Editing Planet (section)

== History and etymology ==
{{Further|History of astronomy|Timeline of Solar System astronomy}}The idea of planets has evolved over the history of astronomy, 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 multitudes of other extrasolar systems. The consensus as to what counts as a planet, as opposed to other objects, has changed several times. It previously encompassed [[asteroid]]s, [[natural satellite|moons]], and [[dwarf planet]]s like [[Pluto]],<ref>{{Cite web |title=What is a Planet? {{!}} Planets |url=https://solarsystem.nasa.gov/planets/in-depth |access-date=2 May 2022 |website=NASA Solar System Exploration |archive-date=26 April 2022 |archive-url=https://web.archive.org/web/20220426123245/https://solarsystem.nasa.gov/planets/in-depth/ |url-status=live }}</ref><ref name="asteroids"/><ref name=metzger22/> and there continues to be some disagreement today.<ref name=metzger22/>

=== Ancient civilizations and classical planets ===
[[File:Apparent retrograde motion of Mars in 2003.gif|thumb|The motion of 'lights' moving across the sky is the basis of the classical definition of planets: wandering stars.]]
The five [[classical planet]]s of the [[Solar System]], being visible to the naked eye, have been known since ancient times and have had a significant impact on [[mythology]], [[religious cosmology]], and ancient [[astronomy]]. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the "[[fixed stars]]", which maintained a constant relative position in the sky.<ref>{{cite web|title=Ancient Greek Astronomy and Cosmology|publisher=[[The Library of Congress]]|url=https://www.loc.gov/collections/finding-our-place-in-the-cosmos-with-carl-sagan/articles-and-essays/modeling-the-cosmos/ancient-greek-astronomy-and-cosmology|access-date=19 May 2016|archive-date=1 May 2015|archive-url=https://web.archive.org/web/20150501051354/https://www.loc.gov/collections/finding-our-place-in-the-cosmos-with-carl-sagan/articles-and-essays/modeling-the-cosmos/ancient-greek-astronomy-and-cosmology|url-status=live}}</ref> Ancient Greeks called these lights {{lang|grc|[[wikt:πλάνης|πλάνητες]] [[wikt:ἀστήρ|ἀστέρες]]}} ({{Transliteration|grc|planētes asteres}}) {{gloss|wandering stars}} or simply {{lang|grc|[[wikt:πλανήτης#Ancient Greek|πλανῆται]]}} ({{Transliteration|grc|planētai}}) {{gloss|wanderers}}<ref>{{LSJ|pla/nhs|πλάνης}}, {{LSJ|planh/ths|πλανήτης|ref}} Retrieved on 11 July 2022.</ref> from which today's word "planet" was derived.<ref>{{cite web |url=http://www.merriam-webster.com/dictionary/planet |title=Definition of planet |publisher=Merriam-Webster OnLine |access-date=23 July 2007 |archive-date=1 June 2012 |archive-url=https://web.archive.org/web/20120601063256/http://www.merriam-webster.com/dictionary/planet |url-status=live }}</ref><ref>{{cite encyclopedia|url=http://dictionary.reference.com/browse/planet|title=''Planet'' Etymology|dictionary=[[dictionary.com]]|access-date=29 June 2015|archive-date=2 July 2015|archive-url=https://web.archive.org/web/20150702151908/http://dictionary.reference.com/browse/planet|url-status=live}}</ref><ref name="oed" /> In [[ancient Greece]], [[History of China#Ancient China|China]], [[Babylon]], and indeed all pre-modern civilizations,<ref>{{cite journal |first=Otto E. |last=Neugebauer |date=1945 |title=The History of Ancient Astronomy Problems and Methods |journal=Journal of Near Eastern Studies |volume=4 |issue=1 |pages=1–38 |doi=10.1086/370729|s2cid=162347339 }}</ref><ref>{{cite book |first=Colin |last=Ronan |title=Astronomy in China, Korea and Japan |editor=Walker, Christopher |chapter=Astronomy Before the Telescope |publisher=British Museum Press |pages=264–265 |date=1996|bibcode=1996abt..conf..245R }}</ref> it was almost universally believed that Earth was the [[Geocentric|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<ref>{{cite book |first=Thomas S. |last=Kuhn |title=The Copernican Revolution |url=https://archive.org/details/copernicanrevolu0008kuhn |url-access=registration |pages=[https://archive.org/details/copernicanrevolu0008kuhn/page/5 5–20] |publisher=Harvard University Press |date=1957 |isbn=978-0-674-17103-9}}</ref> and the apparently [[common sense|common-sense]] perceptions that Earth was solid and stable and that it was not moving but at rest.<ref name="TMU">{{Cite book |last1=Frautschi |first1=Steven C. |title=The Mechanical Universe: Mechanics and Heat |title-link=The Mechanical Universe |last2=Olenick |first2=Richard P. |last3=Apostol |first3=Tom M. |last4=Goodstein |first4=David L. |date=2007 |publisher=Cambridge University Press |isbn=978-0-521-71590-4 |edition=Advanced |location=Cambridge [Cambridgeshire] |oclc=227002144 |author-link=Steven Frautschi |author-link3=Tom M. Apostol |author-link4=David L. Goodstein |page=58}}</ref>

==== Babylon ====
{{Main|Babylonian astronomy}}

The first civilization known to have a functional theory of the planets were the [[Babylonia]]ns, 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.<ref name="practice" /> The [[MUL.APIN]] is a pair of [[cuneiform]] tablets dating from the 7th century BC that lays out the motions of the Sun, Moon, and planets over the course of the year.<ref>{{cite book | first=Francesca | last=Rochberg | chapter=Astronomy and Calendars in Ancient Mesopotamia | title=Civilizations of the Ancient Near East | volume=III | editor=Jack Sasson | date=2000 | page=1930 }}</ref> Late Babylonian astronomy is the origin of Western astronomy and indeed all Western efforts in the [[exact science]]s.<ref name="Aaboe, Asger">{{ Citation | last = Aaboe | first = Asger | author-link = Asger Aaboe | editor-last = Boardman | editor-first = John | editor-link = John Boardman (art historian) | editor2-last = Edwards | editor2-first = I. E. S. | editor2-link = I. E. S. Edwards | editor3-last = Hammond | editor3-first = N. G. L. | editor3-link =  N. G. L. Hammond | editor4-last = Sollberger | editor4-first = E. | editor5-last = Walker | editor5-first = C. B. F | date = 1991 | title = The Assyrian and Babylonian Empires and other States of the Near East, from the Eighth to the Sixth Centuries B.C. | chapter = The culture of Babylonia: Babylonian mathematics, astrology, and astronomy | series = The Cambridge Ancient History | volume = 3 | issue = 2 | publisher = Cambridge University Press | location = Cambridge | pages = 276–292 | isbn = 978-0521227179 }}</ref> The ''[[Enuma anu enlil]]'', written during the [[Neo-Assyrian]] period in the 7th century BC,<ref>{{cite book | volume=8 |series=State Archives of Assyria |title=Astrological reports to Assyrian kings |editor=Hermann Hunger |date=1992 |publisher=Helsinki University Press |isbn=978-951-570-130-5}}</ref> comprises a list of [[omen]]s and their relationships with various celestial phenomena including the motions of the planets.<ref>{{cite journal |title=Babylonian Planetary Omens. Part One. Enuma Anu Enlil, Tablet 63: The Venus Tablet of Ammisaduqa. |first1=W. G. |last1=Lambert |date=1987 |journal=Journal of the American Oriental Society |doi=10.2307/602955 |volume=107 |issue=1 |last2=Reiner |first2=Erica |jstor=602955 |pages=93–96}}</ref><ref name="ancientmes">{{cite journal |url=http://www.folklore.ee/Folklore/vol16/planets.pdf |last1=Kasak |first1=Enn |last2=Veede |first2=Raul |title=Understanding Planets in Ancient Mesopotamia |journal=Electronic Journal of Folklore |access-date=6 February 2008 |volume=16 |date=2001 |pages=7–35 |editor=Mare Kõiva |editor2=Andres Kuperjanov |doi=10.7592/fejf2001.16.planets |citeseerx=10.1.1.570.6778 |archive-date=4 February 2019 |archive-url=https://web.archive.org/web/20190204141254/http://www.folklore.ee/folklore/vol16/planets.pdf |url-status=live }}</ref> The [[inferior planet]]s [[Venus]] and [[Mercury (planet)|Mercury]] and the superior planets [[Mars]], [[Jupiter]], and [[Saturn]] were all identified by [[Babylonian astronomy|Babylonian astronomers]]. These would remain the only known planets until the invention of the [[telescope]] in early modern times.<ref>{{cite journal |title=Babylonian Observational Astronomy |first=A. |last=Sachs |journal=[[Philosophical Transactions of the Royal Society]] |volume=276 |issue=1257 |date=2 May 1974 |pages=43–50 [45 & 48–49] |jstor=74273 |doi=10.1098/rsta.1974.0008 |bibcode=1974RSPTA.276...43S|s2cid=121539390 }}</ref>

