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== Definition == === IAU === The official [[Definition of planet|definition of the term ''planet'']] used by the [[International Astronomical Union]] (IAU) only covers the [[Solar System]] and thus does not apply to exoplanets.<ref>{{cite web |title=IAU 2006 General Assembly: Result of the IAU Resolution votes |date=2006 |url=https://www.iau.org/news/pressreleases/detail/iau0603/ |archive-url=https://web.archive.org/web/20241225044449/https://www.iau.org/news/pressreleases/detail/iau0603/ |access-date=31 March 2025 |archive-date=25 December 2024 }}</ref><ref>{{cite web |author=Brit, R. R. |date=2006 |title=Why Planets Will Never Be Defined |url=http://www.space.com/3142-planets-defined.html |work=[[Space.com]] |access-date=13 February 2008 }}</ref> The IAU Working Group on Extrasolar Planets issued a position statement containing a working definition of "planet" in 2001 and which was modified in 2003.<ref>{{cite web |date=28 February 2003 |title=Working Group on Extrasolar Planets: Definition of a "Planet" |url=http://w.astro.berkeley.edu/~basri/defineplanet/IAU-WGExSP.htm |work=IAU position statement |archive-url=https://web.archive.org/web/20210419012736/http://w.astro.berkeley.edu/~basri/defineplanet/IAU-WGExSP.htm |access-date=31 March 2025 |archive-date=19 April 2021 }}</ref> An ''exoplanet'' was defined by the following criteria: {{blockquote| * Objects with [[true mass]]es below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar [[metallicity]]) that orbit stars or stellar remnants 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 the Solar System. * Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are "[[brown dwarfs]]", no matter how they formed or where they are located. * Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate). }} This working definition 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 |archive-url=https://web.archive.org/web/20250212195344/https://www.iau.org/science/scientific_bodies/commissions/F2/info/documents/ |access-date=31 March 2025 |archive-date=12 February 2025 }}</ref><ref>{{Cite journal |last1=Lecavelier des Etangs |first1=A. |last2=Lissauer |first2=Jack J. |date=June 2022 |title=The IAU working definition of an exoplanet |url=https://linkinghub.elsevier.com/retrieve/pii/S138764732200001X |journal=New Astronomy Reviews |language=en |volume=94 |pages=101641 |doi=10.1016/j.newar.2022.101641|arxiv=2203.09520 |bibcode=2022NewAR..9401641L |s2cid=247065421 }}</ref> The official working definition of an ''exoplanet'' is now as follows: {{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. }} === Alternatives === The IAU's working definition is not always used. One alternate suggestion is that planets should be distinguished from [[brown dwarf]]s on the basis of their formation. It is widely thought that giant planets form through core [[Accretion (astrophysics)|accretion]], which may sometimes produce planets with masses above the deuterium fusion threshold;<ref>{{cite journal |arxiv=0710.5667 |title=Giant Planet Formation by Core Accretion |journal=Extreme Solar Systems |volume=398 |page=235 |bibcode=2008ASPC..398..235M |last1=Mordasini| first1=C. |last2=Alibert |first2=Yann |last3=Benz |first3=Willy |last4=Naef |first4=Dominique |year=2008 }}</ref><ref>{{Cite journal |arxiv=0802.1810 |title=Structure and evolution of super-Earth to super-Jupiter exoplanets. I. Heavy element enrichment in the interior |last1=Baraffe |first1=I. |date=2008 |journal=Astronomy and Astrophysics |volume=482 |issue=1 |pages=315–332 |doi=10.1051/0004-6361:20079321 |bibcode=2008A&A...482..315B |last2=Chabrier |first2=G. |last3=Barman |first3=T. |s2cid=16746688 }}</ref><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> massive planets of that sort may have already been observed.<ref>{{cite journal |doi=10.1051/0004-6361/200912427 |title=The SOPHIE northern extrasolar planets. I. A companion close to the planet/brown-dwarf transition around HD16760 |last1=Bouchy |first1=François |last2=Hébrard |first2=Guillaume |last3=Udry |first3=Stéphane |last4=Delfosse |first4=Xavier |last5=Boisse |first5=Isabelle |last6=Desort |first6=Morgan |last7=Bonfils |first7=Xavier |last8=Eggenberger |first8=Anne |last9=Ehrenreich |first9=David |last10=Forveille |first10=Thierry |last11=Le Coroller |first11=Hervé |last12=Lagrange |first12=Anne-Marie |last13=Lovis |first13=Christophe |last14=Moutou |first14=Claire |last15=Pepe |first15=Francesco |last16=Perrier |first16=Christian |last17=Pont |first17=Frédéric |last18=Queloz |first18=Didier |last19=Santos |first19=Nuno C. |last20=Ségransan |first20=Damien |last21=Vidal-Madjar |first21=Alfred |date=2009 |journal=Astronomy and Astrophysics |volume=505 |issue=2 |pages=853–858 |bibcode=2009A&A...505..853B |doi-access=free }}</ref> Brown dwarfs form like stars from the direct gravitational collapse of clouds of gas, and this formation mechanism also produces objects that are below the {{Jupiter mass|13|jup=y|link=y}} limit and can be as low as {{Jupiter mass|1|jup=y}}.