Planetesimals and Accretion Disk Dynamics

The core formation model predicts a stage of planetary formation which is dominated by planetesimal dynamics. The driving mechanisms at this stage are collisions and relaxation. Collisions are responsible for the evolution of the mass spectrum (source for debris, but also promotes accumulation of mass), as well as for energy dissipation and viscosity. While energy dissipation tends to cool down a protoplanetary disk (i.e. eccentricities and inclinations decrease), it is viscosity that transforms orbital energy into thermal energy (deflection of relative velocities, i.e. eccentricities and inclinations increase). Relaxation on the other hand is a slowly acting, but continuous heating mechanism, in which bodies deflect each others velocities, but never come into contact. Rather they interact over long ranges, but at a high rate. In the long run, relaxation considerably alters phase space density. A planetesimal disk evolves from a collision-dominated to a relaxational regime, in which collisions have become rare events. Gas drag may play a role as well.