==== Greco-Roman astronomy ====
{{See also|Ancient Greek astronomy}}

The [[Ancient Greece|ancient Greeks]] initially did not attach as much significance to the planets as the Babylonians. In the 6th and 5th centuries BC, the [[Pythagoreans]] appear to have developed [[Pythagorean astronomical system|their own independent planetary theory]], which consisted of the Earth, Sun, Moon, and planets revolving around a "Central Fire" at the center of the Universe. [[Pythagoras]] or [[Parmenides]] is said to have been the first to identify the evening star ([[Hesperos]]) and morning star ([[Phosphoros]]) as one and the same ([[Aphrodite]], Greek corresponding to Latin [[Venus]]),<ref name="burnet">{{cite book | first=John |last=Burnet |title= Greek philosophy: Thales to Plato |date=1950 |publisher=Macmillan and Co. |pages=7–11 |url=https://books.google.com/books?id=7yUAmmqHHEgC&pg=PR4 |access-date=7 February 2008 |isbn=978-1-4067-6601-1}}</ref> though this had long been known in Mesopotamia.<ref name=Cooley>{{cite journal |last=Cooley |first=Jeffrey L. |title=Inana and Šukaletuda: A Sumerian Astral Myth |url=https://www.academia.edu/1247599 |journal=KASKAL |volume=5 |pages=161–172 |year=2008 |issn=1971-8608 |quote=The Greeks, for example, originally identified the morning and evening stars with two separate deities, Phosphoros and Hesporos respectively. In Mesopotamia, it seems that this was recognized prehistorically. Assuming its authenticity, a cylinder seal from the Erlenmeyer collection attests to this knowledge in southern Iraq as early as the Late Uruk / Jemdet Nasr Period, as do the archaic texts of the period. [...] Whether or not one accepts the seal as authentic, the fact that there is no epithetical distinction between the morning and evening appearances of Venus in any later Mesopotamian literature attests to a very, very early recognition of the phenomenon. |access-date=26 November 2022 |archive-date=24 December 2019 |archive-url=https://web.archive.org/web/20191224105634/https://www.academia.edu/1247599 |url-status=live }}</ref><ref>{{Cite journal |last=Kurtik |first=G. E. |date=June 1999 |title=The identification of Inanna with the planet Venus: A criterion for the time determination of the recognition of constellations in ancient Mesopotamia |url=http://www.tandfonline.com/doi/abs/10.1080/10556799908244112 |journal=Astronomical & Astrophysical Transactions |language=en |volume=17 |issue=6 |pages=501–513 |doi=10.1080/10556799908244112 |bibcode=1999A&AT...17..501K |issn=1055-6796 |access-date=13 July 2022 |archive-date=16 June 2022 |archive-url=https://web.archive.org/web/20220616151834/https://www.tandfonline.com/doi/abs/10.1080/10556799908244112 |url-status=live }}</ref> In the 3rd century BC, [[Aristarchus of Samos]] proposed a [[Heliocentrism|heliocentric]] system, according to which Earth and the planets revolved around the Sun. The geocentric system remained dominant until the [[Scientific Revolution]].<ref name="TMU"/>

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.<ref name="practice" /><ref name="almagest" /> To the Greeks and Romans, there were seven known planets, each presumed to be [[Geocentric model|circling Earth]] according to the complex laws laid out by Ptolemy. They were, in increasing order from Earth (in Ptolemy's order and using modern names): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn.<ref name="oed">{{cite encyclopedia | url= http://dictionary.oed.com/cgi/entry/50180718?query_type=word&queryword=planet | dictionary= Oxford English Dictionary | title= planet, n | access-date= 7 February 2008 | date= 2007 | archive-date= 3 July 2012 | archive-url= https://web.archive.org/web/20120703072456/http://oed.com/public/redirect/welcome-to-the-new-oed-online | url-status= live }} ''Note: select the Etymology tab ''</ref><ref name="almagest">{{cite journal |first=Bernard R. |last=Goldstein |title=Saving the phenomena: the background to Ptolemy's planetary theory | journal=Journal for the History of Astronomy |volume=28 |issue=1 |date=1997 |pages=1–12 |bibcode=1997JHA....28....1G|doi=10.1177/002182869702800101 |s2cid=118875902 }}</ref><ref>{{cite book |title=Ptolemy's Almagest |author1= Ptolemy |author-link=Ptolemy |author2=Toomer, G. J. |author2-link=G. J. Toomer |publisher=Princeton University Press |date=1998 |isbn=978-0-691-00260-6}}</ref>

=== Medieval astronomy ===
{{Main|Astronomy in the medieval Islamic world|Indian astronomy}}[[File:1660 illustration of Claudius Ptolemy's geocentric model of the Universe.jpg|thumb|upright=1.5|1660 illustration of Claudius Ptolemy's geocentric model]]

After the [[fall of the Western Roman Empire]], astronomy developed further in India and the medieval Islamic world. 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 theorized that the orbits of planets were [[Ellipse|elliptical]].<ref>{{cite web | first1=J. J. | last1=O'Connor | first2=E. F. | last2=Robertson | url=https://mathshistory.st-andrews.ac.uk/Biographies/Aryabhata_I/ | title=Aryabhata the Elder | website=[[MacTutor History of Mathematics archive]] | access-date=10 July 2022 | archive-date=1 February 2022 | archive-url=https://web.archive.org/web/20220201090937/https://mathshistory.st-andrews.ac.uk/Biographies/Aryabhata_I/ | url-status=live }}</ref> 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.<ref>{{cite book | author-link=K. V. Sarma | last=Sarma | first=K. V. | year=1997 |chapter= Astronomy in India| editor-link=Helaine Selin | editor-last=Selin | editor-first=Helaine |title=Encyclopaedia of the History of Science, Technology, and Medicine in Non-Western Cultures | publisher=Kluwer Academic Publishers | isbn=0-7923-4066-3 | page=116 }}</ref>

