Planet Formation: the Difficulty with the Small Guys

Wesley Fraser, Caltech

Planet formation models have an obvious goal; the ability to create
planets on timescales amenable to the short-lived existence of disks
around young stars. A standard picture of planet formation has arisen
in which planetesimals grow by mutual collision and accumulation in a
proto-planetary disk. It has been demonstrated that with reasonable
starting conditions, this process can produce bodies of mass
comparable to the inferred core masses of the gas-giant planets,
terrestrial-mass planets, and bodies with radii similar to the largest
asteroids and Kuiper belt objects.

The planetesimal populations of the Solar system, such as the asteroid
and Kuiper belts, are populations that never achieved planetary
masses. These remnants provide detailed probes of the end-state of
accretion in their locales. While accretion models can produce the
large Solar system bodies, the devil is in the details; a successful
model of planet accretion must account for the properties - albeit
modified by collisional and thermal processes - of the small body
populations.

I will review the standard picture of planet formation, and discuss
how the physical processes involved in this model produce a
characteristic size distribution for the small-body population. I will
present my recent calculations on the post-accretion collisional
evolution of this size distribution, and compare my results to the
observed small body populations of the Solar system. With this, I will
demonstrate that the standard picture of planet formation does not
reproduce many of the observed features in the size distributions of
the small body populations, and conclude that some critical piece of
the puzzle is missing