Brown dwarf

A brown dwarf is an object too massive to be a planet and too light to be a star. Brown dwarfs form like stars, from collapsing clouds of gas, but with less than about 75 times Jupiter's mass (0.07 solar masses) their cores never become hot enough to sustain hydrogen fusion ^{[1] [2]}. They shine instead with the heat of their formation, and a brief flicker of deuterium burning if they have more than about 13 Jupiter masses ^[3], then cool for ever, dimming from the dull red of an L dwarf through the methane-laden T class to the Y dwarfs, the coolest about as warm as a cool day on Earth. The first were confirmed in 1995 ^{[4] [5]}. The latest census finds about one for every four stars ^[6].

Characteristics

Between a star and a planet

The line between the lightest stars and the heaviest brown dwarfs is set by fusion. An object must reach about 0.072 solar masses (about 75 Jupiter masses) at solar composition to fuse hydrogen steadily, and more, up to about 0.083 solar masses, if it is poor in heavy elements ^[1]; a newer equation of state for hydrogen and helium gives 0.075 solar masses, about 78 Jupiter masses ^[2]. Observations place the boundary near spectral type L2.5 and a temperature of about 2,075 K, where the radii of the lowest-mass stars reach a minimum ^[7].

The lower edge, where brown dwarfs meet planets, is set by deuterium, the heavy form of hydrogen, which fuses at lower temperatures. Objects above about 13 Jupiter masses burn most of their deuterium early in life; the exact threshold runs from about 11 to 16 Jupiter masses depending on composition and on how much deuterium must burn to count ^[3]. The International Astronomical Union uses 13 Jupiter masses as the working upper limit for planets, a convention rather than a sharp physical line.

Lithium provides a test. Lithium is destroyed at slightly lower temperatures than hydrogen fuses, so a cool object that still shows lithium in its spectrum must be below about 0.06 solar masses and is a brown dwarf ^[1]. The test, proposed by Rafael Rebolo and colleagues in 1992 ^[8], confirmed Teide 1 in the Pleiades as a brown dwarf ^[9].

Jupiter-sized, whatever the mass

Brown dwarfs are held up mostly by electron degeneracy pressure, the resistance of electrons packed close together, and the result is that their radius barely depends on mass: across roughly two orders of magnitude in mass, from giant planets to the heaviest brown dwarfs, old objects all have about Jupiter's radius ^[10]. A 70 Jupiter-mass brown dwarf is about the size of Jupiter but some 70 times as dense, with a surface gravity 70 to 100 times Jupiter's.

Cooling for ever

With no steady energy source, a brown dwarf cools and fades throughout its life. For older objects the effective temperature follows approximately ^[10]

$$T_\mathrm{eff} \approx 1550\ \mathrm{K} \left(\frac{1\ \mathrm{Gyr}}{t}\right)^{0.32} \left(\frac{M}{0.05\ M_\odot}\right)^{0.83}$$

where $t$ is the age and $M$ the mass. A brown dwarf of 50 Jupiter masses (0.048 solar masses) is about 1,500 K at one billion years old, an L dwarf, but about 900 K at five billion years, a T dwarf. One of 13 Jupiter masses has already cooled to about 490 K at one billion years. So a brown dwarf's spectral type is not a fixed property like a star's: it records a combination of mass and age, and every brown dwarf moves down through the classes as it ages.

The L, T and Y classes

Brown dwarfs and the coolest stars needed spectral classes beyond M.

Temperatures are approximate and vary with the author and with each object's gravity and composition ^{[15] [16]}. The first L and T dwarfs came from infrared sky surveys in the 1990s, and by 2005 more than 450 were known ^[13]. The Y class was established when the WISE satellite found objects with tentative signs of ammonia in their spectra and temperatures of perhaps 300 to 500 K ^[14].