The astronomy of the [[Islamic Golden Age]] mostly took place in the [[Middle East]], [[Central Asia]], [[Al-Andalus]], and [[North Africa]], and later in the [[Far East]] and India. These astronomers, like the polymath [[Ibn al-Haytham]], generally accepted geocentrism, although they did dispute Ptolemy's system of epicycles and sought alternatives. The 10th-century astronomer [[Abu Sa'id al-Sijzi]] accepted that the Earth rotates around its axis.<ref name=Scientia>{{Cite journal| volume = 108| issue = 67| page = 762| first = Alessandro| last = Bausani| title = Cosmology and Religion in Islam| journal = Scientia/Rivista di Scienza| date = 1973}}</ref> In the 11th century, the [[transit of Venus]] was observed by [[Avicenna]].<ref>{{cite encyclopedia |title=Ibn Sīnā: Abū ʿAlī al-Ḥusayn ibn ʿAbdallāh ibn Sīnā |first=Sally P. |last= Ragep |editor=Thomas Hockey |encyclopedia=The Biographical Encyclopedia of Astronomers |publisher=[[Springer Science+Business Media]] |date=2007 |pages=570–572 |doi=10.1888/0333750888/3736 |bibcode=2000eaa..bookE3736. |isbn=978-0-333-75088-9|chapter=Ibn Sina, Abu Ali [known as Avicenna] (980?1037) }}</ref> His contemporary [[Al-Biruni]] devised a method of determining the Earth's radius using [[trigonometry]] that, unlike the older method of [[Eratosthenes]], only required observations at a single mountain.<ref name="The Lost Art of Finding Our Way">{{Cite book|url={{google books |plainurl=y |id=9QAuAAAAQBAJ |page=216}}|title= The Lost Art of Finding Our Way |year=2013|publisher=Harvard University Press|isbn=978-0-674-07282-4 |first=John Edward |last=Huth|pages=216–217}}</ref>

=== Scientific Revolution and discovery of outer planets ===
{{See also|Heliocentrism}}
[[File:The_Solar_System,_with_the_orbits_of_5_remarkable_comets._LOC_2013593161.jpg|thumb|True-scale Solar System poster made by [[Emanuel Bowen]] in 1747. At that time, Uranus, Neptune, and the asteroid belts had all not yet been discovered.]]
With the advent of the [[Scientific Revolution]] and the [[heliocentric model]] of [[Copernicus]], [[Galileo]], and [[Kepler]], use of the term "planet" changed from something that moved around the sky relative to the [[fixed star]] to a body that orbited the Sun, directly (a primary planet) or indirectly (a secondary or satellite planet). Thus the Earth was added to the roster of planets,<ref name="galileo_project" /> and the Sun was removed. The Copernican count of primary planets stood until 1781, when [[William Herschel]] discovered [[Uranus]].<ref name="Dreyer">{{cite book|first=J. L. E. | last=Dreyer |year=1912|title=The Scientific Papers of Sir William Herschel|publisher=Royal Society and Royal Astronomical Society|volume=1|page=100|author-link=J. L. E. Dreyer|url=https://archive.org/details/scientificpapers032804mbp/page/100/mode/2up}}</ref>

When four satellites of Jupiter (the [[Galilean moons]]) and five of Saturn were discovered in the 17th century, they joined Earth's Moon in the category of "satellite planets" or "secondary planets" orbiting the primary planets, though in the following decades they would come to be called simply "satellites" for short. Scientists generally considered planetary satellites to also be planets until about the 1920s, although this usage was not common among non-scientists.<ref name=metzger22>{{cite journal |last1=Metzger |first1=Philip T. |author-link1=Philip T. Metzger |last2=Grundy |first2=W. M. |first3=Mark V. |last3=Sykes |first4=Alan |last4=Stern |first5=James F. |last5=Bell III |first6=Charlene E. |last6=Detelich |first7=Kirby |last7=Runyon |first8=Michael |last8=Summers |date=2022 |title=Moons are planets: Scientific usefulness versus cultural teleology in the taxonomy of planetary science |url=https://www.sciencedirect.com/science/article/pii/S0019103521004206 |journal=Icarus |volume=374 |issue= |page=114768 |doi=10.1016/j.icarus.2021.114768 |arxiv=2110.15285 |bibcode=2022Icar..37414768M |s2cid=240071005 |access-date=8 August 2022 |archive-date=11 September 2022 |archive-url=https://web.archive.org/web/20220911060134/https://www.sciencedirect.com/science/article/pii/S0019103521004206 |url-status=live }}</ref>

In the first decade of the 19th century, four new 'planets' were discovered: [[Ceres (dwarf planet)|Ceres]] (in 1801), [[2 Pallas|Pallas]] (in 1802), [[3 Juno|Juno]] (in 1804), and [[4 Vesta|Vesta]] (in 1807). It soon became apparent that they were rather different from previously known planets: they shared the same general region of space, between Mars and Jupiter (the [[asteroid belt]]), with sometimes overlapping orbits. This was an area where only one planet had been expected, and they were much smaller than all other planets; indeed, it was suspected that they might be shards of a larger planet that had broken up. Herschel called them ''[[asteroid]]s'' (from the Greek for "starlike") because even in the largest telescopes they resembled stars, without a resolvable disk.<ref name="asteroids" /><ref>{{cite OED|asteroid}}</ref>

The situation was stable for four decades, but in the 1840s several additional asteroids were discovered ([[5 Astraea|Astraea]] in 1845; [[6 Hebe|Hebe]], [[7 Iris|Iris]], and [[8 Flora|Flora]] in 1847; [[9 Metis|Metis]] in 1848; and [[10 Hygiea|Hygiea]] in 1849). New "planets" were discovered every year; as a result, astronomers began tabulating the asteroids ([[minor planet]]s) separately from the major planets and assigning them numbers instead of abstract [[planetary symbol]]s,<ref name="asteroids">{{cite web | last =Hilton |first =James L. |date = 17 September 2001 |url =http://aa.usno.navy.mil/faq/docs/minorplanets.php |title =When Did the Asteroids Become Minor Planets? |publisher =U.S. Naval Observatory |access-date = 8 April 2007 |archive-url = https://web.archive.org/web/20070921162818/http://aa.usno.navy.mil/faq/docs/minorplanets.php |archive-date = 21 September 2007}}</ref> although they continued to be considered as small planets.<ref name=metzger19/>

[[Discovery of Neptune|Neptune was discovered in 1846]], its position having been predicted thanks to its gravitational influence upon Uranus. Because the orbit of Mercury appeared to be affected in a similar way, it was believed in the late 19th century that there might be [[Vulcan (hypothetical planet)|another planet even closer to the Sun]]. However, the discrepancy between Mercury's orbit and the predictions of Newtonian gravity was instead explained by an improved theory of gravity, Einstein's [[general relativity]].<ref>{{cite book | first1=Richard P. | last1=Baum  | first2=William | last2=Sheehan | title=In Search of Planet Vulcan: The Ghost in Newton's Clockwork | publisher=Basic Books | year=2003 | page=264 | isbn=978-0738208893 }}</ref><ref>{{cite journal | last1 = Park | first1 = Ryan S. |  last2 = Folkner  | first2= William M. |last3 = Konopliv |first3 = Alexander S. | last4 = Williams | first4 = James G. |last5 = Smith |first5 = David E. |last6 =  Zuber | first6 = Maria T.| display-authors = 4 | year = 2017 | title = Precession of Mercury's Perihelion from Ranging to the MESSENGER Spacecraft | journal = The Astronomical Journal | volume = 153 | issue = 3| page = 121 | doi=10.3847/1538-3881/aa5be2| bibcode = 2017AJ....153..121P | hdl = 1721.1/109312 | s2cid = 125439949 | hdl-access = free | doi-access = free }}</ref>

