an ''exoplanet'' is now as follows:

{{quote|
* 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 [[Lagrange point#Stability|L4/L5 instability]] (M/M<sub>central</sub> < 2/(25+{{math|{{radical|621}}}}) are "planets" (no matter how they formed).
* The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our Solar System.
}}

The IAU noted that this definition could be expected to evolve as knowledge improves.

[[File:hubbledwarf.jpg|left|thumb|upright|CHXR 73 b, an object which lies at the border between planet and brown dwarf]]
This definition makes location, rather than formation or composition, the determining characteristic for planethood. A free-floating object with a mass below 13 Jupiter masses is a "sub-brown dwarf", whereas such an object in orbit around a fusing star is a planet, even if, in all other respects, the two objects may be identical. Further, in 2010, a paper published by Burrows, David S. Spiegel and John A. Milsom called into question the 13-Jupiter-mass criterion, showing that a brown dwarf of three times solar [[metallicity]] could fuse deuterium at as low as 11 Jupiter masses.<ref name=spiegel/>

Also, the 13 Jupiter-mass cutoff does not have precise physical significance. Deuterium fusion can occur in some objects with mass below that cutoff. The amount of deuterium fused depends to some extent on the composition of the object.<ref name=spiegel>{{Cite journal|arxiv=1008.5150 |author1=David S. Spiegel|author2=Adam Burrows |author3=John A. Milsom|title=The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets |journal=The Astrophysical Journal|volume=727|issue=1|pages=57|year=2010|doi=10.1088/0004-637X/727/1/57|bibcode=2011ApJ...727...57S|s2cid=118513110}}</ref> As of 2011 the [[Extrasolar Planets Encyclopaedia]] included objects up to 25 Jupiter masses, saying, "The fact that there is no special feature around {{Jupiter mass|13|jup=y}} in the observed mass spectrum reinforces the choice to forget this mass limit".<ref>{{cite journal|last1=Schneider |first1=J. |last2=Dedieu |first2=C. |last3=Le Sidaner |first3=P. |last4=Savalle |first4=R. |last5=Zolotukhin |first5=I. |title=Defining and cataloging exoplanets: The exoplanet.eu database| date=2011| volume=532| issue=79| journal=[[Astronomy & Astrophysics]] |arxiv=1106.0586| doi=10.1051/0004-6361/201116713|pages=A79 |bibcode=2011A&A...532A..79S|s2cid=55994657 }}</ref> 
As of 2016 this limit was increased to 60 Jupiter masses<ref>{{Cite book|last=Schneider|first=Jean|date=2016-04-04|title=Exoplanets versus brown dwarfs: the CoRoT view and the future|chapter=III.8 Exoplanets versus brown dwarfs: The CoRoT view and the future |page=157 |doi=10.1051/978-2-7598-1876-1.c038|arxiv=1604.00917 |isbn=978-2-7598-1876-1 |s2cid=118434022 }}</ref> based on a study of mass–density relationships.<ref>{{cite journal |arxiv=1506.05097|last1= Hatzes Heike Rauer|first1= Artie P.|title= A Definition for Giant Planets Based on the Mass-Density Relationship|year= 2015|doi=10.1088/2041-8205/810/2/L25|volume=810|issue= 2|journal=The Astrophysical Journal|page=L25|bibcode = 2015ApJ...810L..25H |s2cid= 119111221}}</ref> 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]]."<ref name=eod>{{cite arXiv|eprint=1012.5676v1|author=Wright, J. T.|title=The Exoplanet Orbit Database|class=astro-ph.SR|year=2010|display-authors=etal}}</ref> The [[NASA Exoplanet Archive]] includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses.<ref>{{Cite web|title=Exoplanet Criteria for Inclusion in the Exoplanet Archive|url=https://exoplanetarchive.ipac.caltech.edu/docs/exoplanet_criteria.html|access-date=2023-03-29|website=exoplanetarchive.ipac.caltech.edu}}</ref>

