Pre - Planetary Disks and Planet Formation

Abstract

The behaviour of dust grains in a simple model of cool pre-planetary disks is discussed. Obviously grains are a most important ingredient for planetary formation, considering the fact that the terrestial planets and the presumed rocky cores of the giant planets are made of solid material. Grains drift, diffuse, melt and recondense in a turbulent disk. In particular they also coagulate to form larger, mm - to cm - size conglomerates. These processes occur while a central body forms on account of the inward mass and the outward angular momentum transport. If the turbulence in a pre - planetary disk is due to thermal convection, then grain growth can stabilize the disk by reducing its opacity. Subsequent sedimentation of the bigger conglomerates leads to a dense subdisk of grains which can fragment gravitationally to form km - size solid planetesimals. These later accumulate into solid planets or, in the massive presence of cold gas, they first form rocky cores which can acquire a big gaseous component.