Effects of Type I Migration on Terrestrial Planet Formation

Planetary embryos embedded in a gas disk suffer a decay in semimajor axis—type I migration—due to the asymmetric torques produced by the interior and exterior wakes raised by the body. This presents a challenge for standard oligarchic approaches to forming the terrestrial planets, as the timescale to grow the progenitor objects near 1 AU is longer than that for them to decay into the Sun. In this paper we investigate the middle and late stages of oligarchic growth using both semianalytic methods (based on Thommes and coworkers) and N-body integrations and vary gas properties such as dissipation timescale in different models of the protoplanetary disk. We conclude that even for near-nominal migration efficiencies and gas dissipation timescales of ∼1 Myr, it is possible to maintain sufficient mass in the terrestrial region to form Earth and Venus if the disk mass is enhanced by factors of ∼2–4 over the minimum-mass model. The resulting configurations differ in several ways from the initial conditions used in previous simulations of the final stages of terrestrial accretion, chiefly in (1) larger interembryo spacings, (2) larger embryo masses, and (3) up to ∼0.4 M⊕ of material left in the form of planetesimals when the gas vanishes. The systems we produce are reasonably stable for ∼100 Myr and therefore require an external source to stir up the embryos sufficiently to produce final systems resembling the terrestrial planets.