View source for Brown dwarf

{{short description|Type of substellar object more massive than a planet}}
{{multiple image
| direction         = vertical
| align             = right
| width             = 275
| image1            = Artist’s conception of a brown dwarf like 2MASSJ22282889-431026.jpg <!-- keep curly apostrophe: it is part of the file name -->
| caption1          = Artist's concept of a T-type brown dwarf
| image2            = Brown_Dwarf_Comparison_2020.png
| caption2          = Comparison: most brown dwarfs are slightly larger in volume than Jupiter (15–20%),<ref name="SorahanaYamamura2013">{{cite journal |last1=Sorahana |first1=Satoko |last2=Yamamura |first2=Issei |last3=Murakami |first3=Hiroshi <!-- |display-authors=1 -->|title=On the Radii of Brown Dwarfs Measured with ''AKARI'' Near-infrared Spectroscopy |journal=[[The Astrophysical Journal]] |volume=767 |issue=1 |year=2013 |pages=77 |doi=10.1088/0004-637X/767/1/77 |arxiv=1304.1259 |bibcode=2013ApJ...767...77S |doi-access=free |quote=We find that the brown dwarf radius ranges between 0.64–1.13 R<sub>J</sub> with an average radius of 0.83&nbsp;''R''<sub>J</sub>.}}</ref> but are still up to 80 times more massive due to greater density. Image is to scale, with Jupiter's radius being 11 times that of Earth, and the Sun's radius 10 times that of Jupiter.
}}

'''Brown dwarfs''' are [[substellar object]]s that have more mass than the biggest [[gas giant]] planets, but less than the least massive [[main sequence|main-sequence]] [[star]]s.  Their mass is approximately 13 to 80 [[Jupiter mass|times that of Jupiter]] ({{Jupiter mass}})<ref name="BossCarnegie">{{cite web |last1=Boss |first1=Alan |last2=McDowell |first2=Tina |date=April 3, 2001 |title=Are They Planets or What? |url=http://www.carnegieinstitution.org/News4-3,2001.html |url-status=dead |archive-url=https://web.archive.org/web/20060928065124/http://www.carnegieinstitution.org/News4-3%2C2001.html |archive-date=September 28, 2006 |access-date=March 31, 2022 |website=Untitled Document |publisher=Carnegie Institution of Washington}}</ref><ref name="Wethington">{{cite news |last=Wethington |first=Nicholos<!-- sic --> |date=October 6, 2008 |title=Dense Exoplanet Creates Classification Calamity |work=[[Universe Today]] |url=http://www.universetoday.com/19237/dense-exoplanet-creates-classification-calamity/ |access-date=March 31, 2022}}</ref>{{mdash}}not big enough to sustain [[nuclear fusion]] of ordinary [[hydrogen]] ([[hydrogen-1|<sup>1</sup>H]]) into helium in their cores, but massive enough to emit some light and heat from the [[deuterium fusion|fusion of deuterium]] ([[deuterium|<sup>2</sup>H]]).  The most massive ones (>&nbsp;{{Jupiter mass|65}}) can [[lithium burning|fuse lithium]] ([[lithium-7|<sup>7</sup>Li]]).<ref name="Wethington" />

Astronomers classify self-luminous objects by [[Stellar classification#Spectral types|spectral type]], a distinction intimately tied to the surface temperature, and brown dwarfs occupy types M, L, T, and Y.<ref name="PT-June2008" /><ref>{{cite book |last=Springer |first=Cham |url=https://link.springer.com/book/10.1007/978-3-319-01162-2#about |title=50 Years of Brown Dwarfs |publisher=SpringerLink |year=2014 |isbn=978-3-319-01162-2 |editor-last=Joergens |editor-first=Viki |series=Astrophysics and Space Science Library |volume=401 |at=XI, 168 |doi=10.1007/978-3-319-01162-2 |eissn=2214-7985 |issn=0067-0057 |access-date=March 31, 2022 |url-access=subscription}}</ref> As brown dwarfs do not undergo stable hydrogen fusion, they cool down over time, progressively passing through later spectral types as they age.

Their name comes not from the color of light they emit but from their low luminosity, falling below that of a [[white dwarf]] star but above the level of a [[Gas giant]]. To the naked eye, brown dwarfs would appear in different colors depending on their temperature.<ref name="PT-June2008" /> The warmest ones are possibly orange or red,<ref name="Cain">{{cite web |last=Cain |first=Fraser |date=January 6, 2009 |title=If Brown Isn't a Color, What Color are Brown Dwarfs? |url=http://www.universetoday.com/23247/if-brown-isnt-a-color-what-color-are-brown-dwarfs/ |access-date=24 September 2013}}</ref> while cooler brown dwarfs would likely appear [[magenta]] or black to the human eye.<ref name="PT-June2008" /><ref name="Burrows2001">{{cite journal |last1=Burrows |first1=Adam |author-link1=Adam Burrows |last2=Hubbard |first2=William B. |last3=Lunine |first3=Jonathan I. |author-link3=Jonathan Lunine |last4=Liebert |first4=James |year=2001 |title=The Theory of Brown Dwarfs and Extrasolar Giant Planets |journal=Reviews of Modern Physics |volume=73 |issue=3 |pages=719–765 |arxiv=astro-ph/0103383 |bibcode=2001RvMP...73..719B |doi=10.1103/RevModPhys.73.719 |s2cid=204927572}}</ref> Brown dwarfs may be fully [[convective]], with no layers or chemical differentiation by depth.<ref>{{cite web |last=O'Neill |first=Ian |date=13 September 2011 |title=Violent Storms Rage on Nearby Brown Dwarf |url=https://www.seeker.com/violent-storms-rage-on-nearby-brown-dwarf-1765411574.html |publisher=Seeker.com}}</ref>

Though their existence was initially theorized in the 1960s, it was not until 1994 that the first unambiguous brown dwarfs were discovered.<ref>{{cite web |last1=Finley |first1=Dave |title=Brown Dwarfs, Stars Share Formation Process, New Study Indicates |url=https://public.nrao.edu/news/brown-dwarf-jets/#:~:text=Theorists%20suggested%20in%20the%201960s,like%20stars%20or%20like%20planets. |website=The National Radio Astronomy Observatory |access-date=11 February 2025}}</ref> As brown dwarfs have relatively low surface temperatures, they are not very bright at visible wavelengths, emitting most of their light in the [[infrared]]. However, with the advent of more capable infrared detecting devices, thousands of brown dwarfs have been identified. The nearest known brown dwarfs are located in the [[Luhman 16]] system, a [[Binary star|binary]] of L- and T-type brown dwarfs about {{Convert|6.5|ly|pc|lk=on|abbr=off}} from the Sun. Luhman 16 is the third closest system to the Sun after [[Alpha Centauri]] and [[Barnard's Star]].

== History ==
[[File:Brown Dwarf Gliese 229B.jpg|thumb|right|The smaller object is Gliese 229B, about 20 to 50 times the mass of Jupiter, orbiting the star [[Gliese 229]]. It is in the constellation [[Lepus (constellation)|Lepus]], about 19 light-years from Earth.]]

=== Early theorizing ===
[[File:PIA23685-Planets-BrownDwarfs-Stars.jpg|thumb|left|200px|Planets, brown dwarfs, stars (not to scale)]]
The objects now called "brown dwarfs" were theorized by Shiv S. Kumar in the 1960s to exist and were originally called [[black dwarf]]s,<ref>{{cite journal |last=Kumar |first=Shiv S. |title=Study of Degeneracy in Very Light Stars |journal=Astronomical Journal |volume=67 |page=579 |date=1962 |doi=10.1086/108658 |bibcode=1962AJ.....67S.579K|doi-access=free }}</ref> a classification for dark substellar objects floating freely in space that were not massive enough to sustain hydrogen fusion. However, (a)&nbsp;the term black dwarf was already in use to refer to a cold [[white dwarf]]; (b)&nbsp;[[red dwarf]]s fuse hydrogen; and (c)&nbsp;these objects may be luminous at visible wavelengths early in their lives. Because of this, alternative names for these objects were proposed, including {{not a typo|planetar}} and [[Substellar object|substar]]. In 1975, [[Jill Tarter]] suggested the term "brown dwarf", using "brown" as an approximate color.<ref name="Cain"/><ref name="JillTarter2014">{{citation |last=Tarter |first=Jill |author-link=Jill Tarter |chapter=Brown is Not a Color: Introduction of the Term 'Brown Dwarf' |pages=19–24 |editor-last=Joergens |editor-first=Viki |title=50 Years of Brown Dwarfs – From Prediction to Discovery to Forefront of Research |series=Astrophysics and Space Science Library |volume=401 |publisher=Springer |publication-date=2014 |isbn=978-3-319-01162-2 |chapter-url=https://www.springer.com/astronomy/book/978-3-319-01161-5|doi=10.1007/978-3-319-01162-2_3 |year=2014 }}</ref><ref>{{cite book |last1=Croswell |first1=Ken |title=Planet Quest: The Epic Discovery of Alien Solar Systems |date=1999 |publisher=Oxford University Press |isbn=978-0-192-88083-3 |pages=118–119 }}</ref>

The term "black dwarf" still refers to a [[white dwarf]] that has cooled to the point that it no longer emits significant amounts of light. However, the time required for even the lowest-mass white dwarf [[White dwarf#Radiation and cooling|to cool to this temperature]] is calculated to be longer than the current age of the universe; hence such objects are expected to not yet exist.<ref>{{cite web |date=April 10, 2020 |title=When will the Sun become a black dwarf? |url=https://astronomy.com/magazine/ask-astro/2020/04/when-will-the-sun-become-a-black-dwarf |access-date=2022-05-02 |website=Astronomy.com }}</ref>

Early theories concerning the nature of the lowest-mass stars and the [[hydrogen-burning limit]] suggested that a [[population I]] object with a mass less than 0.07&nbsp;[[solar mass]]es ({{Solar mass|link=y}}) or a [[population II]] object less than {{Solar mass|0.09}} would never go through normal [[stellar evolution]] and would become a completely [[degenerate star]].<ref name="Kumar1963">{{cite journal |last=Kumar |first=Shiv S. |date=1963 |title=The Structure of Stars of Very Low Mass |journal=Astrophysical Journal |volume=137 |page=1121 |bibcode=1963ApJ...137.1121K |doi=10.1086/147589 }}</ref> The resulting brown dwarf star is sometimes called a '''failed star'''.<ref>{{cite journal |last=Burgasser |first=Adam J. |date=2008-06-01 |title=Brown dwarfs: Failed stars, super jupiters |url=https://pubs.aip.org/physicstoday/article-abstract/61/6/70/931935/Brown-dwarfs-Failed-stars-super-jupitersThe?redirectedFrom=fulltext |journal=Physics Today |volume=61 |issue=6 |pages=70–71 |doi=10.1063/1.2947658 |bibcode=2008PhT....61f..70B |issn=0031-9228}}</ref> The first self-consistent calculation of the hydrogen-burning minimum mass confirmed a value between 0.07 and 0.08&nbsp;solar masses for population I objects.<ref name="Hayashi1963">{{cite journal |last1=Hayashi |first1=Chushiro |author-link1=Chushiro Hayashi |last2=Nakano |first2=Takenori |date=1963 |title=Evolution of Stars of Small Masses in the Pre-Main-Sequence Stages |journal=Progress of Theoretical Physics |volume=30 |pages=460–474 |bibcode=1963PThPh..30..460H |doi=10.1143/PTP.30.460 |number=4 |doi-access=free }}</ref><ref name=Nakano2014>{{citation |last=Nakano |first=Takenori |chapter=Pre-main Sequence Evolution and the Hydrogen-Burning Minimum Mass |pages=5–17 |editor-last=Joergens |editor-first=Viki |title=50 Years of Brown Dwarfs – From Prediction to Discovery to Forefront of Research |series=Astrophysics and Space Science Library |volume=401 |publisher=Springer |publication-date=2014 |isbn=978-3-319-01162-2 |chapter-url=https://www.springer.com/astronomy/book/978-3-319-01161-5|doi=10.1007/978-3-319-01162-2_2 |year=2014 |s2cid=73521636 }}</ref>

=== Deuterium fusion ===
The discovery of [[deuterium burning]] down to {{solar mass|0.013|link=yes}} ({{val|13.6|u=[[Jupiter mass|''M''{{sub|J}}]]}}) and the impact of dust formation in the cool outer [[atmosphere]]s of brown dwarfs in the late 1980s brought these theories into question. However, such objects were hard to find because they emit almost no visible light. Their strongest emissions are in the [[infrared]] (IR) spectrum, and ground-based IR detectors were too imprecise at that time to readily identify any brown dwarfs.

Since then, numerous searches by various methods have sought these objects. These methods included multi-color imaging surveys around field stars, imaging surveys for faint companions of [[main sequence|main-sequence]] dwarfs and [[white dwarfs]], surveys of young [[star clusters]], and [[radial velocity]] monitoring for close companions.

=== GD&nbsp;165B and class L ===
For many years, efforts to discover brown dwarfs were fruitless. In 1988, however, a faint companion to the white dwarf star [[GD 165]] was found in an infrared search of white dwarfs. The spectrum of the companion GD&nbsp;165B was very red and enigmatic, showing none of the features expected of a low-mass [[red dwarf]]. It became clear that GD 165B would need to be classified as a much cooler object than the latest M dwarfs then known. GD 165B remained unique for almost a decade until the advent of the Two Micron All-Sky Survey ([[2MASS]]) in 1997, which discovered many objects with similar colors and spectral features.

Today, GD&nbsp;165B is recognized as the prototype of a class of objects now called "L dwarfs".<ref name=Martin1997>{{cite journal |last1=Martín |first1=Eduardo L. |last2=Basri |first2=Gibor |author-link2=Gibor Basri |last3=Delfosse |first3=Xavier |last4=Forveille |first4=Thierry |title=Keck HIRES spectra of the brown dwarf DENIS-P J1228.2-1547 |journal=[[Astronomy and Astrophysics]] |volume=327 |pages=L29–L32 |date=1997 |bibcode=1997A&A...327L..29M}}</ref><ref name=Kirkpatrick1999>{{cite journal |last1=Kirkpatrick |first1=J. Davy |author-link1=J. Davy Kirkpatrick |last2=Reid |first2=I. Neill |last3=Liebert |first3=James |last4=Cutri |first4=Roc M. |last5=Nelson |first5=Brant |last6=Beichmann |first6=Charles A. |author-link6=Charles A. Beichman |last7=Dahn |first7=Conard C. |last8=Monet |first8=David G. |last9=Gizis |first9=John E. |last10=Skrutskie |first10=Michael F. |title=Dwarfs Cooler than ''M'': The Definition of Spectral Type ''L'' Using Discoveries from the 2 Micron All-Sky Survey (2MASS) |journal=The Astrophysical Journal |volume=519 |issue=2 |pages=802–833 |date=1999 |doi=10.1086/307414 |bibcode=1999ApJ...519..802K |s2cid=73569208 |url=https://trs.jpl.nasa.gov/bitstream/2014/37403/1/99-0060.pdf }}</ref>

Although the discovery of the coolest dwarf was highly significant at the time, it was debated whether GD&nbsp;165B would be classified as a brown dwarf or simply a very-low-mass star, because observationally it is very difficult to distinguish between the two.<ref>{{cite journal |last1=Zuckerman |first1=B. |last2=Becklin |first2=E. E. |date=February 1992 |title=Companions to white dwarfs - Very low-mass stars and the brown dwarf candidate GD 165B |url=http://adsabs.harvard.edu/doi/10.1086/171012 |journal=The Astrophysical Journal |language=en |volume=386 |pages=260 |bibcode=1992ApJ...386..260Z |doi=10.1086/171012 |issn=0004-637X}}</ref><ref>{{cite journal |last1=Kirkpatrick |first1=J. D. |last2=Henry |first2=Todd J. |last3=Liebert |first3=James |date=April 1993 |title=The unique spectrum of the brown dwarf candidate GD 165B and comparison to the spectra of other low-luminosity objects |url=http://adsabs.harvard.edu/doi/10.1086/172480 |journal=The Astrophysical Journal |language=en |volume=406 |pages=701 |bibcode=1993ApJ...406..701K |doi=10.1086/172480 |issn=0004-637X }}</ref>

Soon after the discovery of GD&nbsp;165B, other brown-dwarf candidates were reported. Most failed to live up to their candidacy, however, because the absence of lithium showed them to be stellar objects. True stars [[lithium burning|burn their lithium]] within a little over 100&nbsp;[[Myr]], whereas brown dwarfs (which can, confusingly, have temperatures and luminosities similar to true stars) will not. Hence, the detection of lithium in the atmosphere of an object older than 100&nbsp;Myr ensures that it is a brown dwarf.

=== Gliese 229B and class T ===
The first class "T" brown dwarf was discovered in 1994 by [[Caltech]] astronomers [[Shrinivas Kulkarni]], Tadashi Nakajima, Keith Matthews and [[Rebecca Oppenheimer]],<ref>{{cite web |url=https://hubblesite.org/contents/news-releases/1995/news-1995-48.html |title=Astronomers Announce First Clear Evidence of a Brown Dwarf |website=STScI |access-date=2019-10-23 }}</ref> and [[Johns Hopkins University|Johns Hopkins]] scientists [[Samuel T. Durrance]] and David Golimowski. It was confirmed in 1995 as a [[substellar companion]] to [[Gliese 229]]. Gliese 229b is one of the first two instances of clear evidence for a brown dwarf, along with [[Teide 1]]. Confirmed in 1995, both were identified by the presence of the 670.8&nbsp;nm lithium line. The latter was found to have a temperature and luminosity well below the stellar range.

Its near-infrared spectrum clearly exhibited a methane [[absorption band]] at 2&nbsp;micrometres, a feature that had previously only been observed in the atmospheres of giant planets and that of [[Saturn]]'s moon [[Titan (moon)|Titan]]. Methane absorption is not expected at any temperature of a main-sequence star. This discovery helped to establish yet another spectral class even cooler than '''L'''&nbsp;dwarfs, known as "'''T'''&nbsp;dwarfs", for which Gliese&nbsp;229B is the prototype.

=== Teide&nbsp;1 and class M ===
The first confirmed class "M" brown dwarf was discovered by Spanish astrophysicists [[Rafael Rebolo]] (head of the team), María Rosa Zapatero-Osorio, and Eduardo L. Martín in 1994.<ref>{{cite web |url=http://www.iac.es/ |title=Instituto de Astrofísica de Canarias, IAC |publisher=Iac.es |access-date=2013-03-16 }}</ref> This object, found in the [[Pleiades]] open cluster, received the name [[Teide 1]]. The discovery article was submitted to ''Nature'' in May 1995, and published on 14&nbsp;September 1995.<ref name="RafaelRebolo2014">{{cite book |last=Rebolo |first=Rafael |author-link=Rafael Rebolo |title=50 Years of Brown Dwarfs – From Prediction to Discovery to Forefront of Research |volume=401 |pages=25–50 |publication-date=2014 |year=2014 |editor-last=Joergens |editor-first=Viki |series=Astrophysics and Space Science Library |chapter=Teide 1 and the Discovery of Brown Dwarfs |chapter-url=https://www.springer.com/astronomy/book/978-3-319-01161-5 |publisher=Springer |doi=10.1007/978-3-319-01162-2_4 |bibcode=2014ASSL..401...25R |isbn=978-3-319-01162-2 }}</ref><ref>{{cite journal |title=Discovery of a brown dwarf in the Pleiades star cluster |last1=Rebolo |first1=Rafael |author-link1=Rafael Rebolo |last2=Zapatero-Osorio |first2=María Rosa |last3=Martín |first3=Eduardo L. |journal=Nature |volume=377 |issue=6545 |date=September 1995 |pages=129–131 |bibcode=1995Natur.377..129R |doi=10.1038/377129a0 |s2cid=28029538 }}</ref> ''Nature'' highlighted "Brown dwarfs discovered, official" on the front page of that issue.

[[Teide 1]] was discovered in images collected by the [[Instituto de Astrofísica de Canarias|IAC]] team on 6&nbsp;January 1994 using the 80&nbsp;cm telescope (IAC&nbsp;80) at [[Teide Observatory]], and its spectrum was first recorded in December 1994 using the 4.2&nbsp;m William Herschel Telescope at [[Roque de los Muchachos Observatory]] (La Palma). The distance, chemical composition, and age of Teide&nbsp;1 could be established because of its membership in the young Pleiades star cluster. Using the most advanced stellar and substellar evolution models at that moment, the team estimated for Teide&nbsp;1 a mass of {{Jupiter mass|55 ± 15}},<ref>{{cite journal |last1=Leech |first1=Kieron |last2=Altieri |first2=Bruno |last3=Metcalfe |first3=Liam |last4=Martin |first4=Eduardo L. |last5=Rebolo |first5=Rafael |author-link5=Rafael Rebolo |last6=Zapatero-Osorio |first6=María Rosa |last7=Laureijs |first7=René J. |last8=Prusti |first8=Timo |last9=Salama |first9=Alberto |last10=Siebenmorgen |first10=Ralf |last11=Claes |first11=Peter |last12=Trams |first12=Norman |date=2000 |title=Mid-IR Observations of the Pleiades Brown Dwarfs Teide 1 & Calar 3 |journal=ASP Conference Series |volume=212 |pages=82–87 |bibcode=2000ASPC..212...82L }}</ref> which is below the stellar-mass limit. The object became a reference in subsequent young brown dwarf related works.

In theory, a brown dwarf below {{Jupiter mass|65|link=y}} is unable to burn lithium by thermonuclear fusion at any time during its evolution. This fact is one of the [[lithium]] test principles used to judge the substellar nature of low-luminosity and low-surface-temperature astronomical bodies.

