Editing Brown dwarf (section)

=== Low-mass brown dwarfs versus high-mass planets ===
[[File:Brown Dwarf HD 29587 B.png|thumb|right|An artistic concept of the brown dwarf around the star [[HD 29587]], a companion known as [[HD 29587 b]], estimated to be about 55 Jupiter masses]]

Like stars, brown dwarfs form independently, but, unlike stars, they lack sufficient mass to "ignite" hydrogen fusion. Like all stars, they can occur singly or in close proximity to other stars. Some orbit stars and can, like planets, have eccentric orbits.

==== Size and fuel-burning ambiguities ====
Brown dwarfs are all roughly the same radius as Jupiter. At the high end of their mass range ({{Jupiter mass|60–90}}), the volume of a brown dwarf is governed primarily by [[degenerate matter|electron-degeneracy]] pressure,<ref>{{cite journal |title=Planetesimals to Brown Dwarfs: What is a Planet? |date=2006-08-20 |pages=193–216 |first1=Gibor |last1=Basri |last2=Brown |first2=Michael E. |author-link2=Michael E. Brown |volume=34 |issue=2006 |doi=10.1146/annurev.earth.34.031405.125058 |journal=[[Annual Review of Earth and Planetary Sciences]] |arxiv=astro-ph/0608417 |bibcode=2006AREPS..34..193B|s2cid=119338327 }}</ref> as it is in white dwarfs; at the low end of the range ({{Jupiter mass|10}}), their volume is governed primarily by [[Coulomb barrier|Coulomb pressure]], as it is in planets. The net result is that the radii of brown dwarfs vary by only 10–15% over the range of possible masses. Moreover, the mass–radius relationship shows no change from about one Saturn mass to the onset of hydrogen burning ({{val|0.080|0.008|u=M_Solar}}), suggesting that from this perspective brown dwarfs are simply high-mass Jovian planets.<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 |arxiv=1603.08614 |doi=10.3847/1538-4357/834/1/17 |s2cid=119114880 |doi-access=free |bibcode=2017ApJ...834...17C }}</ref> This can make distinguishing them from planets difficult.

In addition, many brown dwarfs undergo no fusion; even those at the high end of the mass range (over {{Jupiter mass|60}}) cool quickly enough that after 10&nbsp;million years they no longer undergo [[deuterium burning|fusion]].

==== Heat spectrum ====
X-ray and infrared spectra are telltale signs of brown dwarfs. Some emit [[X-ray]]s; and all "warm" dwarfs continue to glow tellingly in the red and [[infrared]] spectra until they cool to planet-like temperatures (under {{val|1000|u=K}}).

[[Gas giant]]s have some of the characteristics of brown dwarfs. Like the Sun, [[Jupiter]] and [[Saturn]] are both made primarily of hydrogen and helium. Saturn is nearly as large as Jupiter, despite having only 30% the mass. Three of the giant planets in the Solar System (Jupiter, Saturn, and [[Neptune]]<!-- Uranus emits only barely more heat than it receives from the Sun, per source -->) emit much more (up to about twice) heat than they receive from the Sun.<ref>{{cite web |url=http://astronomy.nmsu.edu/tharriso/ast105/UranusandNeptune.html |title=The Jovian Planets: Uranus, and Neptune |access-date=2013-03-15 |archive-url=https://web.archive.org/web/20120118184803/http://astronomy.nmsu.edu/tharriso/ast105/UranusandNeptune.html |archive-date=2012-01-18 |url-status=dead }}</ref><ref>{{cite web |url=http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/planets.html |title=Cool Cosmos – Planets and Moons |access-date=2019-02-11 |archive-date=2019-02-21 |archive-url=https://web.archive.org/web/20190221171452/http://coolcosmos.ipac.caltech.edu/cosmic_classroom/cosmic_reference/planets.html |url-status=dead }}</ref> All four giant planets have their own "planetary" systems, in the form of extensive moon systems.

==== Current IAU standard ====
Currently, the [[International Astronomical Union]] considers an object above {{Jupiter mass|13}} (the limiting mass for thermonuclear fusion of deuterium) to be a brown dwarf, whereas an object under that mass (and orbiting a star or stellar remnant) is considered a planet. The minimum mass required to trigger sustained hydrogen burning (about {{Jupiter mass|80}}) forms the upper limit of the definition.<ref>{{cite web |title=Working Group on Extrasolar Planets: Definition of a "Planet" |website=IAU position statement |date=2003-02-28 |url=http://home.dtm.ciw.edu/users/boss/definition.html |access-date=2014-04-28 |archive-url=https://web.archive.org/web/20141216075559/http://home.dtm.ciw.edu/users/boss/definition.html |archive-date=2014-12-16 |url-status=dead }}</ref>

