Early terrestrial planet formation by torque-driven convergent migration of planetary embryos

Massive cores of the giant planets are thought to have formed in a gas disk by accretion of pebble-size particles whose accretional cross-section is enhanced by aerodynamic gas drag (Lambrechts & Johansen 2012, Levison et al. 2015). A commonly held view is that the terrestrial planet system formed later (30-200 Myr after the dispersal of the gas disk) by giant collisions of tens of lunar- to Mars-size