High-quality spectral data acquired by the [[W. M. Keck Observatory|Keck&nbsp;1 telescope]] in November 1995 showed that Teide&nbsp;1 still had the initial lithium abundance of the original molecular cloud from which Pleiades stars formed, proving the lack of thermonuclear fusion in its core. These observations fully confirmed that Teide&nbsp;1 is a brown dwarf, as well as the efficiency of the spectroscopic [[Lithium burning|lithium test]].<ref>{{cite journal |author1=R. Rebolo |author2=E. L. Martín |author3=G. Basri |author4=G. W. Marcy |author5=M. R. Zapatero-Osorio |date=1996 |title=Brown Dwarfs in the Pleiades Cluster Confirmed by the Lithium Test |journal=The Astrophysical Journal |volume=469 |pages=53–56 |arxiv=astro-ph/9607002 |bibcode=1996ApJ...469L..53R |doi=10.1086/310263 |s2cid=119485127}}</ref>

For some time, Teide&nbsp;1 was the smallest known object outside the Solar System that had been identified by direct observation. Since then, over 1,800&nbsp;brown dwarfs have been identified,<ref name="DwarfArchives"/> even some very close to Earth, like [[Epsilon Indi]]&nbsp;Ba and Bb, a pair of brown dwarfs gravitationally bound to a Sun-like star 12&nbsp;light-years from the Sun,<ref>{{cite journal |title=Epsilon Indi Ba/Bb: the nearest binary brown dwarf |first1=Mark J. |last1=McCaughrean |first2=Laird M. |last2=Close |first3=Ralf-Dieter |last3=Scholz |first4=Rainer |last4=Lenzen |first5=Beth A. |last5=Biller |first6=Wolfgang |last6=Brandner |first7=Markus |last7=Hartung |first8=Nicolas |last8=Lodieu |journal=Astronomy & Astrophysics |volume=413 |issue=3 |date=January 2004 |pages=1029–1036 |doi=10.1051/0004-6361:20034292 |arxiv=astro-ph/0309256|s2cid=15407249 }}</ref> and Luhman 16, a binary system of brown dwarfs at 6.5&nbsp;light-years from the Sun.

== Theory ==
{{more citations needed section|date=July 2020}}
{{Star nav}}
The standard mechanism for [[stellar evolution|star birth]] is through the gravitational collapse of a cold interstellar cloud of gas and dust. As the cloud contracts, it heats due to the [[Kelvin–Helmholtz mechanism]]. Early in the process the contracting gas quickly radiates away much of the energy, allowing the collapse to continue. Eventually, the central region becomes sufficiently dense to trap radiation. Consequently, the central temperature and density of the collapsed cloud increase dramatically with time, slowing the contraction, until the conditions are hot and dense enough for thermonuclear reactions to occur in the core of the [[protostar]]. For a typical star, gas and radiation pressure generated by the [[thermonuclear fusion]] reactions within its core will support it against any further gravitational contraction. [[Hydrostatic equilibrium]] is reached, and the star will spend most of its lifetime fusing hydrogen into helium as a main-sequence star.

If, however, the initial<ref name=Forbes_Loeb_2019/> mass of the protostar is less than about {{Solar mass|0.08}},<ref>{{cite journal | title=The theory of brown dwarfs and extrasolar giant planets | last1=Burrows | first1=Adam | last2=Hubbard | first2=W. B. | last3=Lunine | first3=J. I. | last4=Liebert | first4=James | journal=Reviews of Modern Physics | volume=73 | issue=3 | pages=719–765 | date=July 2001 | doi=10.1103/RevModPhys.73.719 | arxiv=astro-ph/0103383 | bibcode=2001RvMP...73..719B | s2cid=204927572 | quote=Hence the HBMM at solar metallicity and Y<sub>α</sub> = 50.25 is 0.07 – 0.074 {{solar mass}}, ... while the HBMM at zero metallicity is 0.092 {{solar mass}} }}</ref> normal hydrogen [[thermonuclear fusion]] reactions will not ignite in the core. Gravitational contraction does not heat the small [[protostar]] very effectively, and before the temperature in the core can increase enough to trigger fusion, the density reaches the point where electrons become closely packed enough to create quantum [[electron degeneracy pressure]]. According to the brown dwarf interior models, typical conditions in the core for density, temperature and pressure are expected to be the following:
* <math>10\,\mathrm{g/cm^3} \,\lesssim\, \rho_c \,\lesssim\, 10^3\,\mathrm{{g}/{cm^{3}}} </math>
* <math>T_c \lesssim 3 \times 10^6\,\mathrm{K} </math>
* <math>P_c \sim 10^5\,\mathrm{Mbar}.</math>
<!-- [[File:tdwarf_art.jpg|thumb|left|alt=A picture of a Brown Dwarf|An artist's impression of a brown dwarf and its moon]] -->
This means that the protostar is not massive or dense enough ever to reach the conditions needed to sustain hydrogen fusion. The infalling matter is prevented, by electron degeneracy pressure, from reaching the densities and pressures needed.

Further gravitational contraction is prevented and the result is a brown dwarf that simply cools off by radiating away its internal thermal energy. Note that, in principle, it is possible for a brown dwarf to slowly accrete mass above the [[hydrogen burning limit]] without initiating hydrogen fusion. This could happen via mass transfer in a binary brown dwarf system.<ref name=Forbes_Loeb_2019/>

=== High-mass brown dwarfs versus low-mass stars ===
[[Lithium]] is generally present in brown dwarfs and not in low-mass stars. Stars, which reach the high temperature necessary for fusing hydrogen, rapidly deplete their lithium. Fusion of [[lithium-7]] and a [[proton]] occurs, producing two [[helium-4]] nuclei. The temperature necessary for this reaction is just below that necessary for hydrogen fusion. Convection in low-mass stars ensures that lithium in the whole volume of the star is eventually depleted. Therefore, the presence of the lithium spectral line in a candidate brown dwarf is a strong indicator that it is indeed a substellar object.

==== Lithium test ====
The use of lithium to distinguish candidate brown dwarfs from low-mass stars is commonly referred to as the '''lithium test''', and was pioneered by [[Rafael Rebolo López|Rafael Rebolo]], [[Eduardo Martín]] and [[Antonio Magazzu]]. However, lithium is also seen in very young stars, which have not yet had enough time to burn it all.

Heavier stars, like the Sun, can also retain lithium in their outer layers, which never get hot enough to fuse lithium, and whose convective layer does not mix with the core where the lithium would be rapidly depleted. Those larger stars are easily distinguishable from brown dwarfs by their size and luminosity.

Conversely, brown dwarfs at the high end of their mass range can be hot enough to deplete their lithium when they are young. Dwarfs of mass greater than {{Jupiter mass|65}} can burn their lithium by the time they are half a billion years old;<ref>{{cite journal |last1=Kulkarni |first1=Shrinivas R. |author-link=Shrinivas Kulkarni |title=Brown Dwarfs: A Possible Missing Link Between Stars and Planets |journal=Science |date=30 May 1997 |volume=276 |issue=5317 |pages=1350–1354 |doi=10.1126/science.276.5317.1350 |bibcode=1997Sci...276.1350K }}</ref> thus the lithium test is not perfect.

==== Atmospheric methane ====
Unlike stars, older brown dwarfs are sometimes cool enough that, over very long periods of time, their atmospheres can gather observable quantities of [[methane]], which cannot form in hotter objects. Dwarfs confirmed in this fashion include [[Gliese 229]]B.

==== Iron, silicate and sulfide clouds ====
Main-sequence stars cool, but eventually reach a minimum [[bolometric luminosity]] that they can sustain through steady fusion. This luminosity varies from star to star, but is generally at least 0.01% that of the Sun.{{citation needed|reason=as in talk page|date=April 2013}} Brown dwarfs cool and darken steadily over their lifetimes; sufficiently old brown dwarfs will be too faint to be detectable.
[[File:Brown dwarf clouds.png|thumb|300x300px|Cloud models for the early T-type brown dwarfs [[SIMP J013656.5+093347|SIMP J0136+09]] and [[2MASS J21392676+0220226|2MASS J2139+02]] (left two panels) and the late T-type brown dwarf 2M0050–3322.]]
Clouds are used to explain the weakening of the [[Iron(I) hydride|iron hydride]] (FeH) spectral line in late L-dwarfs. [[Iron]] clouds deplete FeH in the upper atmosphere, and the cloud layer blocks the view to lower layers still containing FeH. The later strengthening of this chemical compound at cooler temperatures of mid- to late T-dwarfs is explained by disturbed clouds that allows a telescope to look into the deeper layers of the atmosphere that still contains FeH.<ref>{{cite journal |last1=Burgasser |first1=Adam J. |last2=Marley |first2=Mark S. |last3=Ackerman |first3=Andrew S. |last4=Saumon |first4=Didier |last5=Lodders |first5=Katharina |last6=Dahn |first6=Conard C. |last7=Harris |first7=Hugh C. |last8=Kirkpatrick |first8=J. Davy |date=2002-06-01 |title=Evidence of Cloud Disruption in the L/T Dwarf Transition |journal=The Astrophysical Journal |volume=571 |issue=2 |pages=L151–L154 |doi=10.1086/341343 |arxiv=astro-ph/0205051 |bibcode=2002ApJ...571L.151B |issn=0004-637X|doi-access=free }}</ref> Young L/T-dwarfs (L2-T4) show high [[Variable star|variability]], which could be explained with clouds, hot spots, magnetically driven [[aurora]]e or [[Thermochemistry|thermochemical]] instabilities.<ref>{{cite journal |last1=Vos |first1=Johanna M. |last2=Faherty |first2=Jacqueline K. |last3=Gagné |first3=Jonathan |last4=Marley |first4=Mark |last5=Metchev |first5=Stanimir |last6=Gizis |first6=John |last7=Rice |first7=Emily L. |last8=Cruz |first8=Kelle |date=2022-01-01 |title=Let the Great World Spin: Revealing the Stormy, Turbulent Nature of Young Giant Exoplanet Analogs with the Spitzer Space Telescope |journal=The Astrophysical Journal |volume=924 |issue=2 |pages=68 |doi=10.3847/1538-4357/ac4502 |arxiv=2201.04711 |bibcode=2022ApJ...924...68V |issn=0004-637X|doi-access=free }}</ref> The clouds of these brown dwarfs are explained as either iron clouds with varying thickness or a lower thick iron cloud layer and an upper [[silicate]] cloud layer. This upper silicate cloud layer can consist out of [[quartz]], [[enstatite]], [[corundum]] and/or [[Forsterite|fosterite]].<ref>{{cite journal |last1=Vos |first1=Johanna M. |last2=Burningham |first2=Ben |last3=Faherty |first3=Jacqueline K. |last4=Alejandro |first4=Sherelyn |last5=Gonzales |first5=Eileen |last6=Calamari |first6=Emily |last7=Bardalez Gagliuffi |first7=Daniella |last8=Visscher |first8=Channon |last9=Tan |first9=Xianyu |last10=Morley |first10=Caroline V. |last11=Marley |first11=Mark |last12=Gemma |first12=Marina E. |last13=Whiteford |first13=Niall |last14=Gaarn |first14=Josefine |last15=Park |first15=Grace |date=2023-02-01 |title=Patchy Forsterite Clouds in the Atmospheres of Two Highly Variable Exoplanet Analogs |journal=The Astrophysical Journal |volume=944 |issue=2 |pages=138 |doi=10.3847/1538-4357/acab58 |arxiv=2212.07399 |bibcode=2023ApJ...944..138V |issn=0004-637X|doi-access=free }}</ref><ref>{{cite journal |last1=Manjavacas |first1=Elena |last2=Karalidi |first2=Theodora |last3=Vos |first3=Johanna M. |last4=Biller |first4=Beth A. |last5=Lew |first5=Ben W. P. |date=2021-11-01 |title=Revealing the Vertical Cloud Structure of a Young Low-mass Brown Dwarf, an Analog to the β-Pictoris b Directly Imaged Exoplanet, through Keck I/MOSFIRE Spectrophotometric Variability |journal=The Astronomical Journal |volume=162 |issue=5 |pages=179 |doi=10.3847/1538-3881/ac174c |arxiv=2107.12368 |bibcode=2021AJ....162..179M |issn=0004-6256|doi-access=free }}</ref> It is however not clear if silicate clouds are always necessary for young objects.<ref>{{cite journal |last1=Tremblin |first1=P. |last2=Chabrier |first2=G. |last3=Baraffe |first3=I. |last4=Liu |first4=Michael. C. |last5=Magnier |first5=E. A. |last6=Lagage |first6=P. -O. |last7=Alves de Oliveira |first7=C. |last8=Burgasser |first8=A. J. |last9=Amundsen |first9=D. S. |last10=Drummond |first10=B. |date=2017-11-01 |title=Cloudless Atmospheres for Young Low-gravity Substellar Objects |journal=The Astrophysical Journal |volume=850 |issue=1 |pages=46 |doi=10.3847/1538-4357/aa9214 |arxiv=1710.02640 |bibcode=2017ApJ...850...46T |issn=0004-637X|doi-access=free }}</ref> Silicate absorption can be directly observed in the [[Infrared astronomy|mid-infrared]] at 8 to 12 μm. Observations with [[Spitzer Space Telescope#Instruments|Spitzer IRS]] have shown that silicate absorption is common, but not ubiquitous, for L2-L8 dwarfs.<ref>{{cite journal |last1=Suárez |first1=Genaro |last2=Metchev |first2=Stanimir |date=2022-07-01 |title=Ultracool dwarfs observed with the Spitzer infrared spectrograph - II. Emergence and sedimentation of silicate clouds in L dwarfs, and analysis of the full M5-T9 field dwarf spectroscopic sample |journal=Monthly Notices of the Royal Astronomical Society |volume=513 |issue=4 |pages=5701–5726 |doi=10.1093/mnras/stac1205 |doi-access=free |issn=0035-8711|arxiv=2205.00168 |bibcode=2022MNRAS.513.5701S }}</ref> Additionally, [[Mid-Infrared Instrument|MIRI]] has observed silicate absorption in the planetary-mass companion [[VHS J1256–1257|VHS 1256b]].<ref>{{cite journal |last1=Miles |first1=Brittany E. |last2=Biller |first2=Beth A. |last3=Patapis |first3=Polychronis |last4=Worthen |first4=Kadin |last5=Rickman |first5=Emily |last6=Hoch |first6=Kielan K. W. |last7=Skemer |first7=Andrew |last8=Perrin |first8=Marshall D. |last9=Whiteford |first9=Niall |last10=Chen |first10=Christine H. |last11=Sargent |first11=B. |last12=Mukherjee |first12=Sagnick |last13=Morley |first13=Caroline V. |last14=Moran |first14=Sarah E. |last15=Bonnefoy |first15=Mickael |date=2023-03-01 |title=The JWST Early-release Science Program for Direct Observations of Exoplanetary Systems II: A 1 to 20 μm Spectrum of the Planetary-mass Companion VHS 1256-1257 b |journal=The Astrophysical Journal |volume=946 |issue=1 |pages=L6 |doi=10.3847/2041-8213/acb04a |arxiv=2209.00620 |bibcode=2023ApJ...946L...6M |issn=0004-637X|doi-access=free }}</ref>

'''Iron rain''' as part of atmospheric convection processes is possible only in brown dwarfs, and not in small stars. The spectroscopy research into iron rain is still ongoing, but not all brown dwarfs will always have this atmospheric anomaly. In 2013, a heterogeneous iron-containing atmosphere was imaged around the B component in the nearby Luhman 16 system.<ref>{{cite journal |last1=Biller |first1=Beth A. |last2=Crossfield |first2=Ian J. M. |last3=Mancini |first3=Luigi |last4=Ciceri |first4=Simona |last5=Southworth |first5=John |last6=Kopytova |first6=Taisiya G. |last7=Bonnefoy |first7=Mickaël |last8=Deacon |first8=Niall R. |last9=Schlieder |first9=Joshua E. |last10=Buenzli |first10=Esther |last11=Brandner |first11=Wolfgang |last12=Allard |first12=France |last13=Homeier |first13=Derek |last14=Freytag |first14=Bernd |last15=Bailer-Jones |first15=Coryn A. L. |last16=Greiner |first16=Jochen |last17=Henning |first17=Thomas |last18=Goldman |first18=Bertrand |title=Weather on the Nearest Brown Dwarfs: Resolved Simultaneous Multi-Wavelength Variability Monitoring of WISE J104915.57–531906.1AB |journal=[[The Astrophysical Journal Letters]] |date=6 November 2013 |volume=778| issue=1 |pages=L10 |doi=10.1088/2041-8205/778/1/l10 |arxiv=1310.5144 |bibcode=2013ApJ...778L..10B |s2cid=56107487 }}</ref>

For late T-type brown dwarfs only a few variable searches were carried out. Thin cloud layers are predicted to form in late T-dwarfs from [[chromium]] and [[potassium chloride]], as well as several [[sulfide]]s. These sulfides are [[Manganese(II) sulfide|manganese sulfide]], [[sodium sulfide]] and [[zinc sulfide]].<ref>{{cite journal |last1=Morley |first1=Caroline V. |last2=Fortney |first2=Jonathan J. |last3=Marley |first3=Mark S. |last4=Visscher |first4=Channon |last5=Saumon |first5=Didier |last6=Leggett |first6=S. K. |date=2012-09-01 |title=Neglected Clouds in T and Y Dwarf Atmospheres |url=https://ui.adsabs.harvard.edu/abs/2012ApJ...756..172M |journal=The Astrophysical Journal |volume=756 |issue=2 |pages=172 |doi=10.1088/0004-637X/756/2/172 |issn=0004-637X|arxiv=1206.4313 |bibcode=2012ApJ...756..172M |s2cid=118398946 }}</ref> The variable T7 dwarf [[2M0050–3322]] is explained to have a top layer of potassium chloride clouds, a mid layer of sodium sulfide clouds and a lower layer of manganese sulfide clouds. Patchy clouds of the top two cloud layers could explain why the methane and water vapor bands are variable.<ref>{{cite journal |last1=Manjavacas |first1=Elena |last2=Karalidi |first2=Theodora |last3=Tan |first3=Xianyu |last4=Vos |first4=Johanna M. |last5=Lew |first5=Ben W. P. |last6=Biller |first6=Beth A. |last7=Oliveros-Gómez |first7=Natalia |date=2022-08-01 |title=Top-of-the-atmosphere and Vertical Cloud Structure of a Fast-rotating Late T Dwarf |journal=The Astronomical Journal |volume=164 |issue=2 |pages=65 |doi=10.3847/1538-3881/ac7953 |arxiv=2206.07566 |bibcode=2022AJ....164...65M |issn=0004-6256|doi-access=free }}</ref>

At the lowest temperatures of the Y-dwarf [[WISE 0855−0714|WISE 0855-0714]] patchy cloud layers of sulfide and [[Ice|water ice]] clouds could cover 50% of the surface.<ref>{{cite journal |last1=Faherty |first1=Jacqueline K. |last2=Tinney |first2=C. G. |last3=Skemer |first3=Andrew |last4=Monson |first4=Andrew J. |date=2014-09-01 |title=Indications of Water Clouds in the Coldest Known Brown Dwarf |url=https://ui.adsabs.harvard.edu/abs/2014ApJ...793L..16F |journal=The Astrophysical Journal |volume=793 |issue=1 |pages=L16 |doi=10.1088/2041-8205/793/1/L16 |issn=0004-637X|arxiv=1408.4671 |bibcode=2014ApJ...793L..16F |s2cid=119246100 }}</ref>

=== 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 name="Wethington"/><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>

=== Role of other physical properties in the mass estimate ===
While spectroscopic features can help to distinguish between [[Red dwarf|low-mass stars]] and brown dwarfs, it is often necessary to estimate the mass to come to a conclusion. The theory behind the mass estimate is that brown dwarfs with a similar mass form in a similar way and are hot when they form. Some have spectral types that are similar to low-mass stars, such as [[2M1101AB]]. As they cool down the brown dwarfs should retain a range of [[Luminosity|luminosities]] depending on the mass.<ref>{{cite journal |last1=Saumon |first1=Didier |last2=Marley |first2=Mark S. |date=December 2008 |title=The Evolution of L and T Dwarfs in Color-Magnitude Diagrams |journal=Astrophysical Journal |volume=689 |issue=2 |pages=1327–1344 |doi=10.1086/592734 |arxiv=0808.2611 |bibcode=2008ApJ...689.1327S |s2cid=15981010 |issn=0004-637X }}</ref><!-- See Figure 2 for example --> Without the age and luminosity, a mass estimate is difficult; for example, an L-type brown dwarf could be an old brown dwarf with a high mass (possibly a low-mass star) or a young brown dwarf with a very low mass. For Y dwarfs this is less of a problem, as they remain low-mass objects near the [[sub-brown dwarf]] limit, even for relatively high age estimates.<ref name=":2">{{cite journal |last1=Marocco |first1=Federico |last2=Kirkpatrick |first2=J. Davy |last3=Meisner |first3=Aaron M. |last4=Caselden |first4=Dan |last5=Eisenhardt |first5=Peter R. M. |last6=Cushing |first6=Michael C. |last7=Faherty |first7=Jacqueline K. |last8=Gelino |first8=Christopher R. |last9=Wright |first9=Edward L. |year=2020 |title=Improved infrared photometry and a preliminary parallax measurement for the extremely cold brown dwarf CWISEP J144606.62-231717.8 |journal=The Astrophysical Journal |volume=888 |issue=2 |pages=L19 |arxiv=1912.07692 |bibcode=2020ApJ...888L..19M |doi=10.3847/2041-8213/ab6201 |s2cid=209386563 |doi-access=free}}</ref> For L and T dwarfs it is still useful to have an accurate age estimate. The luminosity is here the less concerning property, as this can be estimated from the [[spectral energy distribution]].<ref>{{cite journal |last1=Filippazzo |first1=Joseph C. |last2=Rice |first2=Emily L. |last3=Faherty |first3=Jacqueline K.|author3-link=Jackie Faherty |last4=Cruz |first4=Kelle L. |last5=Van Gordon |first5=Mollie M. |last6=Looper |first6=Dagny L. |date=September 2015 |title=Fundamental Parameters and Spectral Energy Distributions of Young and Field Age Objects with Masses Spanning the Stellar to Planetary Regime |journal=Astrophysical Journal |volume=810 |issue=2 |pages=158 |doi=10.1088/0004-637X/810/2/158 |arxiv=1508.01767 |bibcode=2015ApJ...810..158F |s2cid=89611607 |issn=0004-637X }}</ref> The age estimate can be done in two ways. Either the brown dwarf is young and still has spectral features that are associated with youth, or the brown dwarf co-moves with a star or stellar group ([[star cluster]] or [[Stellar association|association]]), where age estimates are easier to obtain. A very young brown dwarf that was further studied with this method is [[2M1207]] and the companion [[2M1207b]]. Based on the location, [[proper motion]] and spectral signature, this object was determined to belong to the ~8-million-year-old [[TW Hydrae association]], and the mass of the secondary was determined to be 8 ± 2 {{Jupiter mass|link=true}}, below the [[deuterium]] burning limit.<ref>{{cite journal |last1=Mohanty |first1=Subhanjoy |last2=Jayawardhana |first2=Ray |last3=Huélamo |first3=Nuria |last4=Mamajek |first4=Eric |date=March 2007 |title=The Planetary Mass Companion 2MASS 1207-3932B: Temperature, Mass, and Evidence for an Edge-on Disk |journal=Astrophysical Journal |volume=657 |issue=2 |pages=1064–1091 |doi=10.1086/510877 |arxiv=astro-ph/0610550 |bibcode=2007ApJ...657.1064M |s2cid=17326111 |issn=0004-637X }}</ref> An example of a very old age obtained by the co-movement method is the brown dwarf + [[white dwarf]] binary COCONUTS-1, with the white dwarf estimated to be {{Val|7.3|2.8|1.6}} [[billion years]] old. In this case the mass was not estimated with the derived age, but the co-movement provided an accurate distance estimate, using [[Gaia (spacecraft)|Gaia]] [[parallax]]. Using this measurement the authors estimated the radius, which was then used to estimate the mass for the brown dwarf as {{Val|15.4|0.9|0.8}} {{Jupiter mass}}.<ref name=":3">{{cite journal |last1=Zhang |first1=Zhoujian |last2=Liu |first2=Michael C. |last3=Hermes |first3=James J. <!-- https://www.bu.edu/cas/ar/2018/7/ a.k.a. "Jota Jota" (https://jjhermes.es), full name James Joseph Hermes Jr. https://repositories.lib.utexas.edu/bitstream/handle/2152/21608/HERMES-DISSERTATION-2013.pdf -->|last4=Magnier |first4=Eugene A. |last5=Marley |first5=Mark S. |last6=Tremblay |first6=Pier-Emmanuel |last7=Tucker |first7=Michael A. |last8=Do |first8=Aaron |last9=Payne |first9=Anna V. |last10=Shappee |first10=Benjamin J. |date=February 2020 |title=COol Companions ON Ultrawide orbiTS (COCONUTS). I. A High-Gravity T4 Benchmark around an Old White Dwarf and A Re-Examination of the Surface-Gravity Dependence of the L/T Transition |journal=The Astrophysical Journal |volume=891 |issue=2 |page=171 |doi=10.3847/1538-4357/ab765c |arxiv=2002.05723 |bibcode=2020ApJ...891..171Z |s2cid=211126544 |doi-access=free }}</ref>

== Observations ==

=== Classification of brown dwarfs ===

==== Spectral class M ====
[[Image:late-M-dwarf-nasa-hurt.png|thumb|left|Artist's vision of a late-M dwarf]]
These are brown dwarfs with a spectral class of M5.5 or later; they are also called late-M dwarfs. Some scientists regard them as [[red dwarf]]s.{{citation needed|date=February 2020}} All brown dwarfs with spectral type M are young objects, such as [[Teide 1]], which is the first M-type brown dwarf discovered, and [[LP 944-20]], the closest M-type brown dwarf.