Another criterion for separating planets and brown dwarfs, rather than deuterium burning, formation process or location, is whether the core [[pressure]] is dominated by [[Coulomb barrier|Coulomb pressure]] or [[electron degeneracy pressure]].<ref>{{cite journal |doi=10.1146/annurev.earth.34.031405.125058 |journal=Annu. Rev. Earth Planet. Sci. |volume=34 |title=Planetesimals To Brown Dwarfs: What is a Planet? |pages=193–216 |year=2006 |arxiv=astro-ph/0608417 |bibcode=2006AREPS..34..193B|last1=Basri |first1=Gibor |last2=Brown |first2=Michael E. |s2cid=119338327 }}</ref><ref>{{cite journal |author1=Boss, Alan P. |author2=Basri, Gibor |author3=Kumar, Shiv S. |author4=Liebert, James |author5=Martín, Eduardo L. |author6=Reipurth, Bo |author7=Zinnecker, Hans |title=Nomenclature: Brown Dwarfs, Gas Giant Planets, and ? |journal=Brown Dwarfs |volume=211 |page=529 |year=2003 |bibcode=2003IAUS..211..529B }}</ref>

One study suggests that objects above {{Jupiter mass|10|jup=y}} formed through gravitational instability and not core accretion and therefore should not be thought of as planets.<ref>{{Cite journal|last=Schlaufman|first=Kevin C.|date=2018-01-18|title=Evidence of an Upper Bound on the Masses of Planets and its Implications for Giant Planet Formation|journal=The Astrophysical Journal |volume=853 |issue=1 |page=37 |doi=10.3847/1538-4357/aa961c|arxiv=1801.06185 |bibcode=2018ApJ...853...37S |s2cid=55995400 |doi-access=free }}</ref>

A 2016 study shows no noticeable difference between gas giants and brown dwarfs in mass–radius trends: from approximately one Saturn mass to about {{Solar mass|0.080 ± 0.008}} (the onset of hydrogen burning), radius stays roughly constant as mass increases, and no obvious difference occurs when passing {{Jupiter mass|13}}. By this measure, brown dwarfs are more like planets than they are like stars.<ref name=ChenKipping>{{cite journal |last1=Chen |first1=Jingjing |last2=Kipping |first2=David |date=2016 |title=Probabilistic Forecasting of the Masses and Radii of Other Worlds |journal=The Astrophysical Journal |volume=834 |issue=1 |page=17 |doi= 10.3847/1538-4357/834/1/17|arxiv=1603.08614 |s2cid=119114880 |doi-access=free }}</ref>

====Planetary-mass stellar objects====
The ambiguity inherent in the IAU's definition was highlighted in December 2005, when the [[Spitzer Space Telescope]] observed [[Cha 110913-773444]] (above), only eight times Jupiter's mass with what appears to be the beginnings of its own [[planetary system]]. Were this object found in orbit around another star, it would have been termed a planet.<ref>{{cite web| year=2005| author=Clavin, Whitney| title=A Planet With Planets? Spitzer Finds Cosmic Oddball| work=Spitzer Science Center| url=http://www.nasa.gov/vision/universe/starsgalaxies/spitzerf-20051129.html| access-date=May 25, 2006| archive-date=October 11, 2012| archive-url=https://web.archive.org/web/20121011011111/http://www.nasa.gov/vision/universe/starsgalaxies/spitzerf-20051129.html| url-status=dead}}</ref>

In September 2006, the [[Hubble Space Telescope]] imaged [[CHXR 73 b]] (left), an object orbiting a young companion star at a distance of roughly 200 AU. At 12 Jovian masses, CHXR 73 b is just under the threshold for deuterium fusion, and thus technically a planet; however, its vast distance from its parent star suggests it could not have formed inside the small star's [[protoplanetary disc]], and therefore must have formed, as stars do, from gravitational collapse.<ref>{{cite web|title=Planet or failed star? Hubble photographs one of the smallest stellar companions ever seen|work=ESA Hubble page|url=http://www.spacetelescope.org/news/html/heic0610.html|year=2006|access-date=February 23, 2007}}</ref>

In 2012, Philippe Delorme, of the [[Institute of Planetology and Astrophysics]] of [[Grenoble]] in France announced the discovery of [[CFBDSIR 2149-0403]]; an independently moving 4–7 Jupiter-mass object that likely forms part of the [[AB Doradus moving group]], less than 100 light years from Earth. Although it shares its spectrum with a [[T dwarf|spectral class T brown dwarf]], Delorme speculates that it may be a planet.<ref>{{cite journal|title=CFBDSIR2149-0403: a 4–7 Jupiter-mass free-floating planet in the young moving group AB Doradus?|author1=P. Delorme |author2=J. Gagn´e |author3=L. Malo |author4=C. Reyl´e |author5=E. Artigau |author6=L. Albert |author7=T. Forveille |author8=X. Delfosse |author9=F. Allard |author10=D. Homeier |journal=Astronomy & Astrophysics|year=2012|arxiv=1210.0305|doi=10.1051/0004-6361/201219984|bibcode=2012A&A...548A..26D|volume=548|pages=A26|s2cid=50935950 }}</ref>

