Editing Brown dwarf (section)

== 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.