==== Spectral class L ====
{{main|L dwarf}}
[[Image:L-dwarf-nasa-hurt.png|thumb|Artist's concept of an L dwarf]]

The defining characteristic of [[spectral class]] M, the coolest type in the long-standing classical stellar sequence, is an optical spectrum dominated by absorption bands of [[titanium(II) oxide]] (TiO) and [[vanadium(II) oxide]] (VO) molecules. However, [[GD 165]]B, the cool companion to the white dwarf [[GD 165]], had none of the hallmark TiO features of M dwarfs. The subsequent identification of many objects like GD 165B ultimately led to the definition of a new [[spectral class]], the '''L dwarfs''', defined in the red optical region of the spectrum not by metal-oxide absorption bands (TiO, VO), but by metal [[hydride]] emission bands ([[FeH]], [[CrH]], [[magnesium hydride|MgH]], [[calcium hydride|CaH]]) and prominent atomic lines of [[alkali metal]]s (Na, K, Rb, Cs). {{As of|2013}}, over 900 L&nbsp;dwarfs had been identified,<ref name="DwarfArchives"/> most by wide-field surveys: the Two Micron All Sky Survey ([[2MASS]]), the [[Deep Near Infrared Survey of the Southern Sky]] (DENIS), and the [[Sloan Digital Sky Survey]] (SDSS). This spectral class also contains the coolest main-sequence stars (>&nbsp;80&nbsp;''M''<sub>J</sub>), which have spectral classes L2 to L6.<ref>{{cite journal |last1=Smart |first1=Richard L. |last2=Bucciarelli |first2=Beatrice |last3=Jones |first3=Hugh R. A. |last4=Marocco |first4=Federico |last5=Andrei |first5=Alexandre Humberto |last6=Goldman |first6=Bertrand |last7=Méndez |first7=René A. |last8=d'Avila |first8=Victor de A. |last9=Burningham |first9=Ben |last10=Camargo |first10=Julio Ignácio Bueno de |last11=Crosta |first11=Maria Teresa |first12=Mario |last12=Daprà |first13=James S. |last13=Jenkins |first14=Regis |last14=Lachaume |first15=Mario G. |last15=Lattanzi |first16=Jucira L. |last16=Penna |first17=David J. |last17=Pinfield |first18=Dario Nepomuceno |last18=da Silva Neto |first19=Alessandro |last19=Sozzetti |first20=Alberto |last20=Vecchiato |date=December 2018 |title=Parallaxes of Southern Extremely Cool objects III: 118 L and T dwarfs |journal=MNRAS |volume=481 |issue=3 |pages=3548–3562 |doi=10.1093/mnras/sty2520 |doi-access=free |arxiv=1811.00672 |bibcode=2018MNRAS.481.3548S |s2cid=119390019 |issn=0035-8711 }}</ref>

==== Spectral class T ====
{{main|T dwarf}}
[[Image:T-dwarf-nasa-hurt.png|thumb|left|Artist's concept of a T dwarf]]

As GD 165B is the prototype of the L dwarfs, [[Gliese 229]]B is the prototype of a second new spectral class, the '''T dwarfs'''. T&nbsp;dwarfs are pinkish-magenta. Whereas [[near-infrared]] (NIR) spectra of L dwarfs show strong absorption bands of H<sub>2</sub>O and [[carbon monoxide]] (CO), the NIR spectrum of Gliese 229B is dominated by absorption bands from [[methane]] (CH<sub>4</sub>), a feature which in the Solar System is found only in the giant planets and [[Titan (moon)|Titan]]. CH<sub>4</sub>, H<sub>2</sub>O, and molecular [[hydrogen]] (H<sub>2</sub>) collision-induced absorption (CIA) give Gliese 229B blue near-infrared colors. Its steeply sloped red optical spectrum also lacks the FeH and CrH bands that characterize L dwarfs and instead is influenced by exceptionally broad absorption features from the [[alkali]] metals [[sodium|Na]] and [[potassium|K]]. These differences led [[J. Davy Kirkpatrick]] to propose the T spectral class for objects exhibiting H- and K-band CH<sub>4</sub> absorption. {{As of|2013}}, 355 T&nbsp;dwarfs were known.<ref name="DwarfArchives"/> NIR classification schemes for T dwarfs have recently been developed by Adam Burgasser and Tom Geballe. Theory suggests that L dwarfs are a mixture of very-low-mass stars and sub-stellar objects (brown dwarfs), whereas the T dwarf class is composed entirely of brown dwarfs. Because of the absorption of [[sodium]] and [[potassium]] in the green part of the spectrum of T dwarfs, the actual appearance of T dwarfs to human [[visual perception]] is estimated to be not brown, but [[magenta]].<ref name=burrows>{{cite journal |last1=Burrows |first1=Adam |last2=Hubbard |first2=William B. |last3=Lunine |first3=Jonathan I. |last4=Liebert |first4=James |year=2001 |title=The theory of brown dwarfs and extrasolar giant planets |journal=[[Reviews of Modern Physics]] |volume=73 |issue=3 |pages=719–765 |doi=10.1103/RevModPhys.73.719 |bibcode=2001RvMP...73..719B |arxiv=astro-ph/0103383 |s2cid=204927572 }}</ref><ref>[http://spider.ipac.caltech.edu/staff/davy/2mass/science/comparison.html "An Artist's View of Brown Dwarf Types"] {{Webarchive |first=Robert |last=Hurt |newspaper=Infrared Processing and Analysis Center |url=https://web.archive.org/web/20111117180311/http://spider.ipac.caltech.edu/staff/davy/2mass/science/comparison.html |date=2011-11-17 }}</ref> Early observations limited how distant T-dwarfs could be observed. T-class brown dwarfs, such as [[WISE 0316+4307]], have been detected more than 100 light-years from the Sun. Observations with JWST have detected T-dwarfs such as [[UNCOVER-BD-1]] up to 4500 parsec distant from the sun.

==== Spectral class Y ====
{{Main|Y dwarf}}
[[File:WISE 1828+2650 Brown dwarf.jpg|thumb|Artist's vision of a Y dwarf]]

In 2009, the coolest-known brown dwarfs had estimated effective temperatures between {{cvt|500|and(-)|600|K|C F|lk=on}}, and have been assigned the spectral class T9. Three examples are the brown dwarfs [[CFBDS J005910.90–011401.3]], [[ULAS J133553.45+113005.2]] and [[ULAS J003402.77−005206.7]].<ref name=four600k>{{cite journal |doi=10.1088/0004-637X/695/2/1517 |bibcode=2009ApJ...695.1517L |title=The Physical Properties of Four ~600 K T Dwarfs |journal=The Astrophysical Journal |volume=695 |issue=2 |pages=1517–1526 |last1=Leggett |first1=Sandy K. |last2=Cushing |first2=Michael C. |last3=Saumon |first3=Didier |last4=Marley |first4=Mark S. |last5=Roellig |first5=Thomas L. |last6=Warren |first6=Stephen J. |last7=Burningham |first7=Ben |last8=Jones |first8=Hugh R. A. |last9=Kirkpatrick |first9=J. Davy |last10=Lodieu |first10=Nicolas |last11=Lucas |first11=Philip W. |last12=Mainzer |first12=Amy K. |last13=Martín |first13=Eduardo L. |last14=McCaughrean |first14=Mark J. |last15=Pinfield |first15=David J. |last16=Sloan |first16=Gregory C. |last17=Smart |first17=Richard L. |last18=Tamura |first18=Motohide |last19=Van Cleve |first19=Jeffrey |year=2009 |arxiv=0901.4093 |s2cid=44050900 }}.</ref> The spectra of these objects have absorption peaks around 1.55 micrometres.<ref name=four600k/> Delorme et al. have suggested that this feature is due to absorption from [[ammonia]] and that this should be taken as indicating the T–Y transition, making these objects of type Y0.<ref name=four600k/><ref name=tytrans>{{cite journal |doi=10.1051/0004-6361:20079317 |bibcode=2008A&A...482..961D |title=CFBDS J005910.90-011401.3: Reaching the T–Y brown dwarf transition? |journal=Astronomy and Astrophysics |volume=482 |issue=3 |pages=961–971 |year=2008 |last1=Delorme |first1=Philippe |last2=Delfosse |first2=Xavier |last3=Albert |first3=Loïc |last4=Artigau |first4=Étienne |last5=Forveille |first5=Thierry |last6=Reylé |first6=Céline |last7=Allard |first7=France |last8=Homeier |first8=Derek |last9=Robin |first9=Annie C. |last10=Willott |first10=Chris J. |last11=Liu |first11=Michael C. |last12=Dupuy |first12=Trent J. |arxiv=0802.4387 |s2cid=847552 }}</ref> However, the feature is difficult to distinguish from absorption by water and [[methane]],<ref name=four600k/> and other authors have stated that the assignment of class Y0 is premature.<ref name="Burninghametal2008"/>

[[File:WISE2010-040-rotate180.jpg|thumb|right|[[WISEPC J045853.90+643451.9|WISE 0458+6434]] is the first ultra-cool brown dwarf (green dot) discovered by [[Wide-field Infrared Survey Explorer|WISE]]. The green and blue comes from infrared wavelengths mapped to visible colors.]]

The first [[James Webb Space Telescope|JWST]] spectral energy distribution of a Y-dwarf was able to observe several bands of molecules in the atmosphere of the Y0-dwarf [[WISE 0359−5401]]. The observations covered spectroscopy from 1 to 12 μm and photometry at 15, 18 and 21 μm. The molecules water (H<sub>2</sub>O), methane (CH<sub>4</sub>), carbon monoxide (CO), carbon dioxide (CO<sub>2</sub>) and ammonia (NH<sub>3</sub>) were detected in WISE 0359−5401. Many of these features have been observed before in this Y-dwarf and warmer T-dwarfs by other observatories, but JWST was able to observe them in a single spectrum. Methane is the main reservoir of carbon in the atmosphere of WISE 0359−5401, but there is still enough carbon left to form detectable carbon monoxide (at 4.5–5.0 μm) and carbon dioxide (at 4.2–4.35 μm) in the Y-dwarf. Ammonia was difficult to detect before JWST, as it blends in with the absorption feature of water in the near-infrared, as well at 5.5–7.1 μm. At longer wavelengths of 8.5–12 μm the spectrum of WISE 0359−5401 is dominated by the absorption of ammonia. At 3 μm there is an additional newly detected ammonia feature.<ref>{{cite journal |last1=Beiler |first1=Samuel A. |last2=Cushing |first2=Michael C. |last3=Kirkpatrick |first3=J. Davy |last4=Schneider |first4=Adam C. |last5=Mukherjee |first5=Sagnick |last6=Marley |first6=Mark S. |date=2023-07-01 |title=The First JWST Spectral Energy Distribution of a Y Dwarf |journal=The Astrophysical Journal |volume=951 |issue=2 |pages=L48 |doi=10.3847/2041-8213/ace32c |issn=0004-637X|arxiv=2306.11807 |bibcode=2023ApJ...951L..48B |doi-access=free }}</ref>

==== Role of vertical mixing ====
[[File:Reaction methan to CO in brown dwarfs.png|thumb|225x225px|Major chemical pathways linking carbon monoxide and methane. The short-lived radicals are marked with a dot. Adopted from Zahnle & Marley<ref name=":9">{{cite journal |last1=Zahnle |first1=Kevin J. |last2=Marley |first2=Mark S. |date=2014-12-01 |title=Methane, Carbon Monoxide, and Ammonia in Brown Dwarfs and Self-Luminous Giant Planets |url=https://ui.adsabs.harvard.edu/abs/2014ApJ...797...41Z |journal=The Astrophysical Journal |volume=797 |issue=1 |pages=41 |doi=10.1088/0004-637X/797/1/41 |arxiv=1408.6283 |bibcode=2014ApJ...797...41Z |s2cid=118509317 |issn=0004-637X}}</ref>]]
In the hydrogen-dominated atmosphere of brown dwarfs a [[chemical equilibrium]] between [[carbon monoxide]] and [[methane]] exists. Carbon monoxide reacts with [[hydrogen]] molecules and forms methane and [[Hydroxyl radical|hydroxyl]] in this reaction. The hydroxyl radical might later react with hydrogen and form water molecules. In the other direction of the reaction, methane reacts with hydroxyl and forms carbon monoxide and hydrogen. The chemical reaction is tilted towards carbon monoxide at higher temperatures (L-dwarfs) and lower pressure. At lower temperatures (T-dwarfs) and higher pressure the reaction is tilted towards methane, and methane predominates at the T/Y-boundary. However, vertical mixing of the atmosphere can cause methane to sink into lower layers of the atmosphere and carbon monoxide to rise from these lower and hotter layers. The carbon monoxide is slow to react back into methane because of an energy barrier that prevents the breakdown of the [[Carbon–oxygen bond|C-O bonds]]. This forces the observable atmosphere of a brown dwarf to be in a chemical disequilibrium. The L/T transition is mainly defined with the transition from a carbon-monoxide-dominated atmosphere in L-dwarfs to a methane-dominated atmosphere in T-dwarfs. The amount of vertical mixing can therefore push the L/T-transition to lower or higher temperatures. This becomes important for objects with modest surface gravity and extended atmospheres, such as giant [[exoplanet]]s. This pushes the L/T transition to lower temperatures for giant exoplanets. For brown dwarfs this transition occurs at around 1200 K. The exoplanet [[HR 8799 c|HR 8799c]], on the other hand, does not show any methane, while having a temperature of 1100K.<ref name=":9" />

The transition between T- and Y-dwarfs is often defined as 500 K because of the lack of spectral observations of these cold and faint objects.<ref name=":10">{{cite journal |last1=Bardalez Gagliuffi |first1=Daniella C. |last2=Faherty |first2=Jacqueline K. |last3=Schneider |first3=Adam C. |last4=Meisner |first4=Aaron |last5=Caselden |first5=Dan |last6=Colin |first6=Guillaume |last7=Goodman |first7=Sam |last8=Kirkpatrick |first8=J. Davy |last9=Kuchner |first9=Marc |last10=Gagné |first10=Jonathan |last11=Logsdon |first11=Sarah E. |last12=Burgasser |first12=Adam J. |last13=Allers |first13=Katelyn |last14=Debes |first14=John |last15=Wisniewski |first15=John |date=2020-06-01 |title=WISEA J083011.95+283716.0: A Missing Link Planetary-mass Object |journal=The Astrophysical Journal |volume=895 |issue=2 |pages=145 |doi=10.3847/1538-4357/ab8d25 |arxiv=2004.12829 |bibcode=2020ApJ...895..145B |s2cid=216553879 |issn=0004-637X |doi-access=free }}</ref> Future observations with [[James Webb Space Telescope|JWST]] and the [[Extremely large telescope|ELTs]] might improve the sample of Y-dwarfs with observed spectra. Y-dwarfs are dominated by deep spectral features of methane, water vapor and possibly absorption features of [[ammonia]] and [[Ice|water ice]].<ref name=":10" /> Vertical mixing, clouds, metallicity, [[photochemistry]], [[lightning]], impact shocks and metallic [[Catalysis|catalysts]] might influence the temperature at which the L/T and T/Y transition occurs.<ref name=":9" />

==== Secondary features ====
{| class="wikitable floatright" style="width: 26em"
|+ Brown dwarf spectral types
|-
! colspan="2" |Secondary features
|-
|pec
|This suffix (e.g. L2pec) stands for "peculiar".<ref>{{cite web|url=http://simbad.u-strasbg.fr/simbad/sim-display?data=sptypes|title=Spectral type codes|website=simbad.u-strasbg.fr|access-date=2020-03-06}}</ref>
|-
|sd
|This prefix (e.g. sdL0) stands for [[subdwarf]] and indicates a low metallicity and blue color.<ref name=":5">{{cite journal |last1=Burningham |first1=Ben |last2=Smith |first2=Leigh |last3=Cardoso |first3=Cátia V. |last4=Lucas |first4=Philip W. |last5=Burgasser |first5=Adam J. |last6=Jones |first6=Hugh R. A. |last7=Smart |first7=Richard L. |date=May 2014 |title=The discovery of a T6.5 subdwarf |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=440 |issue=1 |pages=359–364 |doi=10.1093/mnras/stu184 |doi-access=free |arxiv=1401.5982 |bibcode=2014MNRAS.440..359B |s2cid=119283917 |issn=0035-8711 }}</ref>
|-
|β
|Objects with the beta (β) suffix (e.g. L4β) have an intermediate surface gravity.<ref name=":6">{{cite journal |last1=Cruz |first1=Kelle L. |last2=Kirkpatrick |first2=J. Davy |last3=Burgasser |first3=Adam J. |date=February 2009 |title=Young L Dwarfs Identified in the Field: A Preliminary Low-Gravity, Optical Spectral Sequence from L0 to L5 |journal=The Astronomical Journal |volume=137 |issue=2 |pages=3345–3357 |doi=10.1088/0004-6256/137/2/3345 |arxiv=0812.0364 |bibcode=2009AJ....137.3345C |s2cid=15376964 |issn=0004-6256 }}</ref>
|-
|γ
|Objects with the gamma (γ) suffix (e.g. L5γ) have a low surface gravity.<ref name=":6"/>
|-
|red
|The red suffix (e.g. L0red) indicates objects without signs of youth, but high dust content.<ref name=":7">{{cite journal |last1=Looper |first1=Dagny L. |last2=Kirkpatrick |first2=J. Davy |last3=Cutri |first3=Roc M. |last4=Barman |first4=Travis |last5=Burgasser |first5=Adam J. |last6=Cushing |first6=Michael C. |last7=Roellig |first7=Thomas |last8=McGovern |first8=Mark R. |last9=McLean |first9=Ian S. |last10=Rice |first10=Emily |last11=Swift |first11=Brandon J. |date=October 2008 |title=Discovery of Two Nearby Peculiar L Dwarfs from the 2MASS Proper-Motion Survey: Young or Metal-Rich? |journal=Astrophysical Journal |volume=686 |issue=1 |pages=528–541 |doi=10.1086/591025 |arxiv=0806.1059 |bibcode=2008ApJ...686..528L |s2cid=18381182 |issn=0004-637X }}</ref>
|-
|blue
|The blue suffix (e.g. L3blue) indicates unusual blue near-infrared colors for L dwarfs without obvious low metallicity.<ref name=":8">{{cite journal |last1=Kirkpatrick |first1=J. Davy|last2=Looper |first2=Dagny L. |last3=Burgasser |first3=Adam J. |last4=Schurr |first4=Steven D. |last5=Cutri |first5=Roc M. |last6=Cushing |first6=Michael C. |last7=Cruz |first7=Kelle L. |last8=Sweet |first8=Anne C. |last9=Knapp |first9=Gillian R. |last10=Barman |first10=Travis S. |last11=Bochanski |first11=John J. |date=September 2010 |title=Discoveries from a Near-infrared Proper Motion Survey Using Multi-epoch Two Micron All-Sky Survey Data |journal=Astrophysical Journal Supplement Series |volume=190 |issue=1 |pages=100–146 |doi=10.1088/0067-0049/190/1/100 |arxiv=1008.3591 |bibcode=2010ApJS..190..100K |s2cid=118435904 |issn=0067-0049 }}</ref>
|}
Young brown dwarfs have low [[Surface gravity|surface gravities]] because they have larger radii and lower masses than the field stars of similar spectral type. These sources are noted by a letter ''beta'' (β) for intermediate surface gravity or ''gamma'' (γ) for low surface gravity. Indicators of low surface gravity include weak CaH, K I and Na I lines, as well as a strong VO line.<ref name=":6"/> ''Alpha'' (α) denotes normal surface gravity and is usually dropped. Sometimes an extremely low surface gravity is denoted by a delta (δ).<ref name=":8"/> The suffix "pec" stands for "peculiar"; this suffix is still used for other features that are unusual, and summarizes different properties, indicating low surface gravity, subdwarfs and unresolved binaries.<ref name="Faherty 10">{{cite journal |last1=Faherty |first1=Jacqueline K. |last2=Riedel |first2=Adric R. |last3=Cruz |first3=Kelle L. |last4=Gagne |first4=Jonathan |last5=Filippazzo |first5=Joseph C. |last6=Lambrides |first6=Erini |last7=Fica |first7=Haley |last8=Weinberger |first8=Alycia |last9=Thorstensen |first9=John R. |last10=Tinney |first10=Chris G. |last11=Baldassare |first11=Vivienne |date=July 2016 |title=Population Properties of Brown Dwarf Analogs to Exoplanets |journal=Astrophysical Journal Supplement Series |volume=225 |issue=1 |pages=10 |doi=10.3847/0067-0049/225/1/10 |arxiv=1605.07927 |bibcode=2016ApJS..225...10F |s2cid=118446190 |issn=0067-0049 |doi-access=free }}</ref> The prefix sd stands for [[subdwarf]] and only includes cool subdwarfs. This prefix indicates a low [[metallicity]] and kinematic properties that are more similar to [[Galactic halo|halo]] stars than to [[Thin disk|disk]] stars.<ref name=":5"/> Subdwarfs appear bluer than disk objects.<ref>{{cite web |url=http://www.stsci.edu/~inr/cmd.html |title=Colour-magnitude data |first=Neill |last=Reid <!-- |date=15/07/02 (unless a typo, dates to ca. 2004, otherwise 2002 --> |website=www.stsci.edu |access-date=2020-03-06 }}</ref> The red suffix describes objects with red color, but an older age. This is not interpreted as low surface gravity, but as a high dust content.<ref name=":7"/><ref name=":8"/> The blue suffix describes objects with blue [[near-infrared]] colors that cannot be explained with low metallicity. Some are explained as L+T binaries, others are not binaries, such as [[2MASS J11263991−5003550]] and are explained with thin and/or large-grained clouds.<ref name=":8"/>

