With the discovery of the companions of 51 Peg, 55 Cnc, Tau Boo,
Upsilon And, 70 Vir, 47 UMa, and Gl 229, evolutionary and spectral models
of gas giants and/or brown dwarfs with masses from 0.3 through 60 times that
of Jupiter assume a new and central role in the emerging field of extra-solar
planetary studies. In this contribution, I describe the structural, atmospheric,
and evolutionary characteristics of such exotic objects, as determined by our
recent theoretical calculations (Burrows et al. 1995; Guillot et al. 1996;
Saumon et al. 1996; Marley et al. 1996). The issue, in part diversionary, of
what distinguishes a brown dwarf from a giant planet will be addressed, as will
the luminosities, effective temperatures, gravities, colors, and spectral
signatures of these beasts as a function of age and mass. Our theoretical
calculations can be used to establish direct search strategies via SIRTF, ISO,
and HST (NICMOS), and via various ground-based adaptive optics and
interferometric platforms planned for the near future.
