Located 19 light-years from the sun in the constellation of Lepus, the brown dwarf Gliese 229B orbits the red dwarf Gliese 229. Gliese 229B is 100,000 times dimmer than the Sun.
Credit: STScI, T. Nakajima, S. Kulkarni, S. Durrance, D. Golimowski, NASA.

Brown dwarfs are sub-stellar objects that have masses between those of stars and planets, generally between 10 and 90 time sthe mass of Jupiter. They do not have enough mass to produce energy by nuclear fusion. Rather, the small amount of energy emitted by these objects comes almost exclusively from the heat stored in them during the collapse of the parent gas cloud from which they formed. Brown dwarfs therefore gradually cool and fade with cosmological time.

Brown dwarfs are sometimes referred to as ‘failed stars’ since they are more massive than planets but have insufficient mass to sustain nuclear fusion in their cores. According to current theories, the mass required to sustain nuclear fusion is about 1/12^th of a solar mass (or about 90 times the mass of Jupiter). This therefore sets the upper mass limit for brown dwarfs. The lower limit for classification as a brown dwarf is somewhat more arbitrary, but generally a mass greater than 1/80^th of a solar mass is required for an object to be classified as a brown dwarf and not a planet.

The core temperatures of brown dwarfs must be below about 3 million degrees, as at this temperature fusion becomes sustainable. Surface temperatures are 1,000 degrees Kelvin or less. In 2011, the NASA Wide-field Infrared Survey Explorer (WISE) discovered six extremely cool brown dwarfs known as Y dwarfs, which have temperatures as low as 300 Kelvin, which is the temperature of the human body! At such cool temperatures the atmospheres of brown dwarfs contain many molecules, including methane and even water.

Because of their low temperatures and small sizes, brown dwarfs have extremely low luminosities (about 1/100,000^th of the solar luminosity). This makes them extremely difficult to observe. Indeed, even with modern telescopes it is almost impossible to observe brown dwarfs more distant than two or three hundred light years. Consequently, it was only in 1995 that the first confirmed brown dwarf was observed.

Due to their extremely low luminosities, brown dwarfs were one of the proposed candidates for baryonic dark matter. However, currently observations suggest that there are insufficient numbers of brown dwarfs to account for a significant fraction of the missing mass of the Milky Way.