=== Spectral and atmospheric properties of brown dwarfs ===
[[File:Anatomy of Brown Dwarf's Atmosphere.jpg|thumb|Artist's illustration of a brown dwarf's interior structure. Cloud layers at certain depths are offset as a result of layer shifting.]]
The majority of flux emitted by L and T dwarfs is in the 1- to 2.5-micrometre near-infrared range. Low and decreasing temperatures through the late-M, -L, and -T dwarf sequence result in a rich near-infrared [[spectrum]] containing a wide variety of features, from relatively narrow lines of neutral atomic species to broad molecular bands, all of which have different dependencies on temperature, gravity, and [[metallicity]]. Furthermore, these low temperature conditions favor condensation out of the gas state and the formation of grains.
[[File:PIA23684-BrownDwarfStar-Wind-SpitzerST-ArtistConcept-20200409.jpg|thumb|left|150px|Wind measured (Spitzer ST; Artist Concept; 9 Apr 2020)<ref name="EA-20200409">{{cite news |author=[[National Radio Astronomy Observatory]] |title=Astronomers measure wind speed on a brown dwarf – Atmosphere, interior rotating at different speeds |url=https://www.eurekalert.org/pub_releases/2020-04/nrao-amw040620.php |date=9 April 2020 |work=[[EurekAlert!]] |access-date=10 April 2020 }}</ref>
]]
Typical atmospheres of known brown dwarfs range in temperature from 2200 down to {{Val|750|ul=K}}.<ref name=burrows/> Compared to stars, which warm themselves with steady internal fusion, brown dwarfs cool quickly over time; more massive dwarfs cool more slowly than less massive ones. There is some evidence that the cooling of brown dwarfs slows down at the transition between spectral classes L and T (about 1000 K).<ref>{{cite journal|doi=10.3847/1538-3881/ac66d2|title=Precise Dynamical Masses of ε Indi Ba and Bb: Evidence of Slowed Cooling at the L/T Transition |year=2022 |last1=Chen |first1=Minghan |last2=Li |first2=Yiting |last3=Brandt |first3=Timothy D. |last4=Dupuy |first4=Trent J. |last5=Cardoso |first5=Cátia V. |last6=McCaughrean |first6=Mark J. |journal=The Astronomical Journal |volume=163 |issue=6 |page=288 |arxiv=2205.08077 |bibcode=2022AJ....163..288C |s2cid=248834536 |doi-access=free }}</ref>

Observations of known brown dwarf candidates have revealed a pattern of brightening and dimming of infrared emissions that suggests relatively cool, opaque cloud patterns obscuring a hot interior that is stirred by extreme winds. The weather on such bodies is thought to be extremely strong, comparable to but far exceeding Jupiter's famous storms.

On January 8, 2013, astronomers using NASA's [[Hubble Space Telescope|Hubble]] and [[Spitzer Space Telescope|Spitzer]] space telescopes probed the stormy atmosphere of a brown dwarf named [[2MASS J22282889–4310262]], creating the most detailed "weather map" of a brown dwarf thus far. It shows wind-driven, planet-sized clouds. The new research is a stepping stone toward a better understanding not only brown dwarfs, but also of the atmospheres of planets beyond the Solar System.<ref>{{cite web |url=http://hubblesite.org/newscenter/archive/releases/star/brown-dwarf/2013/02/ |title=NASA Space Telescopes See Weather Patterns in Brown Dwarf |website=Hubblesite |publisher=NASA |access-date=8 January 2013 |archive-date=2 April 2014 |archive-url=https://web.archive.org/web/20140402162957/http://hubblesite.org/newscenter/archive/releases/star/brown-dwarf/2013/02/ |url-status=dead }}</ref>

In April 2020 scientists reported measuring wind speeds of {{val|+650|310|u=metres per second}} (up to 1,450 miles per hour) on the nearby brown dwarf [[2MASS J10475385+2124234]]. To calculate the measurements, scientists compared the rotational movement of atmospheric features, as ascertained by brightness changes, against the electromagnetic rotation generated by the brown dwarf's interior. The results confirmed previous predictions that brown dwarfs would have high winds. Scientists are hopeful that this comparison method can be used to explore the atmospheric dynamics of other brown dwarfs and extrasolar planets.<ref>{{cite web|url=https://www.cnn.com/2020/04/09/world/brown-dwarf-wind-speed-scn/index.html|title=Astronomers Clock High Winds on Object Outside Our Solar System|website=CNN.com|date=9 April 2020 |publisher=CNN|access-date=11 April 2020}}</ref>

=== Observational techniques ===
[[File:BrownDwarfs Comparison 01.png|thumb|Brown dwarfs {{nowrap|[[Teide 1]]}}, [[Gliese 229]]B, and [[WISE 1828+2650]] compared to red dwarf [[Gliese 229A]], Jupiter and our Sun]]

[[Coronagraph]]s have recently been used to detect faint objects orbiting bright visible stars, including Gliese 229B.

Sensitive telescopes equipped with charge-coupled devices (CCDs) have been used to search distant star clusters for faint objects, including Teide 1.

Wide-field searches have identified individual faint objects, such as [[Kelu-1]] (30 light-years away).

Brown dwarfs are often discovered in surveys to discover [[exoplanet]]s. [[Methods of detecting exoplanets]] work for brown dwarfs as well, although brown dwarfs are much easier to detect.

Brown dwarfs can be powerful emitters of radio emission due to their strong magnetic fields. Observing programs at the [[Arecibo Observatory]] and the [[Very Large Array]] have detected over a dozen such objects, which are also called [[Ultra-cool dwarf|ultracool dwarfs]] because they share common magnetic properties with other objects in this class.<ref name="ReferenceA">{{cite journal |last1=Route |first1=Matthew |last2=Wolszczan |first2=Alexander |title=The Second Arecibo Search for 5 GHz Radio Flares from Ultracool Dwarfs |journal=The Astrophysical Journal |date=20 October 2016 |volume=830 |issue=2 |page=85 |doi=10.3847/0004-637X/830/2/85 |arxiv=1608.02480 |bibcode=2016ApJ...830...85R |s2cid=119279978 |doi-access=free }}</ref> The detection of radio emission from brown dwarfs permits their magnetic field strengths to be measured directly.

=== Milestones ===
* 1995: First brown dwarf verified. [[Teide 1]], an M8 object in the [[Pleiades]] [[star cluster|cluster]], is picked out with a CCD in the Spanish Observatory of Roque de los Muchachos of the [[Instituto de Astrofísica de Canarias]].
* First methane brown dwarf verified. Gliese 229B is discovered orbiting red dwarf [[Gliese 229]]A (20 ly away) using an [[adaptive optics]] coronagraph to sharpen images from the {{Convert|60|in|m|adj=on}} reflecting telescope at [[Palomar Observatory]] on Southern California's [[Mount Palomar]]; follow-up infrared spectroscopy made with their {{Convert|200|in|m|adj=on}} [[Hale Telescope]] shows an abundance of methane.
* 1998: First X-ray-emitting brown dwarf found. Cha Helpha 1, an M8 object in the [[Chamaeleon complex|Chamaeleon I]] dark cloud, is determined to be an X-ray source, similar to convective late-type stars.
* 15 December 1999: First X-ray flare detected from a brown dwarf. A team at the University of California monitoring [[LP 944-20]] ({{Jupiter mass|60}}, 16 ly away) via the [[Chandra X-ray Observatory]], catches a 2-hour flare.<ref>{{cite journal |last1=Rutledge |first1=Robert E. |last2=Basri |first2=Gibor |last3=Martín |first3=Eduardo L. |last4=Bildsten |first4=Lars |title=Chandra Detection of an X-Ray Flare from the Brown Dwarf LP 944-20 |journal=The Astrophysical Journal |date=1 August 2000 |volume=538 |issue=2 |pages=L141–L144 |arxiv=astro-ph/0005559 |bibcode=2000ApJ...538L.141R |doi=10.1086/312817 |s2cid=17800872 }}</ref>
* 27 July 2000: First radio emission (in flare and quiescence) detected from a brown dwarf. A team of students at the [[Very Large Array]] detected emission from LP 944–20.<ref name="Berger2001">{{cite journal |last1=Berger |first1=Edo |last2=Ball |first2=Steven |last3=Becker |first3=Kate M. |last4=Clarke |first4=Melanie |last5=Frail |first5=Dale A. |last6=Fukuda |first6=Therese A. |last7=Hoffman |first7=Ian M. |last8=Mellon |first8=Richard |last9=Momjian |first9=Emmanuel |last10=Murphy |first10=Nathanial W. |last11=Teng |first11=Stacey H. |last12=Woodruff |first12=Timothy |last13=Zauderer |first13=B. Ashley |last14=Zavala |first14=Robert T. <!-- see https://public.nrao.edu/news/first-radio-emission-seen-from-a-brown-dwarf/ for the names -->|title=Discovery of radio emission from the brown dwarf LP944-20 |journal=Nature |date=2001-03-15 |volume=410 |issue=6826 |pages=338–340 |arxiv=astro-ph/0102301 |bibcode=2001Natur.410..338B |doi=10.1038/35066514 |pmid=11268202 |s2cid=4411256 |url=https://cds.cern.ch/record/487607|archive-url=https://web.archive.org/web/20210427121818/https://cds.cern.ch/record/487607 |url-status=dead |archive-date=2021-04-27 |type=Submitted manuscript }}</ref>
* 30 April 2004: First detection of a candidate [[exoplanet]] around a brown dwarf: [[2M1207b]] discovered with the [[Very Large Telescope|VLT]] and the first directly imaged exoplanet.<ref>{{cite press release|first1=Gael |last1=Chauvin |first2=Ben |last2=Zuckerman |first3=Anne-Marie |last3=Lagrange |url=https://www.eso.org/public/news/eso0515/ |title=Yes, it is the Image of an Exoplanet: Astronomers Confirm the First Image of a Planet Outside of Our Solar System |publisher=European Southern Observatory |access-date=2020-02-09 }}</ref>
* 20 March 2013: Discovery of the closest brown dwarf system: Luhman 16.<ref>{{cite journal |last=Luhman |first=Kevin L. |date=April 2013 |title=Discovery of a Binary Brown Dwarf at 2 pc from the Sun |journal=Astrophysical Journal Letters |volume=767 |issue=1 |pages=L1 |doi=10.1088/2041-8205/767/1/L1 |arxiv=1303.2401 |bibcode=2013ApJ...767L...1L |s2cid=8419422 |issn=0004-637X }}</ref>
* 25 April 2014: Coldest-known brown dwarf discovered. [[WISE 0855−0714]] is 7.2 light-years away (seventh-closest system to the Sun) and has a temperature between −48 and −13&nbsp;°C.<ref name="NASA20140425">{{cite web |last1=Clavin |first1=Whitney |last2=Harrington |first2=J. D. <!-- NASA's mouthpiece, impossible to find full name! e-mail: j.d.harrington@nasa.gov Phone: 202-358-5241 --> |date=25 April 2014 |title=NASA's Spitzer and WISE Telescopes Find Close, Cold Neighbor of Sun |url=http://www.nasa.gov/jpl/wise/spitzer-coldest-brown-dwarf-20140425/ |url-status=live |archive-url=https://web.archive.org/web/20140426004939/http://www.nasa.gov/jpl/wise/spitzer-coldest-brown-dwarf-20140425 |archive-date=26 April 2014 |website=[[NASA]].gov}}</ref>

=== Brown dwarfs X-ray sources ===
[[File:Lp94420 duo m.jpg|thumb|300px|[[Chandra X-ray Observatory|Chandra]] image of [[LP 944-20]] before flare and during flare]]

X-ray flares detected from brown dwarfs since 1999 suggest changing [[magnetic field of celestial bodies|magnetic fields]] within them, similar to those in very-low-mass stars.  Although they do not fuse hydrogen into helium in their cores like stars, energy from the fusion of deuterium and gravitational contraction keep their interiors warm and generate strong magnetic fields.  The interior of a brown dwarf is in a rapidly boiling, or convective state. When combined with the rapid rotation that most brown dwarfs exhibit, [[convection]] sets up conditions for the development of a strong, tangled [[magnetic field]] near the surface. The magnetic fields that generated the flare observed by [[Chandra X-ray Observatory|Chandra]] from [[LP 944-20]] has its origin in the turbulent magnetized [[Plasma (physics)|plasma]] beneath the brown dwarf's "surface".

Using NASA's [[Chandra X-ray Observatory]], scientists have detected X-rays from a low-mass brown dwarf in a multiple star system.<ref name=Williams>{{cite web |date=April 14, 2003 |title=X-rays from a Brown Dwarf's Corona |url=http://www.williams.edu/Astronomy/jay/chapter18_etu6.html |access-date=March 19, 2010 |archive-url=https://web.archive.org/web/20101230000830/http://www.williams.edu/Astronomy/jay/chapter18_etu6.html |archive-date=December 30, 2010 |url-status=dead }}</ref> This is the first time that a brown dwarf this close to its parent star(s) (Sun-like stars TWA 5A) has been resolved in X-rays.<ref name=Williams/> "Our Chandra data show that the X-rays originate from the brown dwarf's coronal plasma which is some 3 million degrees Celsius", said Yohko Tsuboi of [[Chuo University]] in Tokyo.<ref name=Williams/> "This brown dwarf is as bright as the Sun today in X-ray light, while it is fifty times less massive than the Sun", said Tsuboi.<ref name=Williams/> "This observation, thus, raises the possibility that even massive planets might emit X-rays by themselves during their youth!"<ref name=Williams/>

=== Brown dwarfs as radio sources ===
[[File:Fig. 2 Epoch 2 composite image of LSR J1835 + 3259 aurora and quiescent emission.png|thumb|Resolved quiescent emission (contours) of [[LSR J1835+3259]], which has a similar shape to [[Magnetosphere of Jupiter|Jupiters radiation belts]]. The dark spot in the middle is radio emission from the right-[[Circular polarization|circularly polarized]] aurora.]]
The first brown dwarf that was discovered to emit radio signals was [[LP 944-20]], which was observed since it is also a source of X-ray emission, and both types of emission are signatures of coronae. Approximately 5–10% of brown dwarfs appear to have strong magnetic fields and emit radio waves, and there may be as many as 40 magnetic brown dwarfs within 25 pc of the Sun based on [[Monte Carlo method|Monte Carlo]] modeling and their average spatial density.<ref>{{cite journal |last1=Route |first1=Matthew |title=Radio-flaring Ultracool Dwarf Population Synthesis |journal=The Astrophysical Journal |date=10 August 2017 |volume=845 |issue=1 |page=66 |doi=10.3847/1538-4357/aa7ede |arxiv=1707.02212 |bibcode=2017ApJ...845...66R |s2cid=118895524 |doi-access=free }}</ref> The power of the radio emissions of brown dwarfs is roughly constant despite variations in their temperatures.<ref name="ReferenceA"/> Brown dwarfs may maintain magnetic fields of up to 6 [[Gauss (unit)|kG]] in strength.<ref>{{cite journal |last1=Kao |first1=Melodie M. |first2=Gregg |last2=Hallinan |first3=J. Sebastian |last3=Pineda |first4=David |last4=Stevenson |first5=Adam J. |last5=Burgasser |title=The Strongest Magnetic Fields on the Coolest Brown Dwarfs |journal=The Astrophysical Journal Supplement Series |date=31 July 2018 |volume=237 |issue=2 |page=25 |doi=10.3847/1538-4365/aac2d5 |arxiv=1808.02485 |bibcode=2018ApJS..237...25K |s2cid=118898602 |doi-access=free }}</ref> Astronomers have estimated brown dwarf [[magnetosphere]]s to span an altitude of approximately 10<sup>7</sup> m given properties of their radio emissions.<ref>{{cite journal |last1=Route |first1=Matthew |title=Is WISEP J060738.65+242953.4 Really A Magnetically Active, Pole-on L Dwarf? |journal=The Astrophysical Journal |date=10 July 2017 |volume=843 |issue=2 |page=115 |doi=10.3847/1538-4357/aa78ab |arxiv=1706.03010 |bibcode=2017ApJ...843..115R |s2cid=119056418 |doi-access=free }}</ref> It is unknown whether the radio emissions from brown dwarfs more closely resemble those from planets or stars. Some brown dwarfs emit regular radio pulses, which are sometimes interpreted as radio emission beamed from the poles but may also be beamed from active regions. The regular, periodic reversal of radio wave orientation may indicate that brown dwarf magnetic fields periodically reverse polarity. These reversals may be the result of a brown dwarf magnetic activity cycle, similar to the [[solar cycle]].<ref>{{cite journal |last1=Route |first1=Matthew |title=The Discovery of Solar-like Activity Cycles Beyond the End of the Main Sequence? |journal=The Astrophysical Journal Letters |date=20 October 2016 |volume=830 |issue=2 |page=L27 |doi=10.3847/2041-8205/830/2/L27 |arxiv=1609.07761 |bibcode=2016ApJ...830L..27R |s2cid=119111063 |doi-access=free }}</ref>

The first brown dwarf of spectral class M found to emit radio waves was [[LP 944-20]], detected in 2001. The first brown dwarf of spectral class L found to emit radio waves was [[2MASS J00361617+1821104|2MASS J0036159+182110]], detected in 2008. The first brown dwarf of spectral class T found to emit radio waves was [[2MASS J10475385+2124234]].<ref>{{cite press release |title=Record-breaking radio waves discovered from ultra-cool star |author=Phys.org |url=https://phys.org/news/2012-04-record-breaking-radio-ultra-cool-star.html}}</ref><ref>{{cite journal|last1=Route|first1=M.|last2=Wolszczan|first2=A.|title=The Arecibo Detection of the Coolest Radio-flaring Brown Dwarf|journal=The Astrophysical Journal Letters|date=10 March 2012|volume=747|issue=2|page=L22|doi=10.1088/2041-8205/747/2/L22|arxiv=1202.1287|bibcode=2012ApJ...747L..22R|s2cid=119290950}}</ref> This last discovery was significant since it revealed that brown dwarfs with temperatures similar to exoplanets could host strong >1.7 kG magnetic fields. Although a sensitive search for radio emission from Y dwarfs was conducted at the [[Arecibo telescope|Arecibo Observatory]] in 2010, no emission was detected.<ref>{{cite journal|last1=Route|first1=Matthew|title=ROME. IV. An Arecibo Search for Substellar Magnetospheric Radio Emissions in Purported Exoplanet-hosting Systems at 5 GHz|journal=The Astrophysical Journal|date=1 May 2024|volume=966|issue=1|page=55|doi=10.3847/1538-4357/ad30ff|arxiv=2403.02226|bibcode=2024ApJ...966...55R|doi-access=free }}</ref>

=== Recent developments ===
[[File:Brown dwarfs in the Sun’s neighborhood.jpg|thumb|A visualization representing a three-dimensional map of brown dwarfs (red dots) that have been discovered within 65 light-years of the Sun<ref>{{cite news |last1=Meisner |first1=Aaron |last2=Kocz |first2=Amanda |title=Mapping Our Sun's Backyard |publisher=NOIRLab |url=https://noirlab.edu/public/news/noirlab2105/ |access-date=1 February 2021}}</ref>]]
Estimates of brown dwarf populations in the solar neighbourhood suggest that there may be as many as six stars for every brown dwarf.<ref name="space2012">{{cite web |last=O'Neill |first=Ian |date=12 June 2012 |title=Brown Dwarfs, Runts of Stellar Litter, Rarer than Thought |url=http://www.space.com/16112-brown-dwarf-stars-sun-rare.html |access-date=2012-12-28 |publisher=Space.com}}</ref> A more recent estimate from 2017 using the young massive star cluster [[RCW 38]] concluded that the Milky Way galaxy contains between 25 and 100 billion brown dwarfs.<ref>{{cite journal |last1=Muzic |first1=Koraljka |last2=Schoedel |first2=Rainer |last3=Scholz |first3=Alexander |last4=Geers |first4=Vincent C. |last5=Jayawardhana |first5=Ray |last6=Ascenso |first6=Joana |last7=Cieza |first7=Lucas A. |date=2017-07-02 |title=The low-mass content of the massive young star cluster RCW 38 |journal=Monthly Notices of the Royal Astronomical Society |volume=471 |issue=3 |pages=3699–3712 |arxiv=1707.00277 |bibcode=2017MNRAS.471.3699M |doi=10.1093/mnras/stx1906 |doi-access=free |issn=0035-8711 |s2cid=54736762}}</ref> (Compare these numbers to the estimates of the number of stars in the Milky Way; 100 to 400 billion.)