[[Pluto]] was discovered in 1930. After initial observations led to the belief that it was larger than Earth,<ref>{{cite book |title=Planet Quest: The Epic Discovery of Alien Solar Systems |first=Ken |last=Croswell |author-link=Ken Croswell |publisher=The Free Press |date=1997 |page=57 |isbn=978-0-684-83252-4 }}</ref> the object was immediately accepted as the ninth major planet. Further monitoring found the body was actually much smaller: in 1936, [[Raymond Arthur Lyttleton|Ray Lyttleton]] suggested that Pluto may be an escaped satellite of [[Neptune]],<ref>{{cite journal | last=Lyttleton |first=Raymond A. |date=1936 |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=97 |issue=2 |pages=108–115 |title= On the possible results of an encounter of Pluto with the Neptunian system |bibcode=1936MNRAS..97..108L |doi=10.1093/mnras/97.2.108|doi-access=free }}</ref> and [[Fred Lawrence Whipple|Fred Whipple]] suggested in 1964 that Pluto may be a comet.<ref>{{cite journal | journal=Proceedings of the National Academy of Sciences of the United States of America |volume=52 |pages=565–594 |last=Whipple |first=Fred |date=1964 |bibcode=1964PNAS...52..565W |title= The History of the Solar System |doi= 10.1073/pnas.52.2.565 | pmid=16591209 | issue=2 | pmc=300311|doi-access=free }}</ref> The discovery of its large moon [[Charon (moon)|Charon]] in 1978 showed that Pluto was only 0.2% the mass of Earth.<ref name="ChristyHarrington1978">{{cite journal| first1 = James W.| last1 = Christy| first2 = Robert Sutton| last2 = Harrington| author-link2 = Robert Sutton Harrington| title = The Satellite of Pluto| journal = Astronomical Journal| date = 1978| volume = 83| issue = 8| pages = 1005–1008| bibcode = 1978AJ.....83.1005C| doi = 10.1086/112284| s2cid = 120501620}}</ref> As this was still substantially more massive than any known asteroid, and because no other [[trans-Neptunian objects]] had been discovered at that time, Pluto kept its planetary status, only officially losing it in 2006.<ref>{{cite journal | journal=Scientific American |date=1996 |pages=46–52 |last1=Luu |first1=Jane X. |last2=Jewitt |first2=David C. |title=The Kuiper Belt |volume=274 |issue=5 |doi=10.1038/scientificamerican0596-46|bibcode = 1996SciAm.274e..46L }}</ref><ref name="Pluto loses status as a planet-2006">{{cite news |date=24 August 2006 |title=Pluto loses status as a planet |url=http://news.bbc.co.uk/1/hi/world/5282440.stm |url-status=live |archive-url=https://web.archive.org/web/20120530155226/http://news.bbc.co.uk/2/hi/5282440.stm |archive-date=30 May 2012 |access-date=23 August 2008 |department=[[BBC News]] |publisher=[[British Broadcasting Corporation]]}}</ref>

In the 1950s, [[Gerard Kuiper]] published papers on the origin of the asteroids. He recognized that asteroids were typically not spherical, as had previously been thought, and that the [[asteroid family|asteroid families]] were remnants of collisions. Thus he differentiated between the largest asteroids as "true planets" versus the smaller ones as collisional fragments. From the 1960s onwards, the term "minor planet" was mostly displaced by the term "asteroid", and references to the asteroids as planets in the literature became scarce, except for the geologically evolved largest three: Ceres, and less often Pallas and Vesta.<ref name=metzger19>{{cite journal |last1=Metzger |first1=Philip T. |author-link1=Philip T. Metzger |last2=Sykes |first2=Mark V. |last3=Stern |first3=Alan |last4=Runyon |first4=Kirby |date=2019 |title=The Reclassification of Asteroids from Planets to Non-Planets |journal=Icarus |volume=319 |pages=21–32 |doi=10.1016/j.icarus.2018.08.026|arxiv=1805.04115 |bibcode=2019Icar..319...21M |s2cid=119206487 }}</ref>

The beginning of Solar System exploration by space probes in the 1960s spurred a renewed interest in planetary science. A split in definitions regarding satellites occurred around then: planetary scientists began to reconsider the large moons as also being planets, but astronomers who were not planetary scientists generally did not.<ref name=metzger22/> (This is not exactly the same as the definition used in the previous century, which classed ''all'' satellites as secondary planets, even non-round ones like Saturn's [[Hyperion (moon)|Hyperion]] or Mars's [[Phobos (moon)|Phobos]] and [[Deimos (moon)|Deimos]].)<ref>{{cite book |last=Hind |first=John Russell |author-link=John Russell Hind |date=1863 |title=An introduction to astronomy, to which is added an astronomical vocabulary |url=https://books.google.com/books?id=d9aWKHamz8sC&dq=%22hyperion%22+%22secondary+planet%22&pg=PA205 |location=London |publisher=Henry G. Bohn |page=204 |isbn= |access-date=25 October 2023 |archive-date=30 October 2023 |archive-url=https://web.archive.org/web/20231030064739/https://books.google.com/books?id=d9aWKHamz8sC&dq=%22hyperion%22+%22secondary+planet%22&pg=PA205 |url-status=live }}</ref><ref>{{cite book |author=<!--Staff writer(s); no by-line.--> |editor-first1=Robert |editor-last1=Hunter |editor-first2=John A. |editor-last2=Williams |editor-first3=S. J. |editor-last3=Heritage |date=1897 |title=The American Encyclopædic Dictionary |url=https://books.google.com/books?id=P_VOAAAAYAAJ&dq=%22phobos%22+%22secondary+planet%22&pg=PA3553 |location=Chicago and New York |publisher=R. S. Peale and J. A. Hill |volume=8 |pages=3553–3554 |isbn= |access-date=25 October 2023 |archive-date=30 October 2023 |archive-url=https://web.archive.org/web/20231030063236/https://books.google.com/books?id=P_VOAAAAYAAJ&dq=%22phobos%22+%22secondary+planet%22&pg=PA3553 |url-status=live }}</ref> All the eight major planets and their planetary-mass moons have since been explored by spacecraft, as have many asteroids and the dwarf planets Ceres and Pluto; however, so far the only planetary-mass body beyond Earth that has been explored by humans is the Moon.{{efn|See [[Timeline of Solar System exploration]].}}

