Editing Planet (section)

=== 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" />