In a study published in Aug 2017 [[NASA]]'s [[Spitzer Space Telescope]] monitored infrared brightness variations in brown dwarfs caused by cloud cover of variable thickness. The observations revealed large-scale waves propagating in the atmospheres of brown dwarfs (similarly to the atmosphere of Neptune and other Solar System giant planets). These atmospheric waves modulate the thickness of the clouds and propagate with different velocities (probably due to differential rotation).<ref>{{cite journal |last1=Apai |first1=Dániel |last2=Karalidi |first2=T. |last3=Marley |first3=Mark S. |last4=Yang |first4=H. |last5=Flateau |first5=D. |last6=Metchev |first6=S. |last7=Cowan |first7=N. B. |last8=Buenzli |first8=E. |last9=Burgasser |first9=Adam J. |last10=Radigan |first10=J. |last11=Artigau |first11=Étienne |last12=Lowrance |first12=P. |year=2017 |title=Zones, spots, and planetary-scale waves beating in brown dwarf atmospheres |journal=Science |volume=357 |issue=6352 |pages=683–687 |bibcode=2017Sci...357..683A |doi=10.1126/science.aam9848 |pmid=28818943 |doi-access=free}}</ref>

In August 2020, astronomers discovered 95 brown dwarfs near the [[Sun]] through the project Backyard Worlds: Planet 9.<ref>{{cite web |last=Gohd |first=Chelsea |date=19 August 2020 |title=Volunteers spot almost 100 cold brown dwarfs near our sun |url=https://www.space.com/citizen-scientists-discover-95-brown-dwarfs.html |website=Space.com}}</ref>

In 2024 the [[James Webb Space Telescope]] provided the most detailed weather report yet on two brown dwarfs, revealing "stormy" conditions. These brown dwarfs, part of a [[binary star]] system named [[Luhman 16]] discovered in 2013, are only 6.5 light-years away from Earth and are the closest brown dwarfs to our sun. Researchers discovered that they have turbulent clouds, likely made of silicate grains, with temperatures ranging from {{Convert|875|°C|°F}} to {{Convert|1026|°C|°F}}. This indicates that hot sand is being blown by winds on the brown dwarfs. Additionally, absorption signatures of carbon monoxide, methane, and water vapor were detected.<ref>[https://www.space.com/brown-dwarf-alien-weather-report-jwst Alien weather report: James Webb Space Telescope detects hot, sandy wind on 2 brown dwarfs; Space.com]</ref>

== Binary brown dwarfs ==

=== Brown dwarf–brown dwarf binaries ===
[[File:Hubble Space Telescope - Brown Dwarf Binaries.gif|thumb|Multi-epoch images of brown dwarf binaries taken with the [[Hubble Space Telescope]]. The binary Luhman 16 AB (left) is closer to the Solar System than the other examples shown here.]]
Brown dwarfs binaries of type M, L, and T are less common with a lower mass of the primary.<ref>{{cite web |last=info@noirlab.edu |title=Are the Coolest Brown Dwarfs Loners? |url=https://www.noirlab.edu/public/announcements/geminiann16003/ |access-date=2023-04-16 |website=www.noirlab.edu }}</ref> L-dwarfs have a binary fraction of about {{Val|24|6|2}}% and the binary fraction for late T, early Y-dwarfs (T5-Y0) is about {{val|8|6|u=%}}.<ref name=":11">{{cite journal |last1=Fontanive |first1=Clémence |last2=Biller |first2=Beth |last3=Bonavita |first3=Mariangela |last4=Allers |first4=Katelyn |date=2018-09-01 |title=Constraining the multiplicity statistics of the coolest brown dwarfs: binary fraction continues to decrease with spectral type |journal=Monthly Notices of the Royal Astronomical Society |volume=479 |issue=2 |pages=2702–2727 |doi=10.1093/mnras/sty1682 |doi-access=free |arxiv=1806.08737 |bibcode=2018MNRAS.479.2702F |issn=0035-8711}}</ref>

Brown dwarf binaries have a higher companion-to-host ratio <math>q=M_B/M_A</math> for lower mass binaries. Binaries with a [[Red dwarf|M-type star]] as a primary have for example a broad distribution of ''q'' with a preference of ''q''&nbsp;≥&nbsp;0.4. Brown dwarfs on the other hand show a strong preference for ''q''&nbsp;≥&nbsp;0.7. The separation is decreasing with mass: M-type stars have a separation peaking at 3–30 [[astronomical unit]]s (au), M-L-type brown dwarfs have a projected separation peaking at 5–8 au and T5–Y0 objects have a projected separation that follows a [[Log-normal distribution|lognormal distribution]] with a peak separation of about 2.9&nbsp;au.<ref name=":11" />

An example is the closest brown dwarf binary Luhman 16 AB with a primary L7.5 dwarf and a separation of 3.5&nbsp;au and ''q''&nbsp;=&nbsp;0.85. The separation is on the lower end of the expected separation for M-L-type brown dwarfs, but the mass ratio is typical.

It is not known if the same trend continues with Y-dwarfs, because their sample size is so small. The Y+Y dwarf binaries should have a high mass ratio q and a low separation, reaching scales of less than one au.<ref>{{cite journal |last1=Opitz |first1=Daniela |last2=Tinney |first2=C. G. |last3=Faherty |first3=Jacqueline |last4=Sweet |first4=Sarah |last5=Gelino |first5=Christopher R. |last6=Kirkpatrick |first6=J. Davy |date=2016-02-24 |title=Searching for Binary Y dwarfs with the Gemini Multi-Conjugate Adaptive Optics System (GeMS) |journal=The Astrophysical Journal |volume=819 |issue=1 |pages=17 |doi=10.3847/0004-637X/819/1/17 |arxiv=1601.05508 |bibcode=2016ApJ...819...17O |s2cid=3208550 |issn=1538-4357 |doi-access=free }}</ref> In 2023, the Y+Y dwarf [[WISE J0336−0143|WISE J0336-0143]] was confirmed as a binary with [[James Webb Space Telescope|JWST]], with a mass ratio of q=0.62±0.05 and a separation of 0.97 astronomical units. The researchers point out that the sample size of low-mass binary brown dwarfs is too small to determine if WISE J0336-0143 is a typical representative of low-mass binaries or a peculiar system.<ref name=":14">{{cite journal |last1=Calissendorff |first1=Per |last2=De Furio |first2=Matthew |last3=Meyer |first3=Michael |last4=Albert |first4=Loïc |last5=Aganze |first5=Christian |last6=Ali-Dib |first6=Mohamad |last7=Gagliuffi |first7=Daniella C. Bardalez |last8=Baron |first8=Frederique |last9=Beichman |first9=Charles A. |last10=Burgasser |first10=Adam J. |last11=Cushing |first11=Michael C. |last12=Faherty |first12=Jacqueline Kelly |last13=Fontanive |first13=Clémence |last14=Gelino |first14=Christopher R. |last15=Gizis |first15=John E. |date=2023-03-29 |title=JWST/NIRCam Discovery of the First Y+Y Brown Dwarf Binary: WISE J033605.05–014350.4 |journal=The Astrophysical Journal Letters |volume=947 |issue=2 |pages=L30 |doi=10.3847/2041-8213/acc86d |arxiv=2303.16923 |bibcode=2023ApJ...947L..30C |s2cid=257833714 |doi-access=free }}</ref>

Observations of the orbit of binary systems containing brown dwarfs can be used to measure the mass of the brown dwarf. In the case of [[2MASSW J0746425+2000321]], the secondary weighs 6% of the solar mass. This measurement is called a dynamical mass.<ref>{{cite press release |first=Hervé |last=Bouy |url=https://www.eso.org/public/news/eso0420/ |title=Weighing Ultra-Cool Stars – Large Ground-Based Telescopes and Hubble Team-Up to Perform First Direct Brown Dwarf Mass Measurement |publisher=European Southern Observatory |access-date=2019-12-11 }}</ref><ref>{{cite journal |last1=Bouy |first1=Hervé |last2=Duchêne |first2=Gaspard |last3=Köhler |first3=Rainer |last4=Brandner |first4=Wolfgang |last5=Bouvier |first5=Jérôme |last6=Martín |first6=Eduardo L. |last7=Ghez |first7=Andrea Mia |last8=Delfosse |first8=Xavier |last9=Forveille |first9=Thierry |last10=Allard |first10=France |last11=Baraffe |first11=Isabelle |first12=Gibor |last12=Basri |first13=Laird M. |last13=Close |first14=Caer E. |last14=McCabe |date=2004-08-01 |title=First determination of the dynamical mass of a binary L dwarf |journal=Astronomy & Astrophysics |volume=423 |issue=1 |pages=341–352 |arxiv=astro-ph/0405111 |doi=10.1051/0004-6361:20040551 |issn=0004-6361 |bibcode=2004A&A...423..341B |s2cid=3149721 }}</ref> The brown dwarf system closest to the Solar System is the binary Luhman 16. It was attempted to search for planets around this system with a similar method, but none were found.<ref>{{cite journal |last1=Bedin |first1=Luigi R. |last2=Pourbaix |first2=Dimitri |last3=Apai |first3=Dániel |last4=Burgasser |first4=Adam J. |last5=Buenzli |first5=Esther |last6=Boffin |first6=Henri M. J. |last7=Libralato |first7=Mattia |date=2017-09-01 |title=Hubble Space Telescope astrometry of the closest brown dwarf binary system – I. Overview and improved orbit |url=https://academic.oup.com/mnras/article/470/1/1140/3896221 |journal=Monthly Notices of the Royal Astronomical Society |volume=470 |issue=1 |pages=1140–1155 |arxiv=1706.00657 |doi=10.1093/mnras/stx1177 |doi-access=free |issn=0035-8711 |hdl=10150/625503 |s2cid=119385778 }}</ref>

=== Unusual brown dwarf binaries ===
[[File:SDSS J1416+1348 legacy dr10.jpg|thumb|The wide brown dwarf binary [[SDSS J141624.08+134826.7|SDSS J1416+1348]]]]
The wide binary system [[2M1101AB]] was the first binary with a separation greater than {{Val|20|ul=AU}}. The discovery of the system gave definitive insights to the formation of brown dwarfs. It was previously thought that wide binary brown dwarfs are not formed or at least are disrupted at ages of 1–10 [[Myr]]. The existence of this system is also inconsistent with the ejection hypothesis.<ref>{{cite journal |last=Luhman |first=Kevin L. |date=2004-10-10 |title=The First Discovery of a Wide Binary Brown Dwarf |journal=The Astrophysical Journal |volume=614 |issue=1 |pages=398–403 |arxiv=astro-ph/0407344 |doi=10.1086/423666 |issn=0004-637X |bibcode=2004ApJ...614..398L |s2cid=11733526 }}</ref> The ejection hypothesis was a proposed hypothesis in which brown dwarfs form in a multiple system, but are ejected before they gain enough mass to burn hydrogen.<ref>{{cite journal |last1=Reipurth|first1=Bo|last2=Clarke|first2=Cathie|date=June 2003|title=Brown Dwarfs as Ejected Stellar Embryos: Observational Perspectives|journal=IAUS|volume=211|pages=13–22|issn=1743-9221|bibcode=2003IAUS..211...13R|arxiv=astro-ph/0209005|doi=10.1017/s0074180900210188|s2cid=16822178 }}</ref>

More recently the wide binary [[W2150AB]] was discovered. It has a similar mass ratio and [[Gravitational binding energy|binding energy]] as 2M1101AB, but a greater age and is located in a different region of the galaxy. While 2M1101AB is in a closely crowded region, the binary W2150AB is in a sparsely-separated field. It must have survived any dynamical interactions in its natal [[star cluster]]. The binary belongs also to a few L+T binaries that can be easily resolved by ground-based observatories. The other two are [[SDSS J1416+1348|SDSS J1416+13AB]] and Luhman 16.<ref>{{cite journal |last1=Faherty |first1=Jacqueline K. |last2=Goodman |first2=Sam |last3=Caselden |first3=Dan |last4=Colin |first4=Guillaume |last5=Kuchner |first5=Marc J. |last6=Meisner |first6=Aaron M. |last7=Gagné |first7=Jonathan |last8=Schneider |first8=Adam C. |last9=Gonzales |first9=Eileen C. |last10=Bardalez Gagliuffi |first10=Daniella C. |last11=Logsdon |first11=Sarah E. |title=WISE2150-7520AB: A very low mass, wide co-moving brown dwarf system discovered through the citizen science project Backyard Worlds: Planet 9 |journal=The Astrophysical Journal |volume=889 |issue=2 |pages=176 |arxiv=1911.04600 |doi=10.3847/1538-4357/ab5303 |year=2020 |bibcode=2020ApJ...889..176F |s2cid=207863267 |doi-access=free }}</ref>

There are other interesting binary systems such as the [[eclipsing binary]] brown dwarf system [[2MASS J05352184–0546085]].<ref name="Stassun2006"/> Photometric studies of this system have revealed that the less massive brown dwarf in the system is hotter than its higher-mass companion.<ref name=":1" />

=== Brown dwarfs around stars ===
{{Main|Brown-dwarf desert}}
Brown dwarfs and massive planets in a close orbit (less than 5 au) around stars are rare and this is sometimes described as the brown dwarf desert. Less than 1% of stars with the mass of the sun have a brown dwarf within 3–5 au.<ref>{{cite journal |last1=Grether |first1=Daniel |last2=Lineweaver |first2=Charles H. |date=2006-04-01 |title=How Dry is the Brown Dwarf Desert? Quantifying the Relative Number of Planets, Brown Dwarfs, and Stellar Companions around Nearby Sun-like Stars |url=https://ui.adsabs.harvard.edu/abs/2006ApJ...640.1051G |journal=The Astrophysical Journal |volume=640 |issue=2 |pages=1051–1062 |doi=10.1086/500161 |arxiv=astro-ph/0412356 |bibcode=2006ApJ...640.1051G |s2cid=8563521 |issn=0004-637X}}</ref>

An example for a star–brown dwarf binary is the first discovered T-dwarf [[Gliese 229|Gliese 229 B]], which orbits around the main-sequence star Gliese 229 A, a red dwarf. Brown dwarfs orbiting [[subgiant]]s are also known, such as [[TOI-1994b]] which orbits its star every 4.03 days.<ref>{{cite journal |last1=Page |first1=Emma |last2=Pepper |first2=Joshua |last3=Kane |first3=Stephen |last4=Zhou |first4=George |last5=Addison |first5=Brett |last6=Wright |first6=Duncan |last7=Wittenmyer |first7=Robert |last8=Johnson |first8=Marshall |last9=Evans |first9=Philip |last10=Collins |first10=Karen |last11=Hellier |first11=Coel |last12=Jensen |first12=Eric |last13=Stassun |first13=Keivan |last14=Rodriguez |first14=Joseph |date=2022-06-01 |title=TOI-1994b: An Eccentric Brown Dwarf Transiting a Subgiant |journal=American Astronomical Society Meeting Abstracts |url=https://ui.adsabs.harvard.edu/abs/2022AAS...24030521P |volume=54 |issue=6 |pages=305.21|bibcode=2022AAS...24030521P }}</ref>

There is also disagreement if some low-mass brown dwarfs should be considered planets. The [[NASA Exoplanet Archive|NASA Exoplanet archive]] includes brown dwarfs with a minimum mass less or equal to 30 Jupiter masses as planets as long as there are other criteria fulfilled (e.g. orbiting a star).<ref>{{cite web |title=Exoplanet Criteria for Inclusion in the Exoplanet Archive |url=https://exoplanetarchive.ipac.caltech.edu/docs/exoplanet_criteria.html |access-date=2023-04-16 |website=exoplanetarchive.ipac.caltech.edu}}</ref> The Working Group on Extrasolar Planets (WGESP) of the [[International Astronomical Union|IAU]] on the other hand only considers planets with a mass below 13 Jupiter masses.<ref>{{cite web |title=Working Group on Extrasolar Planets |url=https://w.astro.berkeley.edu/~basri/defineplanet/IAU-WGExSP.htm |access-date=2023-04-16 |website=w.astro.berkeley.edu}}</ref>

=== White dwarf–brown dwarf binaries ===
[[File:LSPM J0241+2553AB.jpg|thumb|[[LSPM J0241+2553AB]], a wide white dwarf(A)–brown dwarf(B) binary.]]
Brown dwarfs around [[white dwarf]]s are quite rare. [[GD 165|GD 165 B]], the prototype of the L dwarfs, is one such system.<ref>{{cite journal |last1=Farihi |first1=Jay |last2=Christopher |first2=Micol <!-- mc@astro.caltech.edu -->|date=October 2004 |title=A Possible Brown Dwarf Companion to the White Dwarf GD 1400 |journal=The Astronomical Journal |volume=128 |issue=4 |pages=1868 |arxiv=astro-ph/0407036 |doi=10.1086/423919 |issn=1538-3881 |bibcode=2004AJ....128.1868F |s2cid=119530628 }}</ref> Such systems can be useful in determining the age of the system and the mass of the brown dwarf. Other white dwarf-brown dwarf binaries are [[COCONUTS-1|COCONUTS-1 AB]] (7&nbsp;billion years old),<ref name=":3" /> and [[LSPM J0055+5948|LSPM J0055+5948 AB]] (10&nbsp;billion years old),<ref name=":12">{{cite journal |last1=Meisner |first1=Aaron M. |last2=Faherty |first2=Jacqueline K. |last3=Kirkpatrick |first3=J. Davy |last4=Schneider |first4=Adam C. |last5=Caselden |first5=Dan |last6=Gagné |first6=Jonathan |last7=Kuchner |first7=Marc J. |last8=Burgasser |first8=Adam J. |last9=Casewell |first9=Sarah L. |last10=Debes |first10=John H. |last11=Artigau |first11=Étienne |last12=Bardalez Gagliuffi |first12=Daniella C. |last13=Logsdon |first13=Sarah E. |last14=Kiman |first14=Rocio |last15=Allers |first15=Katelyn |date=2020-08-01 |title=Spitzer Follow-up of Extremely Cold Brown Dwarfs Discovered by the Backyard Worlds: Planet 9 Citizen Science Project |journal=The Astrophysical Journal |volume=899 |issue=2 |pages=123 |doi=10.3847/1538-4357/aba633 |arxiv=2008.06396 |bibcode=2020ApJ...899..123M |s2cid=221135837 |issn=0004-637X |doi-access=free }}</ref> [[SDSS J22255+0016|SDSS J22255+0016 AB]] (2&nbsp;billion years old)<ref name=":13">{{cite journal |last1=French |first1=Jenni R. |last2=Casewell |first2=Sarah L. |last3=Dupuy |first3=Trent J. |last4=Debes |first4=John H. |last5=Manjavacas |first5=Elena |last6=Martin |first6=Emily C. |last7=Xu |first7=Siyi |date=2023-03-01 |title=Discovery of a resolved white dwarf–brown dwarf binary with a small projected separation: SDSS J222551.65+001637.7AB |journal=Monthly Notices of the Royal Astronomical Society |volume=519 |issue=4 |pages=5008–5016 |doi=10.1093/mnras/stac3807 |doi-access=free |arxiv=2301.02101 |bibcode=2023MNRAS.519.5008F |issn=0035-8711}}</ref> [[WD 0806−661|WD 0806−661 AB]] (1.5–2.7&nbsp;billion years old).<ref>{{cite journal |last1=Leggett |first1=S. K. |last2=Tremblin |first2=P. |last3=Esplin |first3=T. L. |last4=Luhman |first4=K. L. |last5=Morley |first5=Caroline V. |date=2017-06-01 |title=The Y-type Brown Dwarfs: Estimates of Mass and Age from New Astrometry, Homogenized Photometry, and Near-infrared Spectroscopy |journal=The Astrophysical Journal |volume=842 |issue=2 |pages=118 |doi=10.3847/1538-4357/aa6fb5 |arxiv=1704.03573 |bibcode=2017ApJ...842..118L |s2cid=119249195 |issn=0004-637X |doi-access=free }}</ref>