=== Defining the term ''planet'' ===
{{Further|Definition of planet}}
{{Anchor|Definition and similar concepts}}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.<ref name="plutoplanet">{{cite journal |last1=Basri |first1=Gibor |last2=Brown |first2=Michael E. |date=2006 |title=Planetesimals to Brown Dwarfs: What is a Planet? |url=http://www.gps.caltech.edu/~mbrown/papers/ps/basribrown.pdf |url-status=live |journal=Annual Review of Earth and Planetary Sciences |volume=34 |pages=193–216 |arxiv=astro-ph/0608417 |bibcode=2006AREPS..34..193B |doi=10.1146/annurev.earth.34.031405.125058 |s2cid=119338327 |archive-url=https://web.archive.org/web/20080704213644/http://www.gps.caltech.edu/~mbrown/papers/ps/basribrown.pdf |archive-date=4 July 2008 |access-date=4 August 2008}}</ref> They often referred to the demotion of the asteroids as a precedent, although that had been done based on their geophysical differences from planets rather than their being in a belt.<ref name="metzger22" /> Some of the larger [[trans-Neptunian object]]s, such as [[50000 Quaoar|Quaoar]], [[90377 Sedna|Sedna]], [[Eris (dwarf planet)|Eris]], and [[Haumea]],<ref name="planeta">{{cite web|title=Estados Unidos "conquista" Haumea|work=[[ABC.es|ABC]]|date=20 September 2008|url=http://www.abc.es/20080920/nacional-sociedad/estados-unidos-conquista-haumea-20080920.html|access-date=18 September 2008|language=es|archive-date=6 October 2017|archive-url=https://web.archive.org/web/20171006013056/http://www.abc.es/20080920/nacional-sociedad/estados-unidos-conquista-haumea-20080920.html|url-status=live}}</ref> were heralded in the popular press as the [[tenth planet]].

The announcement of Eris in 2005, an object 27% more massive than Pluto, created the impetus for an official definition of a planet,<ref name="plutoplanet" /> as considering Pluto a planet would logically have demanded that Eris be considered a planet as well. Since different procedures were in place for naming planets versus non-planets, this created an urgent situation because under the rules Eris could not be named without defining what a planet was.<ref name="metzger22" /> At the time, it was also thought that the size required for a trans-Neptunian object to become round was about the same as that required for the moons of the giant planets (about 400&nbsp;km diameter), a figure that would have suggested about 200 round objects in the Kuiper belt and thousands more beyond.<ref name="Brown">{{cite web |author-link=Michael E. Brown |title=The Dwarf Planets |first=Michael E. |last=Brown |publisher=California Institute of Technology, Department of Geological Sciences |url=http://web.gps.caltech.edu/~mbrown/dwarfplanets/ |access-date=26 January 2008 |archive-date=19 July 2011 |archive-url=https://web.archive.org/web/20110719164447/http://web.gps.caltech.edu/~mbrown/dwarfplanets/ |url-status=live }}</ref><ref name="BrownList">{{cite web |last1=Brown |first1=Mike |author1-link=Michael E. Brown |title=How Many Dwarf Planets Are There in the Outer Solar System? |url=http://web.gps.caltech.edu/~mbrown/dps.html |publisher=California Institute of Technology |access-date=11 August 2022 |archive-url=https://web.archive.org/web/20220719141419/http://web.gps.caltech.edu/~mbrown/dps.html |archive-date=19 July 2022 |date=23 February 2021}}</ref> Many astronomers argued that the public would not accept a definition creating a large number of planets.<ref name="metzger22" />

{{Quote box
| title = The [[International Astronomical Union]]'s<br />definition of a planet in the [[Solar System]]
| quote = {{ordered list|
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=24 August 2006|publisher=IAU|access-date=23 June 2009}}
| width = 375px
}}To acknowledge the problem, the [[International Astronomical Union]] (IAU) set about creating the [[IAU definition of planet|definition of planet]] and produced one in August 2006. Under this definition, the Solar System is considered to have eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune). Bodies that fulfill the first two conditions but not the third are classified as [[dwarf planet]]s, provided they are not [[natural satellite]]s of other planets. Originally an IAU committee had proposed a definition that would have included a larger number of planets as it did not include (c) as a criterion.<ref>{{cite news |last=Rincon |first=Paul |date=16 August 2006 |title=Planets plan boosts tally 12 |url=http://news.bbc.co.uk/1/hi/sci/tech/4795755.stm |url-status=live |archive-url=https://web.archive.org/web/20070302051348/http://news.bbc.co.uk/1/hi/sci/tech/4795755.stm |archive-date=2 March 2007 |access-date=23 August 2008 |department=[[BBC News]] |publisher=[[British Broadcasting Corporation]]}}</ref> After much discussion, it was decided via a vote that those bodies should instead be classified as dwarf planets.<ref name="Pluto loses status as a planet-2006" /><ref>{{cite journal |last=Green |first=D. W. E. |date=13 September 2006 |title=(134340) Pluto, (136199) Eris, and (136199) Eris I (Dysnomia) |url=http://www.cbat.eps.harvard.edu/iauc/08700/08747.html |journal=IAU Circular |publisher=Central Bureau for Astronomical Telegrams, International Astronomical Union |issue=8747 |page=1 |bibcode=2006IAUC.8747....1G |id=Circular No. 8747 |archive-url=https://web.archive.org/web/20080624225029/http://www.cfa.harvard.edu/iau/special/08747.pdf |archive-date=24 June 2008 |access-date=5 July 2011}}</ref>

==== Criticisms and alternatives to IAU definition ====
{{See also|List of gravitationally rounded objects of the Solar System}}
[[File:25 solar system objects smaller than Earth.jpg|thumb|upright=1.6|The planetary-mass moons to scale, compared with Mercury, Venus, Earth, Mars, and Pluto. Sub-planetary [[Proteus (moon)|Proteus]] and [[Nereid (moon)|Nereid]] (about the same size as Mimas) have been included for comparison. Unimaged [[Dysnomia (moon)|Dysnomia]] (intermediate in size between Tethys and Enceladus) is not shown; it is in any case probably not a solid body.<ref name="Brown2023">{{cite journal |last1=Brown |first1=Michael E. |last2=Butler |first2=Bryan |date=October 2023 |title=Masses and densities of dwarf planet satellites measured with ALMA |journal=The Planetary Science Journal |volume=4 |issue=10 |pages=6 |arxiv=2307.04848 |bibcode=2023PSJ.....4..193B |doi=10.3847/PSJ/ace52a |s2cid= |id=193 |doi-access=free}}</ref>]]

The IAU definition has not been universally used or accepted. In [[planetary geology]], celestial objects are [[geophysical definition of planet|defined as planets by geophysical characteristics]]. A celestial body may acquire a dynamic (planetary) geology at approximately the mass required for its mantle to become plastic under its own weight. This leads to a state of [[hydrostatic equilibrium]] where the body acquires a stable, round shape, which is adopted as the hallmark of planethood by geophysical definitions. For example:<ref name="Stern_Levison_2002">{{citation
 | last1=Stern | first1=S. Alan
 | last2=Levison | first2=Harold F.
 | editor1-first=H. | editor1-last=Rickman
 | title=Regarding the criteria for planethood and proposed planetary classification schemes
 | journal=Highlights of Astronomy | volume=12
 | pages=205–213 | date=2002
 | publisher=Astronomical Society of the Pacific
 | location=San Francisco
 | bibcode=2002HiA....12..205S
 | isbn=978-1-58381-086-6
 | doi=10.1017/S1539299600013289
 | doi-access=free
 }} See p. 208.</ref>

