Planet Formation in the Outer Limits: Collisional or Collisionless?
It has been suggested (Goldreich et al. 2004, Annual Reviews) that planets like Neptune can form in situ at large stellocentric distances by accreting rapidly from massive disks of small, highly collisional, possibly sub-meter-sized bodies. The small bodies have their velocity dispersions damped by inelastic collisions and/or gas drag, and are accreted by protoplanets in strongly gravitationally focused collisions. Left unsolved is the problem of "cleaning up" the excess disk mass not consumed by planets. We review the status of this proposal, and ask whether observations of extrasolar debris disks such as AU Mic and Fomalhaut support it. We touch on two classic problems related to clean-up: whether resonant Kuiper belt objects incontrovertibly imply capture by a smoothly migrating Neptune, and the role of collisions in delivering Jupiter-family comets from the Kuiper belt.