Extrasolar Planets
Alan Boss, Carnegie Institute of Washington
Searches for extrasolar planets and brown dwarf stars have a long and dismal history. However, recent discoveries have marked the advent of an era of discovery of extrasolar planetary systems. The newly discovered objects appear to be a mixture of gas giant planets, similar to Jupiter, and brown dwarf stars, elusive objects that have long been hypothesized to exist but never seen before. Brown dwarf stars are intermediate in mass between giant planets and the lowest mass stars that can burn hydrogen like the Sun; the atmospheres of cool brown dwarfs may very well resemble those of giant planets, but their highly eccentric orbits are a clue to their origin as stars rather than as planets. Giant planets must form in the relatively cool, outer regions of protoplanetary disks, which is why the discovery of several giant planets with circular orbits very close to their central stars was a shock to theorists. Undoubtedly other theoretical adjustments lie ahead, as we begin to learn at last what other planetary systems are like.

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