<ref name=ShivKumar>{{cite journal| bibcode=2003IAUS..211..529B| title=Nomenclature: Brown Dwarfs, Gas Giant Planets, and ?| last1=Kumar| first1=Shiv S.|volume=211| date=2003| page=532| journal=Brown Dwarfs }}</ref> Objects in this mass range that orbit their stars with wide separations of hundreds or thousands of [[astronomical unit]]s (AU) and have large star/object mass ratios likely formed as brown dwarfs; their atmospheres would likely have a composition more similar to their host star than accretion-formed planets, which would contain increased abundances of heavier elements. Most directly imaged planets as of April 2014 are massive and have wide orbits so probably represent the low-mass end of a brown dwarf formation.<ref>{{Cite journal | doi = 10.1088/0004-637X/794/2/159| title = A Statistical Analysis of Seeds and Other High-Contrast Exoplanet Surveys: Massive Planets or Low-Mass Brown Dwarfs?| journal = The Astrophysical Journal| volume = 794| issue = 2| page = 159| year = 2014| last1 = Brandt | first1 = T. D. | last2 = McElwain | first2 = M. W. | last3 = Turner | first3 = E. L. | last4 = Mede | first4 = K. | last5 = Spiegel | first5 = D. S. | last6 = Kuzuhara | first6 = M. | last7 = Schlieder | first7 = J. E. | last8 = Wisniewski | first8 = J. P. | last9 = Abe | first9 = L.| last10 = Biller | first10 = B.| last11 = Brandner | first11 = W.| last12 = Carson | first12 = J.| last13 = Currie | first13 = T.| last14 = Egner | first14 = S.| last15 = Feldt | first15 = M.| last16 = Golota | first16 = T.| last17 = Goto | first17 = M.| last18 = Grady | first18 = C. A.| last19 = Guyon | first19 = O.| last20 = Hashimoto | first20 = J.| last21 = Hayano | first21 = Y.| last22 = Hayashi | first22 = M.| last23 = Hayashi | first23 = S.| last24 = Henning | first24 = T.| last25 = Hodapp | first25 = K. W.| last26 = Inutsuka | first26 = S.| last27 = Ishii | first27 = M.| last28 = Iye | first28 = M.| last29 = Janson | first29 = M.| last30 = Kandori | first30 = R.| display-authors = etal| bibcode = 2014ApJ...794..159B|arxiv = 1404.5335 | s2cid = 119304898}}</ref> One study suggests that objects above {{Jupiter mass|10|jup=y}} formed through gravitational instability and should not be thought of as planets.<ref>{{Cite journal|last=Schlaufman|first=Kevin C.|date=2018-01-22|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|pages=37|doi=10.3847/1538-4357/aa961c|arxiv=1801.06185|bibcode=2018ApJ...853...37S|s2cid=55995400|issn=1538-4357 |doi-access=free }}</ref> Also, the 13-Jupiter-mass cutoff does not have a precise physical significance. Deuterium fusion can occur in some objects with a mass below that cutoff.<ref name="bodenheimer2013" /> The amount of deuterium fused depends to some extent on the composition of the object.<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> In 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|title=Exoplanets versus brown dwarfs: the CoRoT view and the future|chapter=III.8 Exoplanets versus brown dwarfs: The CoRoT view and the future|year=2016|pages=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 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=J. T.| last2=Fakhouri|first2=O.|last3=Marcy|first3=G. W.| last4=Han| first4=E.| last5=Feng| first5=Y.| last6=Johnson| first6=John Asher| last7=Howard| first7=A. W.| last8=Fischer|first8=D. A.|last9=Valenti |first9=J. A.| last10=Anderson| first10=J.| last11=Piskunov|first11=N.|s2cid=51769219}}</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=2022-01-17|website=exoplanetarchive.ipac.caltech.edu}}</ref> Another criterion for separating planets and brown dwarfs, rather than deuterium fusion, formation process or location, is whether the core [[pressure]] is dominated by [[Coulomb barrier|Coulomb pressure]] or [[electron degeneracy pressure]] with the dividing line at around 5 Jupiter masses.<ref name="whatbasribrown">{{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 |date=2006 |arxiv=astro-ph/0608417 |bibcode=2006AREPS..34..193B|last1=Basri |first1=Gibor |last2=Brown |first2=Michael E. |s2cid=119338327 |url=https://authors.library.caltech.edu/5028/1/BASareps06.pdf |type=Submitted manuscript }}</ref><ref name=JamesLiebert>{{cite journal|bibcode=2003IAUS..211..529B|title=Nomenclature: Brown Dwarfs, Gas Giant Planets, and ?|last1=Liebert|first1=James|volume=211|date=2003|page=533|journal=Brown Dwarfs }}</ref> === Confirmation === An exoplanet is confirmed for NASA's Exoplanet Archive either when "different observation techniques reveal features that can only be explained by a planet"<ref>{{cite web | url=https://www.nasa.gov/missions/kepler/new-deep-learning-method-adds-301-planets-to-keplers-total-count/ | title=New Deep Learning Method Adds 301 Planets to Kepler's Total Count - NASA | date=23 November 2021 }}</ref> or by analytical techniques.<ref name=NASA-20220321/> For the Extrasolar Planets Encyclopedia, "A planet is considered as Confirmed if it is claimed unambiguously in an accepted paper or a professional conference."<ref>{{cite web | url=https://exoplanet.eu/catalog/ | title=Catalogue of Exoplanets | date=1995 }}</ref>
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