{{blockquote|a substellar-mass body that has never undergone nuclear fusion and has enough gravitation to be round due to hydrostatic equilibrium, regardless of its orbital parameters.<ref name=Astronomy>{{cite web
 |url          = http://www.astronomy.com/magazine/2018/05/an-organically-grown-planet-definition
 |title        = An organically grown planet definition — Should we really define a word by voting?
 |last1        = Runyon
 |first1       = Kirby D.
 |last2        = Stern
 |first2       = S. Alan
 |date         = 17 May 2018
 |website      = [[Astronomy (magazine)|Astronomy]]
 |access-date  = 12 October 2019
 |archive-date = 10 October 2019
 |archive-url  = https://web.archive.org/web/20191010153028/http://astronomy.com/magazine/2018/05/an-organically-grown-planet-definition
 |url-status   = live
}}</ref>}}

In the Solar System, this mass is generally less than the mass required for a body to clear its orbit; thus, some objects that are considered "planets" under geophysical definitions are not considered as such under the IAU definition, such as Ceres and Pluto.<ref name=planetarysociety/> (In practice, the requirement for hydrostatic equilibrium is universally relaxed to a requirement for rounding and compaction under self-gravity; Mercury is not actually 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> but is universally included as a planet regardless.)<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> Proponents of such definitions often argue that location should not matter and that planethood should be defined by the intrinsic properties of an object.<ref name=planetarysociety/> Dwarf planets had been proposed as a category of small planet (as opposed to [[Minor planet|planetoid]]s as sub-planetary objects) and planetary geologists continue to treat them as planets despite the IAU definition.<ref name=Grundy2019>{{cite journal
 |first1=W.M. |last1=Grundy
 |first2=K.S. |last2=Noll
 |first3=M.W. |last3=Buie
 |first4=S.D. |last4=Benecchi
 |first5=D. |last5=Ragozzine
 |first6=H.G. |last6=Roe
 | display-authors=4
 |title=The Mutual Orbit, Mass, and Density of Transneptunian Binary Gǃkúnǁʼhòmdímà ({{mp|(229762) 2007 UK|126}})
 |url=http://www2.lowell.edu/~grundy/abstracts/2019.G-G.html
 |journal=Icarus
 |doi=10.1016/j.icarus.2018.12.037
 |date=December 2018
 |volume=334
 |page=30
 |bibcode=2019Icar..334...30G
 |s2cid=126574999
 |archive-url=https://web.archive.org/web/20190407045339/http://www2.lowell.edu/~grundy/abstracts/preprints/2019.G-G.pdf
 |archive-date=7 April 2019}}</ref>

The number of dwarf planets even among known objects is not certain. In 2019, Grundy et al. argued based on the low densities of some mid-sized trans-Neptunian objects that the limiting size required for a trans-Neptunian object to reach equilibrium was in fact much larger than it is for the icy moons of the giant planets, being about 900–1000&nbsp;km diameter.<ref name=Grundy2019/> There is general consensus on Ceres in the asteroid belt<ref>{{Cite journal |last1=Raymond |first1=C. A. |last2=Ermakov |first2=A. I. |last3=Castillo-Rogez |first3=J. C. |last4=Marchi |first4=S. |last5=Johnson |first5=B. C. |last6=Hesse |first6=M. A. |last7=Scully |first7=J. E. C. |last8=Buczkowski |first8=D. L. |last9=Sizemore |first9=H. G. |last10=Schenk |first10=P. M. |last11=Nathues |first11=A. |display-authors=4 |date=August 2020 |title=Impact-driven mobilization of deep crustal brines on dwarf planet Ceres |url=https://www.nature.com/articles/s41550-020-1168-2 |journal=Nature Astronomy |language=en |volume=4 |issue=8 |pages=741–747 |bibcode=2020NatAs...4..741R |doi=10.1038/s41550-020-1168-2 |s2cid=211137608 |issn=2397-3366 |access-date=27 June 2022 |archive-date=21 June 2022 |archive-url=https://web.archive.org/web/20220621225448/https://www.nature.com/articles/s41550-020-1168-2 |url-status=live }}</ref> and on the eight trans-Neptunians that probably cross this threshold—{{dp|Orcus}}, {{dp|Pluto}}, {{dp|Haumea}}, {{dp|Quaoar}}, {{dp|Makemake}}, {{dp|Gonggong}}, {{dp|Eris}}, and {{dp|Sedna}}.<ref>{{Cite journal |last1=Barr |first1=Amy C. |last2=Schwamb |first2=Megan E. |date=1 August 2016 |title=Interpreting the densities of the Kuiper belt's dwarf planets |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=460 |issue=2 |pages=1542–1548 |doi=10.1093/mnras/stw1052 |issn=0035-8711|doi-access=free |arxiv=1603.06224 }}</ref><ref name=JWST>{{cite journal|last1=Emery|first1=J. P. |first2=I. |last2=Wong |first3=R. |last3=Brunetto |first4=J. C. |last4=Cook |first5=N. |last5=Pinilla-Alonso |first6=J. A. |last6=Stansberry |first7=B. J. |last7=Holler |first8=W. M. |last8=Grundy |first9=S. |last9=Protopapa |first10=A. C. |last10=Souza-Feliciano |first11=E. |last11=Fernández-Valenzuela |first12=J. I. |last12=Lunine |first13=D. C. |last13=Hines |author-link=|date=2024|title=A Tale of 3 Dwarf Planets: Ices and Organics on Sedna, Gonggong, and Quaoar from JWST Spectroscopy|journal=Icarus |volume=414 |doi=10.1016/j.icarus.2024.116017 |arxiv=2309.15230|bibcode=2024Icar..41416017E }}</ref>

Planetary geologists may include the nineteen known [[planetary-mass moon]]s as "satellite planets", including Earth's Moon and Pluto's [[Charon (moon)|Charon]], like the early modern astronomers.<ref name=planetarysociety/><ref name="News.discovery.com">{{cite web |url=http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html |title=Should Large Moons Be Called 'Satellite Planets'?|last1=Villard|first1=Ray|website=Discovery News|publisher=[[Discovery, Inc.]] |date=14 May 2010 |access-date=4 November 2011 |archive-date=5 May 2012 |archive-url=https://web.archive.org/web/20120505221146/http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html }}</ref> Some go even further and include as planets relatively large, geologically evolved bodies that are nonetheless not very round today, such as Pallas and Vesta;<ref name="planetarysociety" /> rounded bodies that were completely disrupted by impacts and re-accreted like Hygiea;<ref>{{Cite web|url=https://www.space.com/asteroid-hygiea-may-be-smallest-dwarf-planet.html|title=Asteroid Hygiea May be the Smallest Dwarf Planet in the Solar System|website=[[Space.com]]|publisher=[[Purch Group]]|last1=Urrutia|first1=Doris Elin|date=28 October 2019|access-date=28 August 2022|archive-date=5 November 2019|archive-url=https://web.archive.org/web/20191105002904/https://www.space.com/asteroid-hygiea-may-be-smallest-dwarf-planet.html|url-status=live}}</ref><ref>{{Cite news|url=https://www.sciencenews.org/article/hygiea-may-be-solar-system-smallest-dwarf-planet|title=The solar system may have a new smallest dwarf planet: Hygiea|website=[[Science News]]|publisher=[[Society for Science]]|date=28 October 2019|access-date=28 August 2022|archive-date=31 August 2022|archive-url=https://web.archive.org/web/20220831083918/https://www.sciencenews.org/article/hygiea-may-be-solar-system-smallest-dwarf-planet|url-status=live}}</ref><ref name=Yang2020>{{citation|arxiv=2007.08059|title=Binary asteroid (31) Euphrosyne: Ice-rich and nearly spherical|year=2020|doi=10.1051/0004-6361/202038372|last1=Yang|first1=B.|last2=Hanuš|first2=J.|last3=Carry|first3=B.|last4=Vernazza|first4=P.|last5=Brož|first5=M.|last6=Vachier|first6=F.|last7=Rambaux|first7=N.|last8=Marsset|first8=M.|last9=Chrenko|first9=O.|last10=Ševeček|first10=P.|last11=Viikinkoski|first11=M.|last12=Jehin|first12=E.|last13=Ferrais|first13=M.|last14=Podlewska-Gaca|first14=E.|last15=Drouard|first15=A.|last16=Marchis|first16=F.|last17=Birlan|first17=M.|last18=Benkhaldoun|first18=Z.|last19=Berthier|first19=J.|last20=Bartczak|first20=P.|last21=Dumas|first21=C.|last22=Dudziński|first22=G.|last23=Ďurech|first23=J.|last24=Castillo-Rogez|first24=J.|last25=Cipriani|first25=F.|last26=Colas|first26=F.|last27=Fetick|first27=R.|last28=Fusco|first28=T.|last29=Grice|first29=J.|last30=Jorda|first30=L.|journal=Astronomy & Astrophysics|volume=641|page=A80|bibcode=2020A&A...641A..80Y|s2cid=220546126|display-authors=29}}</ref> or even everything at least the diameter of Saturn's moon [[Mimas (moon)|Mimas]], the smallest planetary-mass moon. (This may even include objects that are not round but happen to be larger than Mimas, like Neptune's moon [[Proteus (moon)|Proteus]].)<ref name=planetarysociety/>

