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Wm. R. Ward (Southwest Research Institute, SwRI), C. B. Agnor (University of Colorado & SwRI), H. Tanaka (Tokyo Institute of Technology & SwRI)
The accretion time scales of planetary objects at large stellar distances (such as Neptune) are considered. The formation of massive, distant companions is inhibited in three important ways: (1) Orbital periods are longer, so that more remote regions are dynamically younger. (2) The volume occupied by accreting material is greater, contributing to low collision frequencies. (3) The low gravitational binding energy to the star makes it easier for a planetary embryo to scatter planetesimals out of the system. We suggest that resonant damping of embryo dispersion velocities by a dissipating remnant of a precursor gas disk may mitigate this situation and make the existence of large, outer planets easier to explain. The embryos drive waves at coorbiting Lindblad resonance sites, and the reaction torques damp embryo eccentricities and inclinations (Ward, Icarus 106, 274, 1993; Papaloizou and Larwood, Mon. Not. R. Astron. Soc. 315, 823, 2000). Lower dispersion velocities result in increased embryo collision cross sections. Only a modest gas surface density (~1 its original value) is required to keep velocities mildly sub-sonic, yielding characteristic growth times of order 108 years. It is also likely that less material is ejected from the system than in a gas-free environment, thereby improving accretion efficiency.