The discovery of the first extrasolar planets, with masses in the range of ~0.5 MJup (MJup = Jupiter mass) to ~3 MJup, demands a reevaluation of theoretical mechanisms for giant planet formation. Here we consider a long-discarded mechanism, forming giant planets through the gravitational instability of a protoplanetary disk. Radiative hydrodynamical calculations of the thermal structure of an axisymmetric protoplanetary disk with a mass of ~0.13 M☉ (inside 10 AU), orbiting a solar-mass star, predict that the outer disk may be cool enough (~100 ± 50 K) to become gravitationally unstable. This possibility is investigated here with a fully three-dimensional hydrodynamics code. Growth of significant nonaxisymmetry occurs within a few rotation periods of the outer disk and can result in the formation of several discrete, multiple-MJup clumps in 103 yr. These giant gaseous protoplanets (GGPPs) are gravitationally bound and tidally stable and so should eventually form giant planets. Modest-sized solid cores may form through dust grain growth and sedimentation prior to the centers of the GGPPs reaching planetary densities. The inner disk remains nearly axisymmetric throughout these phases, suggesting a scenario in which the formation of terrestrial planets occurs slowly through collisional accumulation in the hot inner nebula, while rapid formation of GGPPs occurs in the cooler regions of the nebula. Falling disk surface densities would restrict GGPP formation to an annulus, outside of which icy outer planets would have to form slowly through collisional accumulation. GGPP formation occurs for both locally isothermal and locally adiabatic disk thermodynamics, provided that the Toomre Q stability parameter indicates instability (Qmin ≈ 1). Low-order modes, especially m = 1 and 2, are dominant. Provided that a means can be found for inducing massive protoplanetary disks to undergo the GGPP instability (e.g., clumpy accretion of infalling gas onto a marginally stable disk), the GGPP mechanism appears to be a prompt alternative to the long-favored but protracted core accretion mechanism of giant planet formation. Observations hold the promise of deciding which of these two mechanisms is preferred by young stars.
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