Timing and Timescales in the Grand Tack Model of Planet Formation

Abstract:

Most models of planet formation only model the final, giant impact, stage of planetary accretion and therefore rely on previous work to establish their initial distributions of planetary embryos and planetesimals. However, recent works have suggested that the growth of planetary embryos has a stronger radial dependence than previously thought (inside-out growth; see Minton and Levison 2014), whereby Mars-mass embryos could grow at 1.0 au millions of years before lunar-mass objects grow at 2.0 au. Therefore previous models that assume a uniform distribution of planetary embryos and planetesimals across the inner solar system require re-investigation.

One such model is the Grand Tack scenario (Walsh et al. 2011) that has found success in reproducing the Earth/Mars mass ratio and accretion timescales when migrating giant planets truncate the disk of inner solar system rocky material. We will show that this scenario depends on the existence of planetary embryos out beyond 1.5 au at the time of Jupiter's "tack". Using a Lagrangian planetary accretion code capable of modeling the formation of planets directly from planetesimals (LIPAD; see Levison et al. 2012), we can then place constraints on the timing of the Grand Tack scenario by the dynamical requirements of planetary embryo growth.