Terrestrial Planet Formation I. The Transition from Oligarchic Growth to   Chaotic Growth
  We use a hybrid, multiannulus, n-body-coagulation code to investigate the growth of km-sized planetesimals at 0.4-2 AU around a solar-type star. After a short runaway growth phase, protoplanets with masses of roughly 10^26 g and larger form throughout the grid. When (i) the mass in these `oligarchs' is roughly comparable to the mass in planetesimals and (ii) the surface density in oligarchs exceeds 2-3 g/sq cm at 1 AU, strong dynamical interactions among oligarchs produce a high merger rate which leads to the formation of several terrestrial planets. In disks with lower surface density, milder interactions produce several lower mass planets. In all disks, the planet formation timescale is roughly 10-100 Myr, similar to estimates derived from the cratering record and radiometric data. 
Scott J. Kenyon & Benjamin C. Bromley