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.<ref>{{cite news |last=Netburn |first=Deborah |date=13 November 2015 |title=Why we need a new definition of the word 'planet' |url=http://www.latimes.com/science/sciencenow/la-sci-sn-new-planet-definition-margot-20151113-htmlstory.html |url-status=live |archive-url=https://web.archive.org/web/20210603084521/https://www.latimes.com/science/sciencenow/la-sci-sn-new-planet-definition-margot-20151113-htmlstory.html |archive-date=3 June 2021 |access-date=24 July 2016 |newspaper=[[Los Angeles Times]]}}</ref> The formula produces a value called {{mvar|π}} that is greater than 1 for planets.{{efn|name=not-confuse-π|Margot's parameter<ref name=Margot/> is not to be confused with the [[Pi|famous mathematical constant]] {{nowrap|{{mvar|π}}≈3.14159265 ... .}} }} The eight known planets and all known exoplanets have {{mvar|π}} values above 100, while Ceres, Pluto, and Eris have {{mvar|π}} values of 0.1, or less. Objects with {{mvar|π}} values of 1 or more are expected to be approximately spherical, so that objects that fulfill the orbital-zone clearance requirement around Sun-like stars will also fulfill the roundness requirement<ref name="Margot">
{{cite journal |last=Margot |first=Jean-Luc |author-link=Jean-Luc Margot |year=2015 |title=A quantitative criterion for defining planets |journal=[[The Astronomical Journal]] |volume=150 |issue=6 |page=185 |arxiv=1507.06300 |bibcode=2015AJ....150..185M |doi=10.1088/0004-6256/150/6/185 |s2cid=51684830}}
</ref> – though this may not be the case around very low-mass stars.<ref name="Margot 2024"/> In 2024, Margot and collaborators proposed a revised version of the criterion with a uniform clearing timescale of 10&nbsp;billion years (the approximate main-sequence lifetime of the Sun) or 13.8&nbsp;billion years (the [[age of the universe]]) to accommodate planets orbiting brown dwarfs.<ref name="Margot 2024">{{cite journal |last1=Margot |first1=Jean-Luc |last2=Gladman |first2=Brett |last3=Yang |first3=Tony |title=Quantitative Criteria for Defining Planets |journal=The Planetary Science Journal |date=1 July 2024 |volume=5 |issue=7 |pages=159 |doi=10.3847/PSJ/ad55f3|doi-access=free |arxiv=2407.07590 |bibcode=2024PSJ.....5..159M }}</ref>

=== Exoplanets ===
{{Further|Exoplanet#History of detection|Brown dwarf}}
Even before the discovery of [[exoplanet]]s, there were particular disagreements over whether an object should be considered a planet if it was part of a distinct population such as a [[asteroid belt|belt]], or if it was large enough to generate energy by the [[thermonuclear fusion]] of [[deuterium]].<ref name="plutoplanet" /> Complicating the matter even further, bodies too small to generate energy by fusing deuterium can form by [[nebula|gas-cloud]] collapse just like stars and brown dwarfs, even down to the mass of Jupiter:<ref>{{citation |last1=Boss |first1=Alan P. |title=Nomenclature: Brown Dwarfs, Gas Giant Planets, and ? |journal=Brown Dwarfs |volume=211 |page=529 |year=2003 |bibcode=2003IAUS..211..529B |last2=Basri |first2=Gibor |last3=Kumar |first3=Shiv S. |last4=Liebert |first4=James |last5=Martín |first5=Eduardo L. |last6=Reipurth |first6=Bo |last7=Zinnecker |first7=Hans}}</ref> there was thus disagreement about whether how a body formed should be taken into account.<ref name="plutoplanet" />

In 1992, astronomers [[Aleksander Wolszczan]] and [[Dale Frail]] announced the discovery of planets around a [[pulsar]], [[PSR B1257+12]].<ref name="Wolszczan" /> This discovery is generally considered to be the first definitive detection of a planetary system around another star. Then, on 6 October 1995, [[Michel Mayor]] and [[Didier Queloz]] of the [[Geneva Observatory]] announced the first definitive detection of an exoplanet orbiting an ordinary [[main sequence|main-sequence]] star ([[51 Pegasi]]).<ref name="Mayor">{{cite journal |last1=Mayor |first1=Michel |author2=Queloz, Didier |date=1995 |title=A Jupiter-mass companion to a solar-type star |journal=Nature |volume=378 |issue=6356 |pages=355–359 |bibcode=1995Natur.378..355M |doi=10.1038/378355a0 |s2cid=4339201}}</ref>

The discovery of exoplanets led to another ambiguity in defining a planet: the point at which a planet becomes a star. Many known exoplanets are many times the mass of Jupiter, approaching that of stellar objects known as [[brown dwarf]]s. Brown dwarfs are generally considered stars due to their theoretical ability to fuse [[deuterium]], a heavier isotope of [[hydrogen]]. Although objects more massive than 75 times that of Jupiter fuse simple hydrogen, objects of 13 Jupiter masses can fuse deuterium. Deuterium is quite rare, constituting less than 0.0026% of the hydrogen in the galaxy, and most brown dwarfs would have ceased fusing deuterium long before their discovery, making them effectively indistinguishable from supermassive planets.<ref>{{cite journal |last=Basri |first=Gibor |date=2000 |title=Observations of Brown Dwarfs |journal=Annual Review of Astronomy and Astrophysics |volume=38 |issue=1 |pages=485–519 |bibcode=2000ARA&A..38..485B |doi=10.1146/annurev.astro.38.1.485}}</ref>