Clouds and the L/T transition

In L dwarfs, grains of silicate rock and iron condense into thick clouds. Models in which these clouds form and rain out, with the rate of settling as the key parameter, explain how L dwarfs grow redder and dustier ^[17]. Then, over a narrow range of temperature near 1,200 to 1,350 K, brown dwarfs turn rapidly from red to blue in the near-infrared as the clouds break up and sink below the visible atmosphere. Brown dwarfs appear to linger in this L/T transition: the 20-parsec census shows a pile-up of objects at these temperatures, as if cooling slows there ^[15]. In the coldest Y dwarfs water itself should condense, forming water-ice clouds below about 450 K that become thick below about 350 to 375 K ^[18].

Weather

Brown dwarfs rotate in hours and have weather. Their brightness in the infrared varies as cloudy and clearer patches rotate into view: about 80% of L dwarfs vary by more than 0.2% and about 36% of T dwarfs by more than 0.4% ^[19], and the largest variations, above 2%, are found at the L/T transition, where clouds are breaking up ^[20]. By following how the spectrum of Luhman 16B changed as it rotated, astronomers made the first map of the clouds on a brown dwarf ^[21]. JWST found methane glowing in emission from the isolated cold brown dwarf W1935, which needs a warm upper atmosphere; an aurora is a plausible explanation, which is surprising in an object with no star nearby to power it ^[22].

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Discovery

The first brown dwarfs were confirmed in 1995. Teide 1, in the Pleiades cluster, was announced by Rafael Rebolo's team ^[4] and confirmed by the lithium test ^[9]; Gliese 229B, a companion to a nearby red dwarf, was found the same year ^[5], and its spectrum showed methane, like Jupiter's and unlike any star's, the signature of a surface cooler than about 1,000 K ^[12]. Gliese 229B later posed a puzzle: its orbit gave it about 70 Jupiter masses ^[23], far too much for its faintness. In 2024 it was resolved into two brown dwarfs, Gliese 229 Ba and Bb, of 38.1 and 34.4 Jupiter masses, orbiting each other every 12.1 days at 0.042 AU, sixteen times the distance from the Earth to the Moon ^{[24] [25]}.

How many are there?

A census of the 20 parsecs around the Sun counts 525 L, T and Y dwarfs ^[15], and a full census of about 3,600 stars and brown dwarfs in the same volume gives, after correcting for those not yet found, one brown dwarf for every four stars ^[6]. The coldest are still being discovered, and objects below 400 K must exist in large numbers ^[15]. Brown dwarfs are rare, however, as close companions to Sun-like stars: fewer than 1% of such stars have a brown dwarf on an orbit shorter than about five years, against about 5% with a giant planet and 11% with a stellar companion, a gap called the brown dwarf desert ^[26].

At the other end of the mass range, JWST observations of the young cluster IC 348 found brown dwarfs of only about two Jupiter masses, some showing a 3.4-micrometre absorption band from aliphatic hydrocarbons, for which the authors proposed a new spectral class, H ^[27].

Notable examples

WISE 0855-0714 is the coldest known brown dwarf. It was found in 2014 at about 2 parsecs, moving across the sky at 8 arcseconds a year ^[33]. JWST spectra fit a temperature of about 285 K, with carbon monoxide but no phosphine ^[30], and mid-infrared spectra show water depleted as if condensing, though no clear sign of water-ice clouds has yet been found ^[31].

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In Pax Abyssi

Brown dwarfs are three stellar classes in the simulation, L, T and Y (codes BDL, BDT and BDY), each divided into early, mid and late subtypes and into cloud states: thick, patchy or thin clouds for L and Y dwarfs, and banded, patchy or uniform for T dwarfs. Each class has a physics engine that sets temperature, mass and radius from the subclass, and the galaxy map gives L, T and Y dwarfs classes of their own. No brown dwarf is bright enough to see without a large telescope, so none is in the naked-eye sky that the game's flyable stars are drawn from; they arrive with the generated galaxy, whose Milky Way model holds about 22 billion of them, one for every five stars.

See also

• Stellar classification
• Red dwarf
• Gas giant
• Hot Jupiter
• Planet classification
• White dwarf

References

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