Discovery of a cool brown dwarf

Brown dwarfs are star-like objects with masses less than 0.08 times that of the Sun, which are unable to sustain hydrogen fusion in their interiors. They are very hard to detect, as most of the energy of gravitational contraction is radiated away within ~10^8 yr, leaving only a very low residual luminosity. Accordingly, almost all searches for brown dwarfs have been directed towards clusters of young stars—a strategy that has recently proved successful. But there are only modest observable differences between young brown dwarfs and very low-mass stars, making it difficult to identify the former without appealing to sophisticated models. Older brown dwarfs should have a more distinctive appearance, and if they are companions to nearby stars, their luminosity can be determined unambiguously. Here we report the discovery of a probable companion to the nearby star G1229, with no more than one-tenth the luminosity of the least luminous hydrogen-burning star. We conclude that the companion, G1229B, is a brown dwarf with a temperature of less than 1,200 K, and a mass ~20–50 times that of Jupiter