It is also debated whether brown dwarfs would be better defined by their formation process rather than by theoretical mass limits based on nuclear fusion reactions.<ref name="PT-June2008">{{cite journal |last=Burgasser |first=Adam J. |date=June 2008 |title=Brown dwarfs: Failed stars, super Jupiters |url=http://astro.berkeley.edu/~gmarcy/astro160/papers/brown_dwarfs_failed_stars.pdf |url-status=dead |journal=[[Physics Today]] |location=Cambridge, MA |publisher=Massachusetts Institute of Technology |volume=61 |issue=6 |pages=70–71 |bibcode=2008PhT....61f..70B |doi=10.1063/1.2947658 |archive-url=https://web.archive.org/web/20130508182012/http://astro.berkeley.edu/~gmarcy/astro160/papers/brown_dwarfs_failed_stars.pdf |archive-date=May 8, 2013 |access-date=March 31, 2022 |via=American Institute of Physics}}</ref> Under this interpretation brown dwarfs are those objects that represent the lowest-mass products of the [[star formation]] process, while planets are objects formed in an [[accretion disk]] surrounding a star. The coolest free-floating objects discovered, such as [[WISE 0855]], as well as the lowest-mass young objects known, like [[PSO J318.5−22]], are thought to have masses below {{Jupiter mass|13}}, and as a result are sometimes referred to as [[planetary-mass object]]s due to the ambiguity of whether they should be regarded as [[rogue planets]] or brown dwarfs. There are planetary-mass objects known to orbit brown dwarfs, such as [[2M1207b]], [[2MASS J044144b]] and [[CFHTWIR-Oph 98 b|Oph 98 B]].

The 13-Jupiter-mass cutoff is a rule of thumb rather than a quantity with precise physical significance. Larger objects will burn most of their deuterium and smaller ones will burn only a little, and the 13{{Non breaking hyphen}}Jupiter-mass value is somewhere in between.<ref name=bodenheimer2013>{{cite journal |last1=Bodenheimer |first1=Peter |last2=D'Angelo |first2=Gennaro |last3=Lissauer |first3=Jack J. |author-link3=Jack J. Lissauer |last4=Fortney |first4=Jonathan J. |last5=Saumon |first5=Didier |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 |pages=120 (13 pp.) |doi=10.1088/0004-637X/770/2/120 |arxiv=1305.0980 |bibcode=2013ApJ...770..120B|s2cid=118553341 }}</ref> The amount of deuterium burnt also depends to some extent on the composition of the object, specifically on the amount of [[helium]] and [[deuterium]] present and on the fraction of heavier elements, which determines the atmospheric opacity and thus the radiative cooling rate.<ref name=Spiegel2011>{{cite journal |last1=Spiegel |first1=David S. |last2=Burrows |first2=Adam |last3=Milson |first3=John A. |title=The Deuterium-Burning Mass Limit for Brown Dwarfs and Giant Planets |journal=The Astrophysical Journal |volume=727 |issue=1 |page=57 |date=2011 |doi=10.1088/0004-637X/727/1/57 |arxiv=1008.5150 |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=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> As of 2016, this limit was increased to 60 Jupiter masses,<ref>{{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> based on a study of mass–density relationships.<ref>{{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>

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 [[Minimum mass|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=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> The [[NASA Exoplanet Archive]] includes objects with a mass (or minimum mass) equal to or less than 30 Jupiter masses.<ref>[http://exoplanetarchive.ipac.caltech.edu/docs/exoplanet_criteria.html Exoplanet Criteria for Inclusion in the Archive], NASA Exoplanet Archive</ref>

==== Sub-brown dwarf <span class="anchor" id="Sub-brown dwarf"></span> ====
{{Main|Sub-brown dwarf}}
[[File:Sol Cha-110913-773444 Jupiter.jpg|thumb|A size comparison between the [[Sun]], a young sub-brown dwarf, and [[Jupiter]]. As the sub-brown dwarf ages, it will gradually cool and shrink.]]

Objects below {{Jupiter mass|13}}, called '''sub-brown dwarfs''' or '''planetary-mass brown dwarfs''', form in the same manner as [[star]]s and brown dwarfs (i.e. through the collapse of a [[nebula|gas cloud]]) but have a [[Planetary-mass object|mass below the limiting mass for thermonuclear fusion]] of [[deuterium]].<ref>[http://www.dtm.ciw.edu/boss/definition.html Working Group on Extrasolar Planets – Definition of a "Planet"] {{webarchive|url=https://web.archive.org/web/20120702204018/http://www.dtm.ciw.edu/boss/definition.html |date=2012-07-02 }} Position statement on the definition of a "planet" (IAU)</ref>

Some researchers call them free-floating planets,<ref name="Delorme2012">{{cite journal |last1=Delorme |first1=Philippe |first2=Jonathan |last2=Gagné |first3=Lison |last3=Malo |first4=Céline |last4=Reylé |first5=Étienne |last5=Artigau |first6=Loïc |last6=Albert |first7=Thierry |last7=Forveille |first8=Xavier |last8=Delfosse |first9=France |last9=Allard |first10=Derek |last10=Homeier |title=CFBDSIR2149-0403: a 4–7 Jupiter-mass free-floating planet in the young moving group AB Doradus? |journal=Astronomy & Astrophysics |date=December 2012 |arxiv=1210.0305 |doi=10.1051/0004-6361/201219984 |bibcode=2012A&A...548A..26D |volume=548 |page=A26|s2cid=50935950 }}</ref> whereas others call them planetary-mass brown dwarfs.<ref name="Luhman20140421">{{cite journal |title=Discovery of a ~250 K Brown Dwarf at 2 pc from the Sun |journal=[[The Astrophysical Journal Letters]] |first=Kevin L. |last=Luhman |author-link=Kevin Luhman |volume=786 |issue=2 |page=L18 |date=21 April 2014 |doi=10.1088/2041-8205/786/2/L18 |arxiv=1404.6501 |bibcode=2014ApJ...786L..18L|s2cid=119102654 }}</ref>