In October 2013, astronomers led by Dr. Michael Liu of the [[University of Hawaii]] discovered [[PSO J318.5-22]], a solitary free-floating [[L dwarf]] estimated to possess only 6.5 times the mass of Jupiter, making it the least massive [[sub-brown dwarf]] yet discovered.<ref name="liu_discovery">
{{cite journal | title = The Extremely Red, Young L Dwarf PSO J318-22: A Free-Floating Planetary-Mass Analog to Directly Imaged Young Gas-Giant Planets | journal = Astrophysical Journal Letters | date = October 1, 2013 |author = Liu, Michael C. |author2= Magnier, Eugene A. |author3= Deacon, Niall R. |author4= Allers, Katelyn N. |author5= Dupuy, Trent J. |author6= Kotson, Michael C. |author7= Aller, Kimberly M. |author8= Burgett, W. S. |author9= Chambers, K. C. |author10= Draper, P. W. |author11= Hodapp, K. W. |author12= Jedicke, R. |author13= Kudritzki, R.-P. |author14= Metcalfe, N. |author15= Morgan, J. S. |author16= Kaiser, N. |author17= Price, P. A. |author18= Tonry, J. L. |author19=  Wainscoat, R. J. | volume = 777 | issue = 2 | doi =10.1088/2041-8205/777/2/L20 | arxiv=1310.0457|bibcode = 2013ApJ...777L..20L | pages=L20| s2cid = 54007072 }}</ref>

In 2019, astronomers at the [[Calar Alto Observatory]] in Spain identified GJ3512b, a gas giant about half the mass of Jupiter orbiting around the red dwarf star [[GJ3512]] in 204 days. Such a large gas giant around such a small star at such a wide orbit is highly unlikely to have formed via accretion, and is more likely to have formed by fragmentation of the disc, similar to a star.<ref>{{cite web|title=Exoplanet discovery blurs the line between large planets and small stars|author=Andrew Norton|date=September 27, 2019|url=https://phys.org/news/2019-09-exoplanet-discovery-blurs-line-large.html|publisher=phys.org|access-date=March 13, 2020}}</ref>

=== Semantics ===
Finally, from a purely linguistic point of view, there is the dichotomy that the IAU created between 'planet' and 'dwarf planet'. The term 'dwarf planet' arguably contains two words, a noun (planet) and an adjective (dwarf). Thus, the term could suggest that a dwarf planet is a type of planet, even though the IAU explicitly defines a dwarf planet as ''not'' so being. By this formulation therefore, 'dwarf planet' and '[[minor planet]]' are best considered [[compound noun]]s. [[Benjamin Zimmer]] of [[Language Log]] summarised the confusion: "The fact that the IAU would like us to think of dwarf planets as distinct from 'real' planets lumps the lexical item 'dwarf planet' in with such oddities as '[[Welsh rabbit]]' (not really a rabbit) and '[[Rocky Mountain oysters]]' (not really oysters)."<ref name=langlog>{{cite web | last = Zimmer | first = Benjamin | author-link = Benjamin Zimmer |title=New planetary definition a "linguistic catastrophe"!| work=[[Language Log]]| url=http://itre.cis.upenn.edu/~myl/languagelog/archives/003504.html| access-date=October 4, 2006}}</ref> As [[Dava Sobel]], the historian and popular science writer who participated in the IAU's initial decision in October 2006, noted in an interview with [[National Public Radio]], "A dwarf planet is not a planet, and in astronomy, there are dwarf stars, which are stars, and dwarf galaxies, which are galaxies, so it's a term no one can love, dwarf planet."<ref>{{cite web|title=A Travel Guide to the Solar System |year=2006 |work=National Public Radio |url=http://www.sciencefriday.com/pages/2006/Oct/hour2_102706.html |access-date=November 18, 2006 |url-status=dead |archive-url=https://web.archive.org/web/20061107104814/http://www.sciencefriday.com/pages/2006/Oct/hour2_102706.html |archive-date=November 7, 2006 }}</ref>  Mike Brown noted in an interview with the Smithsonian that "Most of the people in the dynamical camp really did not want the word 'dwarf planet', but that was forced through by the pro-Pluto camp. So you're left with this ridiculous baggage of dwarf planets not being planets."<ref name=smithsonian>{{cite web|title=Pluto's Planethood: What Now? |work=Air and Space |url=http://www.airspacemag.com/issues/2006/august-september/FEATURE-PlutoDebate.php?page=1 |archive-url=https://archive.today/20130101211205/http://www.airspacemag.com/issues/2006/august-september/FEATURE-PlutoDebate.php?page=1 |url-status=dead |archive-date=January 1, 2013 |year=2006 |access-date=August 21, 2007 }}</ref>

