DISCOVERY OF A COOL BROWN DWARF

BROWN dwarfs are star-like objects with masses less than 0.08 times th at of the Sun, which are unable to sustain hydrogen fusion in their in teriors(1-4). They are very hard to detect, as most of the energy of g ravitational contraction is radiated away within similar to 10(8) yr, leaving only a very low residual luminosity. Accordingly, almost all s earches for brown dwarfs have been directed towards clusters of young stars-a strategy that has recently proved successful(5,6). But there a re only modest observable differences between young brown dwarfs and v ery low-mass stars, making it difficult to identify the former without appealing to sophisticated models(7). Older brown dwarfs should have a more distinctive appearance, and if they are companions to nearby st ars, their luminosity can be determined unambiguously. Here we report the discovery of a probable companion to the nearby star GI229, with n o more than one-tenth the luminosity of the least luminous hydrogen-bu rning star. We conclude that the companion, GI229B, is a brown dwarf w ith a temperature of less than 1,200 K, acid a mass similar to 20-50 t imes that of Jupiter.