The Effect of an Early Planetesimal-Driven Migration of the Giant Planets on Terrestrial Planet Formation

The migration of the giant planets due to the scattering of planetesimals causes powerful resonances to move through the asteroid belt and the terrestrial planet region. Exactly when and how the giant planets migrated is not well known. In this paper we present results of an investigation of the formation of the terrestrial planets during and after the migration of the giant planets. The latter is assumed to have occurred immediately after the dissipation of the nebular disk – i.e. “early” with respect to the timing of the late heavy bombardment (LHB). The presumed cause of our modeled early migration of the giant planets is angular mometum transfer between the planets and scattered planetesimals. Our model forms the terrestrial planets from a disk of material which stretchs from 0.3–4.0 AU, evenly split in mass between planetesimals and planetary embryos. Jupiter and Saturn are initially at 5.4 and 8.7 AU respectively, on orbits with eccentricities comparable to the current ones, and migrate to 5.2 and 9.4 AU with an e-folding time of 5 Myr. Unfortunately, the terrestrial planets formed in the simulations are not good analogs for the current solar system, with Mars typically being much too massive. Moreover, the final distribution of the planetesimals remaining in the asteroid belt is inconsistent with the observed distribution of asteroids. This argues that, even if giant planet migration had occurred early, the real evolution of the giant planets would have to have been of the “jumping-Jupiter” type, i.e. the increase in orbital separation between Jupiter and Saturn had to be dominated by encounters between Jupiter and a third, Neptune-mass planet. This result was already demonstrated for late migrations occuring at the LHB time by previous work, and this paper shows those conclusions hold for early migration as well.