Brown dwarfs: a possible missing link between stars and planets

Stars are large spheres of gas that are by mass 73% H, 25% He, and 2% 'metals' (elements with atomic number z [is greater than] 2, such as O, N, [...]
Brown dwarfs are objects with masses between that of stars and planets. Postulated some three decades ago, they remained elusive until recently. Unlike stars, these objects have no sustained energy source and cool as they age. One such cool brown dwarf has been discovered as a companion to a nearby star. The spectrum of this object is similar to that of Jupiter. Single brown dwarfs have been detected in stellar nurseries. Ongoing and planned efforts, from both ground and space, will enable astronomers to decisively determine the population of these elusive and dim objects.