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Session 70 - Searching for Other Planetary Systems.
Display session, Wednesday, January 17
North Banquet Hall, Convention Center
The gas giant planets of our solar system are believed to have formed through a two step process. First, collisional accumulation of a swarm of planetesimals produces a \sim 10 M_øplus core of ice and rock. Second, this core accretes an envelope of hydrogen and helium gas through the hydrodynamic collapse of disk gas onto the growing protoplanet. A necessary condition for the formation of gas giant planets is thus the presence of icy planetesimals, which cannot form inside the ice condensation radius of the disk at the time when planetary accumulation is occurring. The ice condensation radius in protoplanetary disk models thus sets a lower limit on the radius at which Jupiter-like planets could form. Radiative hydrodynamical models have been computed of the thermal structure of low mass protoplanetary disks orbiting a solar-mass protostar, with the disk being heated by mass accretion from the parent dense cloud core. These models predict that the midplane temperature falls below the ice condensation point (\sim 160 K) only for radii of several AU or more, for a wide variety of disk masses (0.01 to 0.13 M_ødot) and for a range of disk mass accretion rates (\sim 10^-7 to \sim 10^-5 M_ødot/yr). Variations in other parameters such as the radial density profile and the dust grain opacity have even less effect on temperatures. The location of the ice condensation radius is restricted to the range of \sim 3 AU to \sim 7 AU in these models. If the companion to 51 Peg (a solar-type star) is a gas giant planet, then the companion could not have formed at its present radius of \sim 0.05 AU, but must have suffered significant orbital evolution after its formation. A likely cause of such orbital decay is gravitational interaction between the companion and a protoplanetary disk that evidently survived longer than the solar nebula did.
