A failed star which is not massive enough to ignite thermonuclear fusion in the core. According to stellar models, the maximum mass a brown dwarf can have is . Young brown dwarfs which are gravitationally contracting can release substantial amounts of gravitational energy, but older stars radiate from remnant internal heat only. For old brown dwarfs, the luminosity is smaller than smallest luminosity possible for a hydrogen-fusing star, . The minimum temperature for a star in order to undergo nuclear fusion is K, so lower temperatures are characteristic of brown dwarfs (or other low temperature objects such as planets). Brown dwarfs are intermediate in size between planets and stars, and may form either by condensation of interstellar gas (as stars do) or from accretion of material in a protoplanetary disk (as planets are thought to).

Magnitude 18.8 Teide 1 in the Pleiades appears to have a mass and a spectral type M9 (Rebolo et al. 1995). A second candidate was identified by methane absorption lines in Gliese 229B, a faint object orbiting the M1 star Gliese 229 in the constellation Lepus, 18.6 light years away. It appears to have a mass of , and to be 44 AU away from Gliese 229 (Glanz 1995, Oppenheimer et al. 1995). The methane lines indicate the surface temperature must be below 1000 K, because below this temperature, carbon is more stable as CO than methane. Gliese 229B was subsequently imaged both from Palomar and with the HST (Sky & Telescope Feb. 1996). H. Jones found lithium in object 296A (located 100 ly away), and identified it as a probable brown dwarf with mass of , surface temperature of 2800 K, and luminosity . H. Jones and M. R. S. Hawkins identified four additional brown dwarf candidates by staking U. K. Schmidt telescope photographic plates (Sci. News 1996).

Planet