Systems with close, [[Tidal locking|tidally locked]] brown dwarfs orbiting around white dwarfs belong to the [[post common envelope binary|post common envelope binaries]] or PCEBs. Only eight confirmed PCEBs containing a white dwarf with a brown dwarf companion are known, including [[WD 0137−349|WD 0137-349]] AB. In the past history of these close white dwarf–brown dwarf binaries, the brown dwarf is engulfed by the star in the [[Red giant|red giant phase]]. Brown dwarfs with a mass lower than 20 [[Jupiter mass]]es would [[Photoevaporation|evaporate]] during the engulfment.<ref>{{cite press release |first1=Pierre |last1=Maxted |first2=Ralf |last2=Napiwotzki |first3=Paul |last3=Dobbie |first4=Matt |last4=Burleigh |url=https://www.eso.org/public/news/eso0628/ |title=A Sub-Stellar Jonah – Brown Dwarf Survives Being Swallowed |publisher=European Southern Observatory |access-date=2019-12-11 }}</ref><ref>{{cite journal |last1=Casewell |first1=Sarah L. |last2=Braker |first2=Ian P. |last3=Parsons |first3=Steven G. |last4=Hermes |first4=James J. |last5=Burleigh |first5=Matthew R. |last6=Belardi |first6=Claudia |last7=Chaushev |first7=Alexander |last8=Finch |first8=Nicolle L. |last9=Roy |first9=Mervyn |last10=Littlefair |first10=Stuart P. |last11=Goad |first11=Mike |last12=Dennihy |first12=Erik |date=31 January 2018 |title=The first sub-70 min non-interacting WD–BD system: EPIC212235321 |journal=Monthly Notices of the Royal Astronomical Society |volume=476 |issue=1 |pages=1405–1411 |doi=10.1093/mnras/sty245 |doi-access=free |issn=0035-8711 |bibcode=2018MNRAS.476.1405C |arxiv=1801.07773 |s2cid=55776991 }}</ref> The dearth of brown dwarfs orbiting close to white dwarfs can be compared with similar observations of brown dwarfs around main-sequence stars, described as the [[brown-dwarf desert]].<ref>{{cite journal |last1=Longstaff |first1=Emma S. |last2=Casewell |first2=Sarah L. |last3=Wynn |first3=Graham A. |last4=Maxted |first4=Pierre F. L. |last5=Helling |first5=Christiane |date=2017-10-21 |title=Emission lines in the atmosphere of the irradiated brown dwarf WD0137−349B |url=https://academic.oup.com/mnras/article/471/2/1728/3974054 |journal=Monthly Notices of the Royal Astronomical Society |volume=471 |issue=2 |pages=1728–1736 |arxiv=1707.05793 |doi=10.1093/mnras/stx1786 |doi-access=free |issn=0035-8711 |bibcode=2017MNRAS.471.1728L |s2cid=29792989 }}</ref><ref>{{cite journal |last1=Grether |first1=Daniel |last2=Lineweaver |first2=Charles H. |date=April 2006 |title=How Dry is the Brown Dwarf Desert? Quantifying the Relative Number of Planets, Brown Dwarfs, and Stellar Companions around Nearby Sun-like Stars |journal=The Astrophysical Journal |volume=640 |issue=2 |pages=1051–1062 |doi=10.1086/500161 |arxiv=astro-ph/0412356 |bibcode=2006ApJ...640.1051G |issn=0004-637X |doi-access=free }}</ref> The PCEB might evolve into a [[cataclysmic variable star]] (CV*) with the brown dwarf as the donor.<ref>{{cite journal |last1=Rappaport |first1=Saul A. |last2=Vanderburg |first2=Andrew |last3=Nelson |first3=Lorne |last4=Gary |first4=Bruce L. |last5=Kaye |first5=Thomas G. |last6=Kalomeni |first6=Belinda |last7=Howell |first7=Steve B. |last8=Thorstensen |first8=John R. |last9=Lachapelle |first9=François-René |last10=Lundy |first10=Matthew |last11=St-Antoine |first11=Jonathan |date=2017-10-11 |title=WD 1202-024: the shortest-period pre-cataclysmic variable |url=https://academic.oup.com/mnras/article/471/1/948/3892367 |journal=Monthly Notices of the Royal Astronomical Society |volume=471 |issue=1 |pages=948–961 |arxiv=1705.05863 |doi=10.1093/mnras/stx1611 |doi-access=free |issn=0035-8711 |bibcode=2017MNRAS.471..948R |s2cid=119349942 }}</ref> Simulations have shown that highly evolved CV* are mostly associated with substellar donors (up to 80%).<ref name=":15">{{cite journal |last1=Neustroev |first1=Vitaly V. |last2=Mäntynen |first2=Iikka |date=2023-08-01 |title=A brown dwarf donor and an optically thin accretion disc with a complex stream impact region in the period-bouncer candidate BW Sculptoris |journal=Monthly Notices of the Royal Astronomical Society |volume=523 |issue=4 |pages=6114–6137 |doi=10.1093/mnras/stad1730 |arxiv=2212.03264 |bibcode=2023MNRAS.523.6114N |issn=0035-8711|doi-access=free }}</ref> A type of CV*, called [[WZ Sagittae|WZ Sge]]-type [[dwarf nova]] often show donors with a mass near the borderline of low-mass stars and brown dwarfs.<ref>{{cite journal |last=Kato |first=Taichi |date=2015-12-01 |title=WZ Sge-type dwarf novae |journal=Publications of the Astronomical Society of Japan |volume=67 |issue=6 |pages=108 |doi=10.1093/pasj/psv077 |arxiv=1507.07659 |bibcode=2015PASJ...67..108K |issn=0004-6264|doi-access=free }}</ref> The binary [[BW Sculptoris]] is such a dwarf nova with a brown dwarf donor. This brown dwarf likely formed when a donor star lost enough mass to become a brown dwarf. The mass loss comes with a loss of the orbital period until it reaches a minimum of 70–80 minutes at which the period increases again. This gives this evolutionary stage the name [[period bouncer]].<ref name=":15" /> There could also exist brown dwarfs that merged with white dwarfs. The nova [[CK Vulpeculae]] might be a result of such a white dwarf–brown dwarf merger.<ref>{{cite web |first1=Nicolás |last1=Lira |first2=Charles E. |last2=Blue |first3=Calum |last3=Turner |first4=Masaaki |last4=Hiramatsu |url=https://www.almaobservatory.org/en/press-release/when-is-a-nova-not-a-nova-when-a-white-dwarf-and-a-brown-dwarf-collide/ |title=When Is a Nova Not a 'Nova'? When a White Dwarf and a Brown Dwarf Collide |website=ALMA Observatory |url-status=dead |archive-url=https://web.archive.org/web/20191022055806/https://www.almaobservatory.org/en/press-release/when-is-a-nova-not-a-nova-when-a-white-dwarf-and-a-brown-dwarf-collide/ |archive-date=2019-10-22 |access-date=2019-11-12 }}</ref><ref>{{cite journal |last1=Eyres |first1=Stewart P. S. |last2=Evans |first2=Aneurin |last3=Zijlstra |first3=Albert |last4=Avison |first4=Adam |last5=Gehrz |first5=Robert D. |last6=Hajduk |first6=Marcin|author7-link=Sumner Starrfield |last7=Starrfield |first7=Sumner |last8=Mohamed |first8=Shazrene |last9=Woodward |first9=Charles E. |last10=Wagner |first10=R. Mark |date=2018-12-21 |title=ALMA reveals the aftermath of a white dwarf–brown dwarf merger in CK Vulpeculae |url=https://academic.oup.com/mnras/article/481/4/4931/5107360 |journal=Monthly Notices of the Royal Astronomical Society |volume=481 |issue=4 |pages=4931–4939 |arxiv=1809.05849 |doi=10.1093/mnras/sty2554 |doi-access=free |bibcode=2018MNRAS.481.4931E |s2cid=119462149 |issn=0035-8711 }}</ref>

== Formation and evolution ==
[[File:M1701117 R-SII-Ha(1).png|thumb|The [[HH 1165]] jet launched by the brown dwarf [[Mayrit 1701117]] in the outer periphery of the [[sigma Orionis]] cluster]]The earliest stage of brown dwarf formation is called proto- or pre-brown dwarf. Proto-brown dwarfs are low-mass equivalents of [[Protostar|protostars]] (class 0/I objects). Additionally Very Low Luminosity Objects (VeLLOs) that have L<sub>int</sub> ≤0.1-0.2 {{Solar luminosity|link=true}} are often proto-brown dwarfs. They are found in nearby [[Star formation|star-forming]] clouds. Around 67 promising proto-brown dwarfs and 26 pre-brown dwarfs are known as of 2024.<ref>{{citation |last1=Palau |first1=Aina |title=Observations of pre- and proto-brown dwarfs in nearby clouds: paving the way to further constraining theories of brown dwarf formation |date=2024-10-01 |url=https://ui.adsabs.harvard.edu/abs/2024arXiv241007769P/abstract |access-date=2024-10-25 |arxiv=2410.07769 |last2=Huelamo |first2=Nuria |last3=Barrado |first3=David |last4=Dunham |first4=Michael M. |last5=Lee |first5=Chang Won|journal=New Astronomy Reviews |volume=99 |doi=10.1016/j.newar.2024.101711 |bibcode=2024NewAR..9901711P }}</ref> As of 2017 there is only one known proto-brown dwarf that is connected with a large [[Herbig–Haro object]]. This is the brown dwarf [[Mayrit 1701117]], which is surrounded by a pseudo-disk and a Keplerian disk.<ref>{{cite journal |last1=Riaz |first1=Basmah |last2=Machida |first2=Masahiro N. |last3=Stamatellos |first3=Dimitris |date=July 2019 |title=ALMA reveals a pseudo-disc in a proto-brown dwarf |journal=Monthly Notices of the Royal Astronomical Society |volume=486 |issue=3 |pages=4114–4129 |doi=10.1093/mnras/stz1032 |doi-access=free |arxiv=1904.06418 |bibcode=2019MNRAS.486.4114R |s2cid=119286540 |issn=0035-8711 }}</ref> Mayrit 1701117 launches the 0.7-light-year-long jet [[HH 1165]], mostly seen in ionized [[sulfur]].<ref>{{cite web |first1=Basmah |last1=Riaz |first2=Joan |last2=Najita |url=http://www.noao.edu/news/2017/pr1701.php |title=Punching Above Its Weight, a Brown Dwarf Launches a Parsec-Scale Jet |website=National Optical Astronomy Observatory |access-date=2020-02-18 |archive-date=2020-02-18 |archive-url=https://web.archive.org/web/20200218195420/https://www.noao.edu/news/2017/pr1701.php |url-status=dead }}</ref><ref name="Riaz 47">{{cite journal |last1=Riaz |first1=Basmah |last2=Briceño |first2=Cesar |last3=Whelan |first3=Emma T. |last4=Heathcote |first4=Stephen |date=July 2017 |title=First Large-scale Herbig-Haro Jet Driven by a Proto-brown Dwarf |journal=Astrophysical Journal |volume=844 |issue=1 |pages=47 |doi=10.3847/1538-4357/aa70e8 |arxiv=1705.01170 |bibcode=2017ApJ...844...47R |s2cid=119080074 |issn=0004-637X |doi-access=free }}</ref>

Brown dwarfs form similarly to stars and are surrounded by [[protoplanetary disk]]s,<ref name=":4">{{cite journal |last1=Apai |first1=Dániel |last2=Pascucci |first2=Ilaria |author2-link=Ilaria Pascucci |last3=Bouwman |first3=Jeroen |last4=Natta |first4=Antonella |last5=Henning |first5=Thomas |last6=Dullemond |first6=Cornelis P. |year=2005 |title=The Onset of Planet Formation in Brown Dwarf Disks |journal=Science |volume=310 |issue=5749 |pages=834–6 |arxiv=astro-ph/0511420 |bibcode=2005Sci...310..834A |doi=10.1126/science.1118042 |pmid=16239438 |s2cid=5181947}}</ref> such as [[Cha 110913−773444]]. [[Circumstellar disc|Disks]] around brown dwarfs have been found to have many of the same features as disks around stars; therefore, it is expected that there will be accretion-formed planets around brown dwarfs.<ref name=":4"/> Given the small mass of brown dwarf disks, most planets will be terrestrial planets rather than gas giants.<ref name="tidalplanets">{{cite journal |last1=Burrows |first1=Adam |last2=Hubbard |first2=William B. |last3=Lunine |first3=Jonathan I. |last4=Liebert |first4=James |year=2011 |title=Tidal evolution of planets around brown dwarfs |journal=Astronomy & Astrophysics |volume=535 |pages=A94 |arxiv=1109.2906 |bibcode=2011A&A...535A..94B |doi=10.1051/0004-6361/201117734 |s2cid=118532416 }}</ref> If a giant planet orbits a brown dwarf across our line of sight, then, because they have approximately the same diameter, this would give a large signal for [[Transit method|detection by transit]].<ref>[[David C. Jewitt|Jewitt, David C.]], [http://isites.harvard.edu/fs/docs/icb.topic541038.files/ay98_reading10.pdf Pan-STARRS Science Overview] {{Webarchive|url=https://web.archive.org/web/20151016070851/http://isites.harvard.edu/fs/docs/icb.topic541038.files/ay98_reading10.pdf |date=2015-10-16 }}</ref> The accretion zone for planets around a brown dwarf is very close to the brown dwarf itself, so tidal forces would have a strong effect.<ref name="tidalplanets"/>

[[File:Nearby_young_brown_dwarf_disk_WISEA_J120037.79-784508.3.png|thumb|left|275px|Artist's depiction of brown dwarf W1200-7845]]
In 2020, the closest brown dwarf with an associated primordial disk (class II disk)&mdash;[[WISEA J120037.79-784508.3]] (W1200-7845)&mdash;was discovered by the [[Disk Detective]] project when classification volunteers noted its infrared excess. It was vetted and analyzed by the science team who found that W1200-7845 had a 99.8% probability of being a member of the ε Chamaeleontis (ε Cha) young [[moving group]] [[stellar association|association]]. Its parallax (using Gaia DR2 data) puts it at a distance of 102 parsecs (or 333 lightyears) from Earth&mdash;which is within the local Solar neighborhood.<ref>{{cite web|url=https://blog.diskdetective.org/2020/08/12/our-new-paper-discovery-of-nearby-young-brown-dwarf-disk/|title=Our New Paper: Discovery of Nearby Young Brown Dwarf Disk!|last=Schutte|first=Maria|date=2020-08-12|website=DiskDetective.org|access-date=2023-09-23}}</ref><ref>{{cite journal |last1=Schutte |first1=Maria C. |last2=Lawson |first2=Kellen D. |last3=Wisniewski |first3=John P. |last4=Kuchner |first4=Marc J. |last5=Silverberg |first5=Steven M. |last6=Faherty |first6=Jacqueline K. |last7=Gagliuffi |first7=Daniella C. Bardalez |last8=Kiman |first8=Rocio |last9=Gagné |first9=Jonathan |last10=Meisner |first10=Aaron |last11=Schneider |first11=Adam C. |last12=Bans |first12=Alissa S. |last13=Debes |first13=John H. |last14=Kovacevic |first14=Natalie |last15=Bosch |first15=Milton K.D. |last16=Luca |first16=Hugo A. Durantini |last17=Holden |first17=Jonathan |last18=Hyogo |first18=Michiharu |date=2020-08-04 |title=Discovery of a Nearby Young Brown Dwarf Disk |journal=The Astrophysical Journal |volume=160 |issue=4 |pages=156 |doi=10.3847/1538-3881/abaccd |arxiv=2007.15735v2 |bibcode=2020AJ....160..156S |s2cid=220920317 |issn=1538-3881 |doi-access=free }}</ref>

[[File:Brown dwarf proplyds Orion Nebula.jpg|thumb|right|brown dwarf proplyds in the Orion Nebula with [[Hubble Space Telescope|Hubble]] and [[James Webb Space Telescope|JWST]].]]
A paper from 2021 studied [[circumstellar disc]]s around brown dwarfs in [[stellar association]]s that are a few million years old and 140 to 200 parsecs away. The researchers found that these disks are not massive enough to form planets in the future. There is evidence in these disks that might indicate that planet formation begins at earlier stages and that planets are already present in these disks. The evidence for disk evolution includes a decreasing disk mass over time, dust grain growth and dust settling.<ref name=":16">{{cite journal |last1=Rilinger |first1=Anneliese M. |last2=Espaillat |first2=Catherine C.|author2-link=Catherine Espaillat |date=2021-11-01 |title=Disk Masses and Dust Evolution of Protoplanetary Disks around Brown Dwarfs |journal=The Astrophysical Journal |volume=921 |issue=2 |pages=182 |doi=10.3847/1538-4357/ac09e5 |arxiv=2106.05247 |bibcode=2021ApJ...921..182R |s2cid=235377000 |issn=0004-637X |doi-access=free }}</ref> Two brown dwarf disks were also found in absorption and at least 4 disks are [[photoevaporation|photoevaporating]] from external UV-ratiation in the [[Orion Nebula]]. Such objects are also called [[proplyd|proplyds]]. [[Proplyd 181−247]], which is a brown dwarf or low-mass star, is surrounded by a disk with a radius of 30 astronomical units and the disk has a mass of 6.2±1.0 {{jupiter mass}}.<ref name="Luhman2024">{{cite journal |last1 = Luhman  |first1 = K. L. |first2=C |last2=Alves de Oliveira |first3=I. |last3=Baraffe |first4=G. |last4=Chabrier |first5=E. |last5=Manjavacas |first6=R. J. |last6=Parker |first7=P. |last7=Tremblin |author-link1 = Kevin Luhman |author-link3=Isabelle Baraffe |date = 13 Oct 2024 |title = JWST/NIRSpec Observations of Brown Dwarfs in the Orion Nebula Cluster |journal = The Astrophysical Journal |volume = 975 |issue = 2 |page = 162 |doi = 10.3847/1538-4357/ad7b19 |doi-access = free |arxiv = 2410.10000 |bibcode = 2024ApJ...975..162L }}</ref> Disks around brown dwarfs usually have a radius smaller than 40 [[astronomical unit]]s, but three disks in the more distant [[Taurus molecular cloud]] have a radius larger than 70 au and were resolved with [[Atacama Large Millimeter Array|ALMA]]. These larger disks are able to form rocky planets with a mass >1 {{Earth mass|link=true}}.<ref>{{cite journal |last1=Ricci |first1=L. |last2=Testi |first2=L. |last3=Natta |first3=A. |last4=Scholz |first4=A. |last5=de Gregorio-Monsalvo |first5=I. |last6=Isella |first6=A. |date=2014-08-01 |title=Brown Dwarf Disks with ALMA |url=https://ui.adsabs.harvard.edu/abs/2014ApJ...791...20R |journal=The Astrophysical Journal |volume=791 |issue=1 |pages=20 |doi=10.1088/0004-637X/791/1/20 |arxiv=1406.0635 |bibcode=2014ApJ...791...20R |hdl=10023/8607 |s2cid=13180928 |issn=0004-637X}}</ref> There are also brown dwarfs with disks in associations older than a few million years,<ref>{{cite journal |last1=Boucher |first1=Anne |last2=Lafrenière |first2=David |last3=Gagné |first3=Jonathan |last4=Malo |first4=Lison |last5=Faherty |first5=Jacqueline K. |last6=Doyon |first6=René |last7=Chen |first7=Christine H. |date=2016-11-01 |title=BANYAN. VIII. New Low-mass Stars and Brown Dwarfs with Candidate Circumstellar Disks |journal=The Astrophysical Journal |volume=832 |issue=1 |pages=50 |doi=10.3847/0004-637X/832/1/50 |arxiv=1608.08259 |bibcode=2016ApJ...832...50B |s2cid=119017727 |issn=0004-637X |doi-access=free }}</ref> which might be evidence that disks around brown dwarfs need more time to dissipate. Especially old disks (>20 Myrs) are sometimes called [[Peter Pan disk]]s. Currently [[2MASS J02265658-5327032]] is the only known brown dwarf that has a Peter Pan disk.<ref>{{cite journal |last1=Silverberg |first1=Steven M. |last2=Wisniewski |first2=John P. |last3=Kuchner |first3=Marc J. |last4=Lawson |first4=Kellen D. |last5=Bans |first5=Alissa S. |last6=Debes |first6=John H. |last7=Biggs |first7=Joseph R. |last8=Bosch |first8=Milton K. D. |last9=Doll |first9=Katharina |last10=Luca |first10=Hugo A. Durantini |last11=Enachioaie |first11=Alexandru |last12=Hamilton |first12=Joshua |last13=Holden |first13=Jonathan |last14=Hyogo |first14=Michiharu |date=2020-02-01 |title=Peter Pan Disks: Long-lived Accretion Disks Around Young M Stars |journal=The Astrophysical Journal |volume=890 |issue=2 |pages=106 |doi=10.3847/1538-4357/ab68e6 |arxiv=2001.05030 |bibcode=2020ApJ...890..106S |s2cid=210718358 |issn=0004-637X |doi-access=free }}</ref>

The brown dwarf [[Cha 110913−773444]], located 500 light-years away in the constellation Chamaeleon, may be in the process of forming a miniature planetary system. Astronomers from [[Pennsylvania State University]] have detected what they believe to be a disk of gas and dust similar to the one hypothesized to have formed the Solar System. Cha 110913−773444 is the smallest brown dwarf found to date ({{Jupiter mass|8}}), and if it formed a planetary system, it would be the smallest-known object to have one.<ref>{{cite journal |last1=Luhman |first1=Kevin L. |last2=Adame |first2=Lucía |last3=d'Alessio |first3=Paola |last4=Calvet |first4=Nuria|author4-link=Nuria Calvet |last5=Hartmann |first5=Lee |last6=Megeath |first6=S. T. |last7=Fazio |first7=G. G. |year=2005 |title=Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk |journal=The Astrophysical Journal |volume=635 |issue=1 |pages=L93–L96 |arxiv=astro-ph/0511807 |bibcode=2005ApJ...635L..93L |doi=10.1086/498868 |s2cid=11685964 }}</ref>

== Planets around brown dwarfs ==
[[File:Artist’s impression of the disc of dust and gas around a brown dwarf.jpg|thumb|Artist's impression of a disc of dust and gas around a brown dwarf<ref>{{cite press release |first1=Luca |last1=Ricci |first2=Leonardo |last2=Testi |first3=Douglas |last3=Pierce-Price |first4=John |last4=Stoke |title=Even Brown Dwarfs May Grow Rocky Planets |url=http://www.eso.org/public/news/eso1248/ |access-date=3 December 2012 |publisher=European Southern Observatory |archive-url=https://web.archive.org/web/20121203042337/http://www.eso.org/public/news/eso1248/ |archive-date=3 December 2012 |url-status=dead }}</ref>]]
According to the IAU working definition (from August 2018) an exoplanet can orbit a brown dwarf. It requires a mass below 13 {{Jupiter mass}} and a mass ratio of ''M''/''M''<sub>central</sub><2/(25+√621). This means that an object with a mass up to 3.2&nbsp;{{Jupiter mass}} around a brown dwarf with a mass of 80&nbsp;{{Jupiter mass}} is considered a planet. It also means that an object with a mass up to 0.52&nbsp;{{Jupiter mass}} around a brown dwarf with a mass of 13&nbsp;{{Jupiter mass}} is considered a planet.<ref>{{cite journal |last1=Lecavelier des Etangs |first1=A. |last2=Lissauer |first2=Jack J. |date=2022-06-01 |title=The IAU working definition of an exoplanet |url=https://ui.adsabs.harvard.edu/abs/2022NewAR..9401641L |journal=New Astronomy Reviews |volume=94 |pages=101641 |doi=10.1016/j.newar.2022.101641 |arxiv=2203.09520 |bibcode=2022NewAR..9401641L |s2cid=247065421 |issn=1387-6473}}</ref>