Conversely, astronomer Robert Cumming of the Stockholm Observatory notes that, "The name 'minor planet' [has] been more or less synonymous with 'asteroid' for a very long time. So it seems to me pretty insane to complain about any ambiguity or risk for confusion with the introduction of 'dwarf planet'."<ref name=langlog />

== See also ==
{{Wiktionary|planet}}
{{cmn|
* [[Geophysical definition of planet|Geophysical definition of ''planet'']]
* [[IAU definition of planet|IAU definition of ''planet'']]
* [[List of gravitationally rounded objects of the Solar System]]
* [[List of former planets]]
* [[Mesoplanet]]
* [[Natural kind]]
* [[Planemo]]
* [[Planetar (astronomy)]]
* [[Planetesimal]]
* [[Planets in astrology]]
* [[Rogue planet]]
* [[Sub-brown dwarf]]
* [[Timeline of discovery of Solar System planets and their moons]]
}}

== Notes ==
{{refbegin}}
<ol type="a">

<li>{{Note label|B|b|none}} Defined as the region occupied by two bodies whose orbits cross a common distance from the Sun, if their orbital periods differ less than an order of magnitude. In other words, if two bodies occupy the same distance from the Sun at one point in their orbits, and those orbits are of similar size, rather than, as a [[comet]]'s would be, extending for several times the other's distance, then they are in the same orbital zone.<ref>{{cite journal| title= What is a Planet?| first=Steven| last=Soter| date=August 16, 2006| doi= 10.1086/508861| journal= The Astronomical Journal| volume= 132| issue= 6| pages= 2513–2519|arxiv=astro-ph/0608359|bibcode = 2006AJ....132.2513S | s2cid=14676169}} submitted to The Astronomical Journal, August 16, 2006</ref></li>
<li>{{Note label|C|c|none}}In 2002, in collaboration with dynamicist Harold Levison, Stern wrote, "we define an ''überplanet'' as a planetary body in orbit around a [[star]] that is dynamically important enough to have cleared its neighboring planetesimals ... And we define an ''unterplanet'' as one that has not been able to do so," and then a few paragraphs later, "our [[Solar System]] clearly contains 8 überplanets and a far larger number of unterplanets, the largest of which are [[Pluto]] and [[Ceres (dwarf planet)|Ceres]]."<ref name="Stern 2002">{{cite journal | last=Stern | first=S. Alan |author2=Levison, Harold F. | year=2002 | title=Regarding the criteria for planethood and proposed planetary classification schemes | url=http://www.boulder.swri.edu/~hal/PDF/planet_def.pdf | journal=Highlights of Astronomy| volume=12 | pages=205–213, as presented at the XXIVth General Assembly of the IAU–2000 [Manchester, UK, August 7–18, 2000]|bibcode = 2002HiA....12..205S | doi=10.1017/S1539299600013289 | doi-access=free }}</ref> While this may appear to contradict Stern's objections, Stern noted in an interview with Smithsonian Air and Space that, unlike the IAU's definition, his definition still allows unterplanets to be planets: "I do think from a dynamical standpoint, there are planets that really matter in the architecture of the solar system, and those that don't. They're both planets. Just as you can have wet and dry planets, or life-bearing and non-life-bearing planets, you can have dynamically important planets and dynamically unimportant planets."<ref name=smithsonian /></li>
<li>{{Note label|D|d|none}}The density of an object is a rough guide to its composition: the lower the density, the higher the fraction of ices, and the lower the fraction of rock. The denser objects, Vesta and Juno, are composed almost entirely of rock with very little ice, and have a density close to the [[Moon]]'s, while the less dense, such as Proteus and Enceladus, are composed mainly of ice.<ref>{{cite journal | year=1997 |author1=Righter, Kevin |author2=Drake, Michael J. | title=A magma ocean on Vesta: Core formation and petrogenesis of eucrites and diogenites | bibcode=1997M&PS...32..929R | doi=10.1111/j.1945-5100.1997.tb01582.x | journal=Meteoritics & Planetary Science | volume=32 | issue=6 | pages=929–944|s2cid=128684062 | doi-access=free }}</ref><ref>{{cite web |year=2003 |author1=Johanna Torppa |author2=Mikko Kaasalainen |author3=Tadeusz Michałowski |author4=Tomasz Kwiatkowski |author5=Agnieszka Kryszczyńska |author6=Peter Denchev |author7=Richard Kowalski |title=Shapes and rotational properties of thirty asteroids from photometric data |work=Astronomical Observatory, Adam Mickiewicz University |url=http://www.rni.helsinki.fi/~mjk/thirty.pdf |access-date=May 25, 2006 |archive-date=November 6, 2015 |archive-url=https://web.archive.org/web/20151106103549/http://www.rni.helsinki.fi/~mjk/thirty.pdf |url-status=dead }}</ref></li></ol>
{{refend}}

