Paper:	Progress in Giant Planet Formation
Volume:	294, Scientific Frontiers in Research on Extrasolar Planets
Page:	269
Authors:	Boss, A. P.
Abstract:	Two very different mechanisms have been proposed for the formation of the gas and ice giant planets. The conventional explanation for the formation of gas giant planets, core accretion, presumes that a gaseous envelope collapses upon a roughly 10 Earth-mass, solid core that was formed by the collisional accumulation of planetary embryos orbiting in a gaseous disk. The more radical explanation, disk instability, hypothesizes that the gaseous portion of protoplanetary disks undergoes a gravitational instability, leading to the formation of self-gravitating clumps, within which dust grains coagulate and settle to form cores. Core accretion appears to require several million years or more to form a gas giant planet, implying that only long-lived disks would form gas giants. Disk instability, on the other hand, is so rapid (thousands of years), that gas giants could form in even the shortest-lived disks. Core accretion has severe difficulty in explaining the formation of the ice giant planets, unless two extra protoplanets are formed in the gas giant planet region and thereafter migrate outward. Recently, an alternative mechanism for ice giant planet formation has been proposed, based on observations of protoplanetary disks in the Orion nebula cluster: disk instability leading to the formation of four gas giant protoplanets with cores, followed by photoevaporation of the disk and gaseous envelopes of the protoplanets outside about 10 AU by a nearby OB star, producing ice giants. In this scenario, Jupiter survives unscathed, while Saturn is a transitional planet. These two basic mechanisms have very different predictions for gas and ice giant extrasolar planets, both in terms of their frequency and epoch of formation, suggesting a number of astronomical tests which could determine the dominant mechanism for giant planet formation.

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