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Session 27 - Planets.
Oral session, Wednesday, January 07
Lincoln,
The standard model of giant planet formation requires the accumulation of a \sim 10 M_øplus ice/rock core that can accrete hydrodynamically an envelope of H and He gas prior to the removal of the solar nebula. Recent models of Jovian planet interiors have revised downward considerably their estimated core masses, removing much of the rationale for the core accretion model. This trend toward a more solar-like composition for Jupiter and Saturn suggests that a reappraisal of an alternative mechanism is appropriate, giant gaseous protoplanet formation (GGPP) by gravitational instability of an intermediate mass (\sim 0.14 M_ødot) solar nebula. Three dimensional hydrodynamical models of the evolution of such a disk, starting from a realistic, non-power-law, radial thermal profile, have shown that disks with Toomre Q stability parameters of \sim 1 can break-up into multiple-Jupiter-mass GGPPs within a few hundred years, regardless of the thermodynamics of the instability (1997, Science, 276, 1836-1839). These models fixed the location of the solar-mass protostar at the center of the disk, which discouraged the growth of m = 1 modes. New models which allow the central protostar to wobble back and forth to preserve the center of mass of the star-disk system show that the resulting preferential excitation of m = 1 modes leads to the formation of two GGPPs, at 5 AU and 10 AU from the central star, similar to the distances of Jupiter and Saturn. Giant planets could only form by the GGPP mechanism in regions of the disk with sufficiently cool temperatures, however, so like the core accretion mechanism, the GGPP mechanism offers no easy explanation for the formation in situ of the hot Jupiters (e.g., 51 Pegasi B); in either case, post-formational orbital migration must be invoked to explain the present location of hot Jupiters. Jupiter and Saturn, on the other hand, apparently experienced little or no orbital migration.