== References ==
{{reflist|30em}}

== Bibliography and external links ==
{{refbegin}}
* [https://web.archive.org/web/20071015191648/http://www.sciam.com/article.cfm?chanID=sa006&articleID=93385350-E7F2-99DF-3FD6272BB4959038&pageNumber=2&catID=2 What is a planet?] -Steven Soter
* [http://www.space.com/scienceastronomy/061121_exoplanet_definition.html Why Planets will never be defined: Robert Roy Britt on the outcome of the IAU's decision]
* {{cite news|last=Nunberg|first=Earth|title=Dwarfing Pluto|newspaper=NPR.org|publisher=NPR|url=https://www.npr.org/templates/story/story.php?storyId=5723794|date=August 28, 2006|access-date=April 13, 2007}} An examination of the redefinition of Pluto from a linguistic perspective.
* [[The Pluto Files]] by [[Neil deGrasse Tyson]] The Rise and Fall of America's Favorite Planet   
* [http://news.bbc.co.uk/1/hi/sci/tech/4798205.stm Q&A New planets proposal] Wednesday, August 16, 2006, 13:36 GMT 14:36 UK
* [http://www2.ess.ucla.edu/~jewitt/kb/pluto.html David Jewitt's Kuiper Belt page- Pluto]
* [http://www.icq.eps.harvard.edu/ICQPluto.html Dan Green's webpage: What is a planet?]
* [https://web.archive.org/web/20050912085702/http://www.space.com/scienceastronomy/solarsystem/planet_confusion_001101-2.html What is a Planet? Debate Forces New Definition]
* [http://www.infoplease.com/ipa/A0776605.html?mail-07-29 The Flap Over Pluto]
* [https://web.archive.org/web/20050805001650/http://slate.msn.com/id/2123839/ "You Call That a Planet?: How astronomers decide whether a celestial body measures up."]
* David Darling. ''The Universal Book of Astronomy, from the Andromeda Galaxy to the Zone of Avoidance.'' 2003. John Wiley & Sons Canada ({{ISBN|0-471-26569-1}}), p.&nbsp;394
* ''Collins Dictionary of Astronomy'', 2nd ed. 2000. HarperCollins Publishers ({{ISBN|0-00-710297-6}}), p.&nbsp;312-4.
* [https://arxiv.org/abs/astro-ph/0607184 Catalogue of Planetary Objects. Version 2006.0] O.V. Zakhozhay, V.A. Zakhozhay, Yu.N. Krugly, 2006
* [http://kuffner-sternwarte.at/im_brennp/archiv2006/Planeten_Definition_Resolution_5_6_7.html The New Proposal, Resolution 5, 6 and 7] 2006-08-22
* [https://web.archive.org/web/20081208140503/http://www.astronomy2006.com/media-stream-archive.php IAU 2006 General Assembly: video-records of the discussion and of the final vote on the Planet definition.]
* Boyle, Alan, ''The Case for Pluto'' The Case for Pluto Book by MSNBC Science Editor and author of "Cosmic Log"
* Croswell, Dr. Ken "Pluto Question" [http://kencroswell.com/PlutoQuestion.html Pluto Question]
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