Liquid metallic hydrogen and the structure of brown dwarfs and giant planets

Electron-degenerate, pressure-ionized hydrogen (usually referred to as metallic hydrogen) is the principal constituent of brown dwarfs, the long-sought objects which lie in the mass range between the lowest-mass stars (about eighty times the mass of Jupiter) and the giant planets. The thermodynamics and transport properties of metallic hydrogen are important for understanding the properties of these objects, which, unlike stars, continually and slowly cool from initial nondegenerate (gaseous) states. Within the last year, a brown dwarf (Gliese 229 B) has been detected and its spectrum observed and analyzed, and several examples of extrasolar giant planets have been discovered. The brown dwarf appears to have a mass of about forty to fifty Jupiter masses and is now too cool to be fusing hydrogen or deuterium, although we predict that it will have consumed all of its primordial deuterium. This paper reviews the current understanding of the interrelationship between its interior properties and its observed spectrum, and also discusses the current status of research on the structure of giant planets, both in our solar system and elsewhere.