The Search for Brown Dwarfs

We know a lot about stars and planets. We know about stars because they are easy to detect and because we can observe them in the various stages of their life cycles. We know about planets because we live on a planet, because there are nearby planets that can be visited by spacecraft and observed in detail, and because the known planets and other solar system materials preserve some evidence of their history. However, the planets and stars we know about are separated in mass by about two orders of magnitude or more, and the history of astronomy suggests that we should adopt an Ecological Principle: All imaginable physical niches are probably occupied. The objects of intermediate mass, known by various names but here called brown dwarfs (Tarter 1975, 1986) fill this niche. The evidence for their existence is currently rather weak and confusing (Fienberg 1990): New discoveries seem scarcely more frequent than retractions or reinterpretations of old discoveries! From that point of view, this review might be premature by a few years. However, our theoretical understanding is rather well developed and there are enough tantalizing observations that an interim review and perspective seems worthwhile. Certainly, the observational situation is extremely dynamic and the interest level is very high in several parts of the astronomical and planetary communities