==== IAU working definition of exoplanets ====
The 2006 IAU definition presents some challenges for exoplanets because the language is specific to the Solar System and the criteria of roundness and orbital zone clearance are not presently observable for exoplanets.<ref name="exodef">{{Cite journal |last1=Lecavelier des Etangs |first1=A. |last2=Lissauer |first2=Jack J. |date=1 June 2022 |title=The IAU working definition of an exoplanet |url=https://www.sciencedirect.com/science/article/pii/S138764732200001X |journal=New Astronomy Reviews |language=en |volume=94 |page=101641 |doi=10.1016/j.newar.2022.101641 |arxiv=2203.09520 |bibcode=2022NewAR..9401641L |s2cid=247065421 |issn=1387-6473 |access-date=13 May 2022 |archive-date=13 May 2022 |archive-url=https://web.archive.org/web/20220513110848/https://www.sciencedirect.com/science/article/pii/S138764732200001X |url-status=live }}</ref> In 2018, this definition was reassessed and updated as knowledge of exoplanets increased.<ref name="iauexo">{{cite journal |last1=Lecavelier des Etangs |first1=A. |last2=Lissauer |first2=Jack J. |date=2022 |title=The IAU working definition of an exoplanet |journal=New Astronomy Reviews |volume=94 |issue= |page=101641 |arxiv=2203.09520 |bibcode=2022NewAR..9401641L |doi=10.1016/j.newar.2022.101641 |s2cid=247065421}}</ref> The current official working definition of an exoplanet is as follows:<ref name="exoworkdef" />

{{blockquote|
# 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.
# Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor 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).<ref name=exoworkdef/>
}}

The IAU noted that this definition could be expected to evolve as knowledge improves.<ref name="exoworkdef">{{cite web |title=Official Working Definition of an Exoplanet |url=https://www.iau.org/science/scientific_bodies/commissions/F2/info/documents/ |access-date=29 November 2020 |work=IAU position statement |archive-date=3 July 2022 |archive-url=https://web.archive.org/web/20220703184850/https://www.iau.org/science/scientific_bodies/commissions/F2/info/documents/ |url-status=live }}</ref> A 2022 review article discussing the history and rationale of this definition suggested that the words "in young star clusters" should be deleted in clause 3, as such objects have now been found elsewhere, and that the term "sub-brown dwarfs" should be replaced by the more current "free-floating planetary mass objects". The term "[[Planetary-mass object|planetary mass object]]" has also been used to refer to ambiguous situations concerning exoplanets, such as objects with mass typical for a planet that are free-floating or orbit a brown dwarf instead of a star.<ref name="iauexo" /> Free-floating objects of planetary mass have sometimes been called planets anyway, specifically [[rogue planet]]s.<ref name="eso2120">{{cite web | title = ESO telescopes help uncover largest group of rogue planets yet | url = https://www.eso.org/public/news/eso2120/ | publisher = [[European Southern Observatory]] | date = 22 December 2021 | access-date = 22 December 2021}}</ref>

The limit of 13 Jupiter masses is not universally accepted. Objects below this mass limit can sometimes burn deuterium, and the amount of deuterium that is burned depends on an object's composition.<ref name="bodenheimer2013">{{cite journal |title=Deuterium Burning in Massive Giant Planets and Low-mass Brown Dwarfs Formed by Core-nucleated Accretion |journal=The Astrophysical Journal |date=2013 |volume=770 |issue=2 |page=120 |doi=10.1088/0004-637X/770/2/120 |arxiv=1305.0980 |bibcode=2013ApJ...770..120B |last1=Bodenheimer |first1=Peter |last2=D'Angelo |first2=Gennaro |last3=Lissauer |first3=Jack J. |last4=Fortney |first4=Jonathan J. |last5=Saumon |first5=Didier |s2cid=118553341 }}</ref><ref>{{Cite journal | doi = 10.1088/0004-637X/727/1/57| title = The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets| journal = The Astrophysical Journal| volume = 727| issue = 1| page = 57| year = 2011| last1 = Spiegel | first1 = D. S. |last2=Burrows |first2=Adam | last3 = Milsom | first3 = J. A. | bibcode = 2011ApJ...727...57S|arxiv = 1008.5150 | s2cid = 118513110}}</ref> Furthermore, deuterium is quite scarce, so the stage of deuterium burning does not actually last very long; unlike hydrogen burning in a star, deuterium burning does not significantly affect the future evolution of an object.<ref name="Hatzes" /> The relationship between mass and radius (or density) show no special feature at this limit, according to which brown dwarfs have the same physics and internal structure as lighter Jovian planets, and would more naturally be considered planets.<ref name="Hatzes">{{cite journal |arxiv=1506.05097 |last1=Hatzes |first1=Artie P. |author-link1=Artie P. Hatzes |last2=Rauer |first2=Heike |author-link2=Heike Rauer |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><ref name="ChenKipping" />

Thus, many catalogues of exoplanets include objects heavier than 13 Jupiter masses, sometimes going up to 60 Jupiter masses.<ref>{{cite journal |last1=Schneider |first1=Jean |last2=Dedieu |first2=Cyril |last3=Le Sidaner |first3=Pierre |last4=Savalle |first4=Renaud |last5=Zolotukhin |first5=Ivan |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><ref name="corot">{{cite book |last=Schneider |first=Jean |arxiv=1604.00917 |chapter=Exoplanets versus brown dwarfs: the CoRoT view and the future |title=The CoRoT Legacy Book |date=July 2016 |page=157 |doi=10.1051/978-2-7598-1876-1.c038 |isbn=978-2-7598-1876-1|s2cid=118434022 }}</ref><ref name="eod">{{cite journal |arxiv=1012.5676 |title=The Exoplanet Orbit Database |date=2010 |bibcode=2011PASP..123..412W |doi=10.1086/659427 |volume=123 |issue=902 |journal=[[Publications of the Astronomical Society of the Pacific]] |pages=412–422 |last1=Wright |first1=Jason T. |last2=Fakhouri |first2=Onsi |last3=Marcy |first3=Geoffrey W. |author-link3=Geoffrey Marcy |last4=Han |first4=Eunkyu |last5=Feng |first5=Y. Katherina |last6=Johnson |first6=John Asher |author-link6=John Johnson (astronomer) |last7=Howard |first7=Andrew W. |last8=Fischer |first8=Debra A. |author-link8=Debra Fischer |last9=Valenti |first9=Jeff A. |last10=Anderson |first10=Jay |last11=Piskunov |first11=Nikolai |s2cid=51769219 }}</ref><ref>[http://exoplanetarchive.ipac.caltech.edu/docs/exoplanet_criteria.html Exoplanet Criteria for Inclusion in the Archive] {{Webarchive|url=https://web.archive.org/web/20150127102447/http://exoplanetarchive.ipac.caltech.edu/docs/exoplanet_criteria.html |date=27 January 2015 }}, NASA Exoplanet Archive</ref> (The limit for hydrogen burning and becoming a [[red dwarf]] star is about 80 Jupiter masses.)<ref name="Hatzes" /> The situation of main-sequence stars has been used to argue for such an inclusive definition of "planet" as well, as they also differ greatly along the two orders of magnitude that they cover, in their structure, atmospheres, temperature, spectral features, and probably formation mechanisms; yet they are all considered as one class, being all hydrostatic-equilibrium objects undergoing nuclear burning.<ref name="Hatzes" />