astronomical object that is intermediate between a planet and a star. Brown dwarfs have a mass less than 0.08 solar mass, and their surface temperatures are below 2,500 K (the lower limit of the temperature of red dwarfs). They are not brown in colour but red. Brown dwarfs were first hypothesized in 1963 by American astronomer Shiv Kumar, who called these objects “black” dwarfs. American astronomer Jill Tarter proposed the name “brown dwarf” in 1975. Searches for brown dwarfs in the 1980s and 1990s found several candidates; however, none could be definitively identified as a brown dwarf. In 1995 astronomers at Palomar Observatory and Johns Hopkins University found Gliese 229 B. Its extremely low luminosity showed it had a mass less than 0.05 solar mass, and the detection of methane in its spectrum showed that it had a surface temperature less than 1,200 K. Hence, Gliese 229 B was the first object that was definitively identified as a brown dwarf.

Sometimes described as failed stars, brown dwarfs are believed to form in the same way that stars form; fragments of an interstellar cloud contract into smaller, denser clouds. Unlike stars, however, brown dwarfs do not have mass enough to generate the internal heat that in stars ignites hydrogen and establishes thermonuclear fusion reactions, which are the source of stellar energies. Though they generate some heat and some light, brown dwarfs also cool rapidly and shrink. Similar in appearance to high-mass planets, brown dwarfs may be distinguishable from planets only in their formation mechanism.