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 kilometer-sized planetesimals at 0.4-2 AU around a solar-type star. After a short runaway growth phase, protoplanets with masses of ∼10 26 g and larger form throughout the grid. When (1) the mass in these "oligarchs" is roughly comparable to the mass in planetesimals and (2) the surface density in oligarchs exceeds 2-3 g cm -2 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 ∼10-100 Myr, similar to estimates derived from the cratering record and radiometric data.