The [[super-Jupiter]] planetary-mass objects [[2M1207b]], [[2MASS J044144]] and Oph 98 B that are orbiting brown dwarfs at large orbital distances may have formed by [[cloud collapse]] rather than accretion and so may be [[sub-brown dwarf]]s rather than [[planet]]s, which is inferred from relatively large masses and large orbits. The first discovery of a low-mass companion orbiting a brown dwarf ([[ChaHα8]]) at a small orbital distance using the [[Doppler spectroscopy|radial velocity technique]] paved the way for the detection of planets around brown dwarfs on orbits of a few AU or smaller.<ref>{{cite journal |last1=Joergens |first1=Viki |last2=Müller |first2=André |title=16–20 MJup Radial Velocity Companion Orbiting the Brown Dwarf Candidate Cha Hα 8 |journal=The Astrophysical Journal |year=2007 |volume=666 |issue=2 |pages=L113–L116 |doi=10.1086/521825 |bibcode=2007ApJ...666L.113J |arxiv=0707.3744|s2cid=119140521 }}</ref><ref>{{cite journal |last1=Joergens |first1=Viki |last2=Müller |first2=André |last3=Reffert |first3=Sabine |title=Improved radial velocity orbit of the young binary brown dwarf candidate Cha Hα 8 |journal=Astronomy and Astrophysics |date=2010 |volume=521 |issue=A24 |pages=A24 |doi=10.1051/0004-6361/201014853 |bibcode=2010A&A...521A..24J |arxiv=1006.2383|s2cid=54989533 }}</ref> However, with a mass ratio between the companion and primary in [[ChaHα8]] of about 0.3, this system rather resembles a binary star. Then, in 2008, the first planetary-mass companion in a relatively small orbit ([[MOA-2007-BLG-192Lb]]) was discovered orbiting a brown dwarf.<ref name="bennett2008">{{cite journal |last1=Bennet |first1=David P. |first2=Ian A. |last2=Bond |first3=Andrzej |last3=Udalski |first4=Takahiro |last4=Sumi |first5=Fumio |last5=Abe |first6=Akihiko |last6=Fukui |first7=Kei |last7=Furusawa |first8=John B. |last8=Hearnshaw |first9=Sarah |last9=Holderness |first10=Yoshitaka |last10=Itow |first11=Koki |last11=Kamiya |first12=Aarno V. |last12=Korpela |first13=Pamela M. |last13=Kilmartin |first14=Wei |last14=Lin |first15=Cho Hong |last15=Ling |first16=Kimiaki |last16=Masuda |first17=Yutaka |last17=Matsubara |first18=Noriyuki |last18=Miyake |first19=Yasushi |last19=Muraki |first20=Maiko |last20=Nagaya |first21=Teppei |last21=Okumura |first22=Kouji |last22=Ohnishi |first23=Yvette C. |last23=Perrott |first24=Nicholas J. |last24=Rattenbury |first25=Takashi |last25=Sako |first26=Toshiharu |last26=Saito |first27=S. |last27=Sato |first28=Ljiljana |last28=Skuljan |first29=Denis J. |last29=Sullivan |first30=Winston L. |last30=Sweatman |first31=Paul J. |last31=Tristram |first32=Philip C. M. |last32=Yock |first33=Marcin |last33=Kubiak |first34=Michał K. |last34=Szymański |first35=Grzegorz |last35=Pietrzyński |first36=Igor |last36=Soszyński 
|first37=O. |last37=Szewczyk |first38=Łukasz |last38=Wyrzykowski |first39=Krzysztof |last39=Ulaczyk |first40=Virginie |last40=Batista |first41=Jean-Philippe |last41=Beaulieu |first42=Stéphane |last42=Brillant |first43=Arnaud |last43=Cassan |first44=Pascal |last44=Fouqué |first45=Pierre |last45=Kervella |first46=Daniel |last46=Kubas |first47=Jean-Baptiste |last47=Marquette |arxiv=0806.0025 |title=A Low-Mass Planet with a Possible Sub-Stellar-Mass Host in Microlensing Event MOA-2007-BLG-192 |journal=The Astrophysical Journal |date=30 May 2008 |volume=684 |issue=1 |pages=663–683 |doi=10.1086/589940 |bibcode=2008ApJ...684..663B |s2cid=14467194 }}</ref>
<!-- Then, in 2013, the first planetary-mass companion (OGLE-2012-BLG-0358L b) in a relatively small orbit was discovered orbiting a brown dwarf.<ref>{{cite magazine |url=http://www.technologyreview.com/view/517556/first-planet-discovered-orbiting-a-brown-dwarf/ |title=First Planet Discovered Orbiting a Brown Dwarf |magazine=MIT Technology Review |date=29 July 2013 |access-date=29 July 2013}}</ref>
In 2015, the first terrestrial-mass planet orbiting a brown dwarf was found, OGLE-2013-BLG-0723LBb.<ref>{{cite journal |arxiv=1507.02388 |last1=Burrows |first1=Adam |title=A Venus-Mass Planet Orbiting a Brown Dwarf: Missing Link between Planets and Moons |journal=The Astrophysical Journal |volume=812 |issue=1 |pages=47 |last2=Hubbard |first2=William B. |last3=Lunine |first3=Jonathan I. |last4=Liebert |first4=James |last5=Kozłowski |first5=S. |last6=Skowron |first6=J. |last7=Poleski |first7=R. |last8=Soszyński |first8=I. |last9=Pietrukowicz |first9=P. |last10=Mróz |first10=P. |last11=Szymański |first11=M. K. |last12=Wyrzykowski |first12=Ł. |last13=Ulaczyk |first13=K. |last14=Pietrzyński |first14=G. |last15=Shvartzvald |first15=Y. |last16=Maoz |first16=D. |last17=Kaspi |first17=S. |last18=Gaudi |first18=B. S. |last19=Hwang |first19=K.-H. |last20=Choi |first20 = J.-Y. |last21=Shin |first21=I.-G. |last22=Park |first22=H. |last23=Bozza |first23=V. |year=2015 |doi=10.1088/0004-637X/812/1/47 |bibcode=2015ApJ...812...47U }}</ref> -->

Planets around brown dwarfs are likely to be [[carbon planet]]s depleted of water.<ref>{{cite journal |arxiv=1311.1228 |last1=Burrows |first1=Adam |title=The Atomic and Molecular Content of Disks Around Very Low-mass Stars and Brown Dwarfs |journal=The Astrophysical Journal |volume=779 |issue=2 |pages=178 |last2=Hubbard |first2=William B. |last3=Lunine |first3=Jonathan I. |last4=Liebert |first4=James |year=2013 |doi=10.1088/0004-637X/779/2/178 |bibcode=2013ApJ...779..178P |s2cid=119001471 }}</ref>

A 2017 study, based upon observations with [[Spitzer Space Telescope|Spitzer]] estimates that 175 brown dwarfs need to be monitored in order to guarantee (95%) at least one detection of a below earth-sized planet via the transiting method.<ref>{{cite journal |last1=He |first1=Matthias Y. |last2=Triaud |first2=Amaury H. M. J. |last3=Gillon |first3=Michaël |date=January 2017 |title=First limits on the occurrence rate of short-period planets orbiting brown dwarfs |journal=Monthly Notices of the Royal Astronomical Society |volume=464 |issue=3 |pages=2687–2697 |arxiv=1609.05053 |bibcode=2017MNRAS.464.2687H |doi=10.1093/mnras/stw2391 |doi-access=free |s2cid=53692008 }}</ref> JWST could potentially detect smaller planets. The orbits of planets and moons in the [[Solar System|solar system]] often align with the orientation of the host star/planet they orbit. Assuming the orbit of a planet is aligned with the [[Rotation|rotational axis]] of a brown dwarf or [[Rogue planet|planetary-mass object]], the geometric transit probability of an object similar to [[Io (moon)|Io]] can be calculated with the formula cos(79.5°)/cos([[Orbital inclination|inclination]]).<ref name=":122">{{cite journal |last1=Limbach |first1=Mary Anne |last2=Vos |first2=Johanna M. |last3=Winn |first3=Joshua N. |last4=Heller |first4=René |last5=Mason |first5=Jeffrey C. |last6=Schneider |first6=Adam C. |last7=Dai |first7=Fei |date=2021-09-01 |title=On the Detection of Exomoons Transiting Isolated Planetary-mass Objects |journal=The Astrophysical Journal |volume=918 |issue=2 |pages=L25 |arxiv=2108.08323 |bibcode=2021ApJ...918L..25L |doi=10.3847/2041-8213/ac1e2d |issn=0004-637X |doi-access=free}}</ref> The inclination was estimated for several brown dwarfs and planetary-mass objects. [[SIMP J013656.5+093347|SIMP 0136]] for example has an estimated inclination of 80°±12.<ref>{{cite journal |last1=Vos |first1=Johanna M. |last2=Allers |first2=Katelyn N. |last3=Biller |first3=Beth A. |date=2017-06-01 |title=The Viewing Geometry of Brown Dwarfs Influences Their Observed Colors and Variability Amplitudes |journal=The Astrophysical Journal |volume=842 |issue=2 |pages=78 |arxiv=1705.06045 |bibcode=2017ApJ...842...78V |doi=10.3847/1538-4357/aa73cf |doi-access=free |issn=0004-637X}}</ref> Assuming the lower bound of i≥68° for SIMP 0136, this results in a transit probability of ≥48.6% for close-in planets. It is however not known how common close-in planets are around brown dwarfs and they might be more common for lower-mass objects, as disk sizes seem to decrease with mass.<ref name=":16" />

=== Habitability ===

Habitability for hypothetical planets [[orbit]]ing brown dwarfs has been studied. Computer models suggesting conditions for these bodies to have [[habitable planet]]s are very stringent, the [[habitable zone]] being narrow, close (T dwarf 0.005&nbsp;au) and decreasing with time, due to the cooling of the brown dwarf (they fuse for at most 10 million years).    The orbits there would have to be of extremely low [[Eccentricity (mathematics)|eccentricity]] (on the order of 10 to the minus 6) to avoid strong [[tidal force]]s that would trigger a [[runaway greenhouse effect]] on the planets, rendering them uninhabitable. There would also be no moons.<ref>
{{cite journal |bibcode=2013AsBio..13..279B |arxiv=1211.6467 |doi=10.1089/ast.2012.0867 |title=Habitable Planets Around White and Brown Dwarfs: The Perils of a Cooling Primary |date=2011 |last1=Barnes |first1=Rory |last2=Heller |first2=René |journal=Astrobiology |volume=13 |issue=3 |pages=279–291 |pmid=23537137 |pmc=3612282}}</ref>

== Superlative brown dwarfs ==
{{Main|List of brown dwarfs}}
In 1984, it was postulated by some astronomers that the Sun may be orbited by an undetected brown dwarf (sometimes referred to as [[Nemesis (hypothetical star)|Nemesis]]) that could interact with the [[Oort cloud]] just as [[List of nearest stars and brown dwarfs#Distant future and past encounters|passing stars]] can. However, this hypothesis has fallen out of favor.<ref name="Morrison">{{cite web |date=2 August 2011 |title=Scientists today no longer think an object like Nemesis could exist |publisher=NASA Ask An Astrobiologist |first=David |last=Morrison |author-link=David Morrison (astrophysicist) |url=http://astrobiology2.arc.nasa.gov/ask-an-astrobiologist/question/?id=16790 |archive-url=https://archive.today/20121213101422/http://astrobiology2.arc.nasa.gov/ask-an-astrobiologist/question/?id=16790 |url-status=dead |archive-date=13 December 2012 |access-date=2011-10-22 }}</ref>

=== Table of firsts ===
{| class="wikitable" style="text-align:center;"
|- style="background:#efefef;"
! Record
! Name
! Spectral type
! RA/Dec
! Constellation
! Notes
|-
| First discovered
| [[Gliese 569]] Bab (Companions of M3 field star)
| M8.5 and M9
| 14<sup>h</sup>54<sup>m</sup>29.2<sup>s</sup> +16° 06{{prime}} 04{{pprime}}
| [[Bootes (constellation)|Bootes]]
| Imaged in 1985 published in 1988 weighed in 2004
|-
| First imaged with coronography
| [[Gliese 229 B]]
| T6.5
| 06<sup>h</sup>10<sup>m</sup>34.62<sup>s</sup> −21°51'52.1"
| [[Lepus (constellation)|Lepus]]
| Discovered 1994
|-
| First with [[Planet#Planetary-mass objects|planemo]]
| [[2M1207]]
| M8
| 12<sup>h</sup>07<sup>m</sup>33.47<sup>s</sup> −39°32'54.0"
| [[Centaurus]]
|Planet discovered in 2004
|-
| First with a [[Protoplanetary disk|circumstellar disk]]
| [[ChaHα1]]
| M7.5
| 11<sup>h</sup>07<sup>m</sup>17.0<sup>s</sup> −77° 35{{prime}} 54{{pprime}}
| [[Chamaeleon]]
| Disk discovered in 2000, first disk around a [[Good faith|bona fide]] brown dwarf, also first x-ray emitting<ref>{{cite journal |last1=Comerón |first1=F. |last2=Neuhäuser |first2=R. |last3=Kaas |first3=A. A. |date=2000-07-01 |title=Probing the brown dwarf population of the Chamaeleon I star forming region |url=https://ui.adsabs.harvard.edu/abs/2000A&A...359..269C |journal=Astronomy and Astrophysics |volume=359 |pages=269–288 |bibcode=2000A&A...359..269C |issn=0004-6361}}</ref>
|-
| First with [[bipolar outflow]]
|[[Rho-Oph 102]] (SIMBAD: [GY92] 102)
|
|16 26 42.758 −24 41 22.24
|Ophiuchus
|partly resolved outflow<ref>{{cite journal|last1=Whelan|first1=Emma T.|last2=Ray|first2=Thomas P.|last3=Bacciotti|first3=Francesca|last4=Natta|first4=Antonella|last5=Testi|first5=Leonardo|last6=Randich|first6=Sofia|date=June 2005|title=A resolved outflow of matter from a brown dwarf|journal=Nature|volume=435|issue=7042|pages=652–654|doi=10.1038/nature03598|pmid=15931217|arxiv=astro-ph/0506485|bibcode=2005Natur.435..652W|s2cid=4415442|issn=0028-0836}}</ref>
|-
|First with large-scale [[Herbig–Haro object|Herbig-Haro object]]
|[[Mayrit 1701117]] 
(Herbig-Haro object: [[HH 1165]])
|proto-BD
|05 40 25.799 −02 48 55.42
|Orion
|projected length of the Herbig-Haro object: 0.8 [[light-year]]s (0.26 [[Parsec|pc]])<ref name="Riaz 47"/>
|-
| First field type (solitary)
| [[Teide 1]]
| M8
| 3<sup>h</sup>47<sup>m</sup>18.0<sup>s</sup> +24° 22{{prime}} 31{{pprime}}
| Taurus
| 1995
|-
| First as a companion to a normal star
| [[Gliese 229 B]]
| T6.5
| 06<sup>h</sup>10<sup>m</sup>34.62<sup>s</sup> −21°51'52.1"
| Lepus
| 1995
|-
| First spectroscopic binary brown dwarf
| [[PPL 15 A, B]]<ref name=":0">{{cite journal |arxiv=astro-ph/9908015 |title=[astro-ph/9908015] PPl 15: The First Brown Dwarf Spectroscopic Binary |year=1999 |last1=Basri |first1=Gibor |last2=Martín |first2=Eduardo L. |doi=10.1086/301079 |journal=The Astronomical Journal |volume=118 |issue=5 |pages=2460–2465 |bibcode=1999AJ....118.2460B|s2cid=17662168 }}</ref>
| M6.5
|03<sup>h</sup> 48<sup>m</sup> 4.659<sup>s</sup> +23° 39' 30.32{{pprime}}
| Taurus
| Basri and Martín 1999
|-
| First eclipsing binary brown dwarf
| [[2M0535-05]]<ref name="Stassun2006"/><ref name=":1">{{cite journal |arxiv=0704.3106 |title=A Surprising Reversal of Temperatures in the Brown-Dwarf Eclipsing Binary 2MASS J05352184-0546085 |year=2007 |last1=Stassun |first1=Keivan G. |last2=Mathieu |first2=Robert D. |last3=Valenti |first3=Jeff A. |doi=10.1086/519231 |journal=The Astrophysical Journal |volume=664 |issue=2 |pages=1154–1166 |bibcode=2007ApJ...664.1154S|s2cid=15144741 }}</ref>
| M6.5 
|{{RA|15|35|21.84732}} {{Dec|-05|46|08.5714}}
| Orion
| Stassun 2006, 2007 (distance ~450 pc)
|- 
| First binary brown dwarf of T Type
| [[Epsilon Indi|Epsilon Indi Ba, Bb]]<ref>{{cite press release |first1=Ralf-Dieter |last1=Scholz |first2=Mark |last2=McCaughrean |url=http://www.eso.org/outreach/press-rel/pr-2003/pr-01-03.html |title=eso0303 – Discovery of Nearest Known Brown Dwarf |date=2003-01-13 |publisher=European Southern Observatory |access-date=2013-03-16 |url-status=dead |archive-url=https://web.archive.org/web/20081013074256/http://www.eso.org/outreach/press-rel/pr-2003/pr-01-03.html |archive-date=October 13, 2008}}</ref>
| T1 + T6
|22<sup>h</sup> 03<sup>m</sup> 21.65363<sup>s</sup> −56° 47{{prime}} 09.5228{{pprime}}
| Indus
| Distance: 3.626pc
|-
|First binary brown dwarf of Y Type
|[[WISE J0336−0143]]
|Y+Y
|03<sup>h</sup> 36<sup>m</sup> 05.052<sup>s</sup> −01° 43{{prime}} 50.48{{pprime}}
|[[Eridanus (constellation)|Eridanus]]
|2023<ref name=":14" />
|-
| First trinary brown dwarf
| [[DENIS-P J020529.0-115925]] A/B/C
| L5, L8 and T0
| 02<sup>h</sup>05<sup>m</sup>29.40<sup>s</sup> −11°59'29.7"
| [[Cetus]]
| Delfosse et al. 1997<ref>{{cite journal |arxiv=astro-ph/0410226 |last1=Burrows |first1=Adam |title=A possible third component in the L dwarf binary system DENIS-P J020529.0-115925 discovered with the Hubble Space Telescope |journal=The Astronomical Journal |volume=129 |issue=1 |pages=511–517 |last2=Hubbard |first2=William B. |last3=Lunine |first3=Jonathan I. |last4=Liebert |first4=James |year=2004 |doi=10.1086/426559 |bibcode=2005AJ....129..511B |s2cid=119336794 }}</ref>
|-
| First halo brown dwarf
| [[2MASS J05325346+8246465]]
| [[Subdwarf|sd]]L7
| 05<sup>h</sup>32<sup>m</sup>53.46<sup>s</sup> +82°46'46.5"
| [[Gemini (constellation)|Gemini]]
| Burgasser et al. 2003<ref>{{cite journal |last1=Burgasser |first1=Adam J. |first2=J. Davy |last2=Kirkpatrick |first3=Adam |last3=Burrows |first4=James |last4=Liebert |first5=I. Neill |last5=Reid |first6=John E. |last6=Gizis |first7=Mark R. |last7=McGovern |first8=Lisa |last8=Prato |first9=Ian S. |last9=McLean |year=2003 |title=The First Substellar Subdwarf? Discovery of a Metal-Poor L Dwarf with Halo Kinematics |journal=The Astrophysical Journal |volume=592 |issue=2 |pages=1186–1192 |doi=10.1086/375813 |arxiv=astro-ph/0304174 |bibcode=2003ApJ...592.1186B |s2cid=11895472 }}</ref>
|-
| First with late-M spectrum
| [[Teide 1]]
| M8
| 3<sup>h</sup>47<sup>m</sup>18.0<sup>s</sup> +24° 22{{prime}} 31{{pprime}}
| Taurus
| 1995
|-
| First with L spectrum
|[[GD 165]]B
|L4
|{{RA|14|24|39.144}} {{DEC|09|17|13.98}}
|[[Boötes]]
|1988
|-
| First with T spectrum
| [[Gliese 229]] B
| T6.5
| 06<sup>h</sup>10<sup>m</sup>34.62<sup>s</sup> −21°51'52.1"
| Lepus
| 1995
|-
| Latest-T spectrum
| [[ULAS J003402.77−005206.7]]
| T9<ref name="Burninghametal2008">{{cite journal |last1=Burningham |first1=Ben |last2=Pinfield |first2=David J. |last3=Leggett |first3=Sandy K. |last4=Tamura |first4=Motohide |last5=Lucas |first5=Philip W. |last6=Homeier |first6=Derek |last7=Day-Jones |first7=Avril |last8=Jones |first8=Hugh R. A. |last9=Clarke |first9=J. R. A. |last10=Ishii |first10=Miki |last11=Kuzuhara |first11=Masayuki |last12=Lodieu |first12=Nicolas |last13=Zapatero-Osorio |first13=María Rosa |last14=Venemans |first14=Bram Pieter |last15=Mortlock |first15=Daniel J. |last16=Barrado y Navascués |first16=David |last17=Martín |first17=Eduardo L. |last18=Magazzù |first18=Antonio |title=Exploring the substellar temperature regime down to ~550K |date=2008 |doi=10.1111/j.1365-2966.2008.13885.x |journal=Monthly Notices of the Royal Astronomical Society |volume=391 |issue=1 |pages=320–333 |doi-access=free |arxiv=0806.0067 |bibcode=2008MNRAS.391..320B |s2cid=1438322 }}</ref>
|
| Cetus
| 2007
|-
| First with Y spectrum
| [[CFBDS J005910.90–011401.3|CFBDS0059]]<ref name="tytrans"/>
| ~Y0
|{{RA|00|59|10.83}} {{DEC|-01|14|01.3}}
|Cetus
| 2008; this is also classified as a T9 dwarf, due to its close resemblance to other T dwarfs.<ref name="Burninghametal2008"/>
|-
| First X-ray-emitting
| [[ChaHα1]]
| M8
|
| Chamaeleon
| 1998
|-
| First X-ray flare
| LP 944–20
| M9V
| 03<sup>h</sup>39<sup>m</sup>35.22<sup>s</sup> −35°25'44.1"
| [[Fornax]]
| 1999
|-
| First radio emission (in flare and quiescence)
| [[LP 944-20]]
| M9V
| 03<sup>h</sup>39<sup>m</sup>35.22<sup>s</sup> −35°25'44.1"
| Fornax
| 2000<ref name="Berger2001"/>
|-
| First potential brown dwarf auroras discovered
| [[LSR J1835+3259]]<!-- Might be an ultracool dwarf star according to Saur et al. 2018. -->
| M8.5
| 
| Lyra
| 2015
|-
| First detection of differential rotation in a brown dwarf
|[[TVLM 513-46546]]
| M9
| 15<sup>h</sup>01<sup>m</sup>08.3<sup>s</sup> +22° 50{{prime}} 02{{pprime}}
| Boötes
| Equator rotates faster than poles by 0.022 radians / day<ref>{{cite journal |arxiv=1404.4682 |last1=Wolszczan |first1=Alexander |last2=Route |first2=Matthew |title=Timing Analysis of the Periodic Radio and Optical Brightness Variations of the Ultracool Dwarf, TVLM 513-46546 |journal=The Astrophysical Journal |volume=788 |issue=1 |pages=23 |year=2014 |doi=10.1088/0004-637X/788/1/23 |bibcode=2014ApJ...788...23W |s2cid=119114679 }}</ref>
|-
| First confirmed brown dwarf to have survived the primary's [[red giant]] phase
| [[WD 0137−349]] B<ref>{{cite journal |title=Survival of a brown dwarf after engulfment by a red giant star |doi=10.1038/nature04987 |last1=Maxted |first1=Pierre F. L. |last2=Napiwotzki |first2=Ralf |last3=Dobbie |first3=Paul D. |last4=Burleigh |first4=Matthew R. |date=2006 |journal=Nature |volume=442 |pages=543–5 |pmid=16885979 |issue=7102 |arxiv=astro-ph/0608054 |bibcode=2006Natur.442..543M |url=https://cds.cern.ch/record/976227 |archive-url=https://web.archive.org/web/20210427120858/https://cds.cern.ch/record/976227 |url-status=dead |archive-date=2021-04-27 |type=Submitted manuscript |hdl=2299/1227 |s2cid=4368344 }}</ref>
| L8
| {{RA|01|39|42.847}} {{DEC|-34|42|39.32}}
| [[Sculptor (constellation)]]
|
|}

=== Table of extremes ===
{{Incomplete list|date=August 2008}}
{| class="wikitable" style="text-align:center;"
|- style="background:#efefef;"
! Record
! Name
! Spectral type
! RA/Dec
! Constellation
! Notes
|-
| Oldest
|
[[LSPM J0055+5948|LSPM J0055+5948 B]]

[[Wolf 1130|Wolf 1130 C]]

[[CWISE J060202.17-462447.8|CWISE J0602-4624]] 
|
T8

sdT8

L8
|
{{RA|00|55|58.300}} {{Dec|+59|48|02.53}} or

{{RA|20|05|02.1951}} {{Dec|+54|26|03.234}} or

{{RA|06|02|02.17}} {{Dec|-46|24|47.8}}
|[[Cassiopeia (constellation)|Cassiopeia]], [[Cygnus (constellation)|Cygnus]] or [[Pictor]]
|three of the few examples with a good age estimate:

LSPM J0055B: 10±3 [[billion years]]<ref name=":12" /><ref name=":13" />

Wolf 1130C: >10 billion years<ref>{{cite journal |last1=Mace |first1=Gregory N. |last2=Mann |first2=Andrew W. |last3=Skiff |first3=Brian A. |last4=Sneden |first4=Christopher |last5=Kirkpatrick |first5=J. Davy |last6=Schneider |first6=Adam C. |last7=Kidder |first7=Benjamin |last8=Gosnell |first8=Natalie M. |last9=Kim |first9=Hwihyun |last10=Mulligan |first10=Brian W. |last11=Prato |first11=L. |last12=Jaffe |first12=Daniel |date=2018-02-01 |title=Wolf 1130: A Nearby Triple System Containing a Cool, Ultramassive White Dwarf |journal=The Astrophysical Journal |volume=854 |issue=2 |pages=145 |doi=10.3847/1538-4357/aaa8dd |arxiv=1802.04803 |bibcode=2018ApJ...854..145M |s2cid=56431008 |issn=0004-637X |doi-access=free }}</ref>

CWISE J0602-4624: {{Val|10.9|2.6|2.0}} billion years<ref>{{cite journal |last1=Rothermich |first1=Austin |last2=Faherty |first2=Jacqueline K. |author-link2=Jackie Faherty |last3=Bardalez-Gagliuffi |first3=Daniella |last4=Schneider |first4=Adam C. |last5=Kirkpatrick |first5=J. Davy |author-link5=J. Davy Kirkpatrick |last6=Meisner |first6=Aaron M. |last7=Burgasser |first7=Adam J. |last8=Kuchner |first8=Marc |author-link8=Marc Kuchner |last9=Allers |first9=Katelyn |last10=Gagné |first10=Jonathan |last11=Caselden |first11=Dan |last12=Calamari |first12=Emily |last13=Popinchalk |first13=Mark |last14=Gerasimov |first14=Roman |last15=Aganze |first15=Christian |date=7 Mar 2024 |title=89 New Ultracool Dwarf Co-Moving Companions Identified With The Backyard Worlds: Planet 9 Citizen Science Project |journal=AJ |arxiv=2403.04592 |last16=Softich |first16=Emma |last17=Hsu |first17=Chin-Chun |last18=Karpoor |first18=Preethi |last19=Theissen |first19=Christopher A. |last20=Rees |first20=Jon |last21=Cecilio-Flores-Elie |first21=Rosario |last22=Cushing |first22=Michael C. |last23=Marocco |first23=Federico |last24=Casewell |first24=Sarah |last25=Hamlet |first25=Les |last26=Allen |first26=Michaela B. |last27=Beaulieu |first27=Paul |first28=Guillaume |last28=Colin |first29=Jean Marc |last29=Gantier |first30=Leopold |last30=Gramaize |first31=Peter |last31=Jałowiczor |first32=Martin |last32=Kabatnik |first33=Frank |last33=Kiwi |first34=David W. |last34=Martin |first35=Billy |last35=Pendrill |first36=Ben |last36=Pumphrey |first37=Arttu |last37=Sainio |first38=Jörg |last38=Schümann |first39=Nikolaj |last39=Stevnbak |first40=Guoyou |last40=Sun |first41=Christopher |last41=Tanner |first42=Vinod |last42=Thakur |first43=Melina |last43=Thévenot |first44=Zbigniew |last44=Wedracki|volume=167 |issue=6 |page=253 |doi=10.3847/1538-3881/ad324e |doi-access=free |bibcode=2024AJ....167..253R }}</ref>
|-
| Youngest
|2MASS J05413280-0151272
|M8.5
|{{RA|05|41|32.801}} {{DEC|-01|51|27.20}}
|[[Orion (constellation)|Orion]]
|One brown dwarf member of the about 0.5 Myr-old [[Flame Nebula]]. {{Jupiter mass|20.9}} object<ref>{{cite journal |last1=Levine |first1=Joanna L. |last2=Steinhauer |first2=Aaron |last3=Elston |first3=Richard J. |last4=Lada |first4=Elizabeth A. |date=2006-08-01 |title=Low-Mass Stars and Brown Dwarfs in NGC 2024: Constraints on the Substellar Mass Function |url=https://ui.adsabs.harvard.edu/abs/2006ApJ...646.1215L |journal=The Astrophysical Journal |volume=646 |issue=2 |pages=1215–1229 |doi=10.1086/504964 |arxiv=astro-ph/0604315 |bibcode=2006ApJ...646.1215L |s2cid=118955538 |issn=0004-637X |quote=Table 3: FLMN_J0541328-0151271}}</ref> 
|-
| {{anchor|SDSS J0104+1535}}Most massive
| SDSS J010448.46+153501.8<ref name="zhang2017">{{cite journal |title=Primeval very low-mass stars and brown dwarfs – II. The most metal-poor substellar object |journal=Monthly Notices of the Royal Astronomical Society |volume=468 |issue=1 |pages=261 |date=2017-06-11 |arxiv=1702.02001 |last1=Zhang |first1=ZengHua |last2=Homeier |first2=Derek |last3=Pinfield |first3=David J. |last4=Lodieu |first4=Nicolas |last5=Jones |first5=Hugh R. A. |last6=Pavlenko |first6=Yakiv V. |doi=10.1093/mnras/stx350|doi-access=free |bibcode = 2017MNRAS.468..261Z |s2cid=54847595 }}</ref>
| [[Subdwarf|usd]]L1.5
| 01<sup>h</sup>04<sup>m</sup>48.46<sup>s</sup> +15°35'01.8"
| [[Pisces (constellation)|Pisces]]
| distance is ~180–290&nbsp;pc, mass is ~{{Jupiter mass|88.5–91.7}}. Transitional brown dwarfs.
|-
| Metal-rich
|
|
|
|
|
|-
| Metal-poor
| SDSS J010448.46+153501.8<ref name="zhang2017"/>
| [[Subdwarf|usd]]L1.5
| 01<sup>h</sup>04<sup>m</sup>48.46<sup>s</sup> +15°35'01.8"
| [[Pisces (constellation)|Pisces]]
| distance is ~180–290&nbsp;pc, metallicity is ~0.004&nbsp;[[Metallicity|Z]]<sub>[[Sun|Sol]]</sub>. Transitional brown dwarfs.
|-
| Least massive
|
|
|
|
|
|-
| Largest
|[[2MASS J05352184−0546085|2MASS J0535 A]]
|M6.5
|{{RA|15|35|22}} {{Dec|-05|46|09}}
|[[Orion (constellation)|Orion]]
|Radius is {{val|6.71|ul=Jupiter radius}}<ref>{{cite journal |last1=Gómez Maqueo Chew |first1=Yilen |last2=Stassun |first2=Keivan G. |last3=Prša |first3=Andrej |last4=Mathieu |first4=Robert D. |date=2009-07-10 |title=Near-Infrared Light Curves of the Brown Dwarf Eclipsing Binary 2Mass J05352184-0546085: Can Spots Explain the Temperature Reversal? |url=https://iopscience.iop.org/article/10.1088/0004-637X/699/2/1196 |journal=The Astrophysical Journal |volume=699 |issue=2 |pages=1196–1208 |arxiv=0905.0491 |bibcode=2009ApJ...699.1196G |doi=10.1088/0004-637X/699/2/1196 |issn=0004-637X}}</ref>
|
|-
| Smallest
|[[ZTF J1406+1422|ZTF J1406+1222 B]]
|
|{{RA|14|06|56}} {{Dec|-12|22|43}}
|[[Boötes]]<ref>{{Cite web |title=Find the constellation which contains given sky coordinates |url=https://djm.cc/constellation.html |access-date=2025-01-16 |website=djm.cc}}</ref>
|Radius is {{val|0.029|ul=Solar radius}} (~20,200&nbsp;km)<ref name="ZTF J1406+1422">{{cite journal |last1=Burdge |first1=Kevin B. |last2=Marsh |first2=Thomas R. |last3=Fuller |first3=Jim |last4=Bellm |first4=Eric C. |last5=Caiazzo |first5=Ilaria |last6=Chakrabarty |first6=Deepto |last7=Coughlin |first7=Michael W. |last8=De |first8=Kishalay |last9=Dhillon |first9=V. S. |last10=Graham |first10=Matthew J. |last11=Rodríguez-Gil |first11=Pablo |last12=Jaodand |first12=Amruta D. |last13=Kaplan |first13=David L. |last14=Kara |first14=Erin |last15=Kong |first15=Albert K. H. |date=May 2022 |title=A 62-minute orbital period black widow binary in a wide hierarchical triple |url=https://www.nature.com/articles/s41586-022-04551-1 |journal=Nature |volume=605 |issue=7908 |pages=41–45 |arxiv=2205.02278 |doi=10.1038/s41586-022-04551-1 |pmid=35508781 |bibcode=2022Natur.605...41B |issn=1476-4687}}</ref>
|-
| Fastest rotating
| [[2MASS J03480772−6022270]]
| T7
| 03<sup>h</sup>48<sup>m</sup>07.72<sup>s</sup> –60°22'27.1"
| [[Reticulum]]
| Rotational period of {{val|1.080|0.004|0.005}} hours<ref name="Tannock2021">{{cite journal |title=Weather on Other Worlds. V. The Three Most Rapidly Rotating Ultra-cool Dwarfs |first1=Megan E. |last1=Tannock |first2=Stanimir |last2=Metchev |first3=Aren |last3=Heinze |first4=Paulo A. |last4=Miles-Páez |first5=Jonathan |last5=Gagné |first6=Adam J. |last6=Burgasser |first7=Mark S. |last7=Marley |first8=Dániel |last8=Apai |first9=Genaro |last9=Suárez |first10=Peter |last10=Plavchan|journal=The Astronomical Journal |date=March 2021 |volume=161 |issue=5 |page=224 |doi=10.3847/1538-3881/abeb67 |arxiv=2103.01990 |bibcode=2021AJ....161..224T |s2cid=232105126 |doi-access=free }}</ref>
|-
| Farthest
|Candidate brown dwarfs in the [[Small Magellanic Cloud]]
|
|{{RA|01|29|32}} {{DEC|–73|33|38}}{{Efn|Location of the open cluster [[NGC 602]] where these brown dwarfs were found.}}
|[[Hydrus]]
|Distance: {{Convert|60900|pc|ly|disp=number}} light-years<ref>{{cite journal | doi=10.3847/1538-4357/ad779e | doi-access=free | title=Discovering Subsolar Metallicity Brown Dwarf Candidates in the Small Magellanic Cloud | date=2024 | last1=Zeidler | first1=Peter | last2=Sabbi | first2=Elena | last3=Nota | first3=Antonella | last4=Manjavacas | first4=Elena | last5=Jones | first5=Olivia C. | last6=Pacifici | first6=Camilla | journal=The Astrophysical Journal | volume=975 | issue=1 | page=18 | bibcode=2024ApJ...975...18Z }}</ref>
|-
| Nearest
| [[Luhman 16]] AB
| L7.5 + T0.5 ± 1
|{{RA|10|49|18.723}} {{DEC|−53|19|09.86}}
| [[Vela (constellation)|Vela]]
| Distance: ~6.5 ly
|-

| Brightest
|[[LP 944-20]]
|opt: M9beta,
IR: L0:
|{{RA|03|39|35.220}} {{Dec|-35|25|44.09}}
|[[Fornax]]
|According to the ultracool fundamental properties<ref>{{cite journal |last1=Sanghi |first1=Aniket |last2=Liu |first2=Michael C. |last3=Best |first3=William M. J. |last4=Dupuy |first4=Trent J. |last5=Siverd |first5=Robert J. |last6=Zhang |first6=Zhoujian |last7=Hurt |first7=Spencer A. |last8=Magnier |first8=Eugene A. |last9=Aller |first9=Kimberly M. |last10=Deacon|first10=Niall R. |title=Table of Ultracool Fundamental Properties |url=https://zenodo.org/records/10086810 |website=Zenodo|date=2023 |doi=10.5281/zenodo.10086810 }}</ref> this object shows signs of youth and could therefore be a brown dwarf with 19.85±13.02 {{Jupiter mass}} and J<sub>MKO</sub>=10.68±0.03 mag
|-
| Dimmest
|[[WD 0806-661 B|L 97-3B]]
| Y1
|{{RA|08|06|53.736}} {{DEC|-66|18|16.74}}
|[[Volans]]
| jmag=25.42, planetary-mass object
|-
| Hottest
| [[ZTF J1406+1222|ZTF J1406+1222 B]]
|
|{{RA|14|06|56}} {{Dec|-12|22|43}}
|[[Boötes]]
|Temperature: {{cvt|10462|K|C F}} in dayside<ref name="ZTF J1406+1422"/>
|-
| Coolest
| [[WISE 0855−0714]]<ref name="NASA20140425"/>
| Y4
|{{RA|08|55|10.83}} {{DEC|-07|14|42.5}}
|[[Hydra (constellation)|Hydra]]
| Temperature: {{cvt|−48 to −13|C|K F}}
|-
| Coolest radio-flaring
| [[WISEPA J062309.94-045624.6|WISE J062309.94-045624.6]]
| T8 
| 06<sup>h</sup>23<sup>m</sup>09.28<sup>s</sup> −04°56'22.8"
| [[Monoceros (constellation)|Monoceros]]
| {{cvt|699|K|C F}} brown dwarf with 4.17 mJy bursts<ref>{{cite journal |arxiv=2306.15219 |last1=Rose |first1=Kovi |last2=Pritchard |first2=Joshua |last3=Murphy |first3=Tara |last4=Caleb |first4=Manisha |author4-link=Manisha Caleb|last5=Dobie |first5=Dougal |last6=Driessen |first6=Laura  |last7=Duchesne |first7=Stefan |last8=Kaplan |first8=David |last9=Lenc |first9=Emil |last10=Wang |first10=Ziteng      |title= Periodic Radio Emission from the T8 Dwarf WISE J062309.94−045624.6 |journal=The Astrophysical Journal Letters|year=2023 |volume=951 |issue=2 |pages=L43 |doi=10.3847/2041-8213/ace188 |bibcode=2023ApJ...951L..43R |s2cid=259262475 |doi-access=free }}</ref>
|-
| Most dense
| [[TOI-569b]]<ref>{{cite web |url=https://aasnova.org/2020/06/24/transiting-brown-dwarfs-from-tess/ |access-date=2013-03-16 |title=Transiting Brown Dwarfs from TESS 2 |author=[[Astrobites]] |date=24 June 2020 |publisher=AAS Nova }}</ref>
|
|07<sup>h</sup> 40<sup>m</sup> 24.658<sup>s</sup> −42° 09{{prime}} 16.74{{pprime}}
|[[Puppis]]
| [[List of transiting extrasolar planets|Transiting]], has {{jupiter mass|link=yes|64.1}} with a diameter 0.79 ± 0.02 times that of Jupiter. Density is 171.3g/cm<sup>3</sup>.
|-
| Least dense
|
|
|
|
|
|}

== See also ==
* [[Fusor (astronomy)]]
* {{annotated link|Brown-dwarf desert}}
* {{annotated link|Blue dwarf (red-dwarf stage)}}
* {{annotated link|Dark matter}}
* {{annotated link|Exoplanet}}
* [[Stellification]]
* [[WD 0032−317#WD 0032−317 b|WD 0032-317 b]]
* [[List of brown dwarfs]]
* [[List of Y-dwarfs]]

== Footnotes ==
{{notelist}}

== References ==
{{reflist|25em|refs=

<ref name="DwarfArchives">
{{cite web
 |first1=J. Davy |last1=Kirkpatrick |author1-link=J. Davy Kirkpatrick
 |first2=Adam J. |last2=Burgasser
 |date=6 November 2012
 |title=Photometry, spectroscopy, and astrometry of M, L, and T dwarfs
 |website=DwarfArchives.org
 |publisher=California Institute of Technology
 |place=Pasadena, CA
 |url=http://spider.ipac.caltech.edu/staff/davy/ARCHIVE/index.shtml
 |access-date=2012-12-28
}} (M=536, L=918, T=355, Y=14)
</ref>

<ref name=Forbes_Loeb_2019>{{citation
 | title=On the Existence of Brown Dwarfs More Massive than the Hydrogen Burning Limit
 | last1=Forbes | first1=John C. | last2=Loeb | first2=Abraham
 | journal=The Astrophysical Journal
 | volume=871 | issue=2 | id=227 | pages=11
 | date=February 2019 | doi=10.3847/1538-4357/aafac8
 | arxiv=1805.12143 | bibcode=2019ApJ...871..227F | s2cid=119059288 | doi-access=free }}</ref>

<ref name="Stassun2006">{{cite journal|last1=Stassun|first1=Keivan G.|last2=Mathieu|first2=Robert D.|last3=Valenti|first3=Jeff A.|date=March 2006|title=Discovery of two young brown dwarfs in an eclipsing binary system|journal=Nature|volume=440|issue=7082|pages=311–314|doi=10.1038/nature04570|pmid=16541067|bibcode=2006Natur.440..311S|s2cid=4310407|issn=0028-0836}}</ref>

}}

== External links ==
{{Wiktionary}}
{{commons}}
* [http://hubblesite.org/newscenter/archive/releases/2013/02/image/a/ HubbleSite newscenter – Weather patterns on a brown dwarf]
* {{cite journal |last1=Allard |first1=France |last2=Homeier |first2=Derek |year=2007 |title=Brown dwarfs |journal=[[Scholarpedia]] |volume=2 |issue=12 |page=4475 |doi=10.4249/scholarpedia.4475 |bibcode=2007SchpJ...2.4475A |doi-access=free }}

=== History ===
* Kumar, Shiv S.; ''Low-Luminosity Stars''. Gordon and Breach, London, 1969—an early overview paper on brown dwarfs
* [https://web.archive.org/web/20050114164835/http://www.bartleby.com/65/br/browndwa.html The Columbia Encyclopedia: "Brown Dwarfs"]

=== Details ===
* [http://www.dwarfarchives.org/ A current list of L and T dwarfs]
* [https://web.archive.org/web/20050224110208/http://astron.berkeley.edu/~stars/bdwarfs/structbd.html A geological definition of brown dwarfs], contrasted with stars and planets (via Berkeley)
* I. Neill Reid's pages at the [[Space Telescope Science Institute]]:
** [https://archive.today/20121215095626/http://www-int.stsci.edu/~inr/ldwarf1.html On spectral analysis] of [[M dwarf]]s, [[L dwarf]]s, and [[T dwarf]]s
** [https://archive.today/20130521055905/http://www-int.stsci.edu/~inr/ldwarf3.html Temperature and mass characteristics] of low-temperature dwarfs
* [https://archive.today/20121206015750/http://www.spaceref.com/news/viewpr.html?pid=2192 First X-ray from brown dwarf observed], Spaceref.com, 2000
* Montes, David; [http://www.ucm.es/info/Astrof/recopilaciones/enanasmarrones.html "Brown Dwarfs and ultracool dwarfs (late-M, L, T)"], UCM
* [http://www.space.com/scienceastronomy/060703_mystery_monday.html Wild Weather: Iron Rain on Failed Stars]—scientists are investigating astonishing weather patterns on brown dwarfs, Space.com, 2006
* [http://www.nasa.gov/vision/universe/starsgalaxies/brown_dwarf_detectives.html NASA Brown dwarf detectives] {{Webarchive|url=https://web.archive.org/web/20141017075014/http://www.nasa.gov/vision/universe/starsgalaxies/brown_dwarf_detectives.html |date=2014-10-17 }}—Detailed information in a simplified sense
* [http://www.darkstar1.co.uk/ds3.htm Brown Dwarfs]—Website with general information about brown dwarfs (has many detailed and colorful artist's impressions)

=== Stars ===
* [https://web.archive.org/web/20051201132115/http://ssc.spitzer.caltech.edu/documents/compendium/cha_halpha1/ Cha Halpha 1] stats and history
* [https://web.archive.org/web/20050212200244/http://astron.berkeley.edu/~stars/bdwarfs/observbd.html "A census of observed brown dwarfs"] (not all confirmed), 1998
* {{cite journal |last1=Luhman |first1=Kevin L. |last2=Adame |first2=Lucía |last3=d'Alessio |first3=Paola |last4=Calvet |first4=Nuria |last5=Hartmann |first5=Lee |last6=Megeath |first6=S. Thomas |last7=Fazio |first7=Giovanni G. |year=2005 |title=Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk |journal=The Astrophysical Journal |volume=635 |issue=1 |pages=L93–L96 |arxiv=astro-ph/0511807 |bibcode=2005ApJ...635L..93L |doi=10.1086/498868 |s2cid=11685964 }}
* [https://web.archive.org/web/20070609130324/http://www.gemini.edu/index.php?option=content&task=view&id=232 Michaud, Peter; Heyer, Inge; Leggett, Sandy K.; and Adamson, Andy; "Discovery Narrows the Gap Between Planets and Brown Dwarfs", Gemini and Joint Astronomy Centre, 2007]
* {{cite journal |arxiv=astro-ph/0607305 |doi=10.1111/j.1365-2966.2006.10795.x |doi-access=free |title=The possiblity of detection of ultracool dwarfs with the UKIRT Infrared Deep Sky Survey |date=2006 |last1=Deacon |first1=N. R. |last2=Hambly |first2=N. C. |journal=Monthly Notices of the Royal Astronomical Society |volume=371 |issue=4 |pages=1722–1730 |bibcode=2006MNRAS.371.1722D }}

{{Star}}
{{Exoplanet}}
{{Portal bar|Astronomy|Stars|Outer space}}
{{Authority control}}

{{DEFAULTSORT:Brown Dwarf}}
[[Category:Brown dwarfs| ]]
[[Category:Definition of planet]]
[[Category:Star types]]
[[Category:Stellar phenomena]]
[[Category:Substellar objects]]
[[Category:Types of planet]]