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A. P. Boss (DTM/CIW)
The first confirmed detections of extrasolar planets and brown dwarf stars mark the beginning of a new era for the discovery of low mass companions to solar-type stars. The newly-discovered objects appear to be a mixture of gas giant planets, presumably similar to Jupiter, and brown dwarf stars, stars with masses less than 80 Jupiters. Brown dwarfs are too low in mass to undergo hydrogen fusion, making their detection nearly as difficult as that of extrasolar giant planets. In most cases, the detection techniques yield only basic properties for the companions -- their masses (usually only a lower limit on the mass), their orbital periods and separations, and their orbital eccentricities. These properties are sufficient, however, to begin at least a provisional sorting of the objects into either the giant planet or brown dwarf category, based in part on theoretical expectations for the masses and orbital eccentricities of giant planets and brown dwarfs. Brown dwarf stars are believed to form on highly eccentric orbits, with masses no less than about 3 Jupiters, at the same time that the primary star forms. Giant planets are thought to form after the primary star has gained most of its mass, within a protoplanetary disk. Two competing mechanisms exist for giant planet formation, (1) slow collisional accumulation of a solid core followed by accretion of disk gas, and (2) rapid gravitational instability of the disk gas followed by sedimentation of dust grains to form a core. Both mechanisms have a long list of advantages and disadvantages, making it hard to predict theoretically the dominant mechanism. However, a definitive observational test exists for determining which mechanism predominantly forms giant planets. If the youngest stars (less than 1 Myr old) already have giant planets, then the disk instability mechanism must be responsible, because the core accretion mechanism evidently requires greater than 1 Myr to form a planet of Jupiter-mass.