Editing Brown dwarf (section)

== 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's concept; 9 April 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|date=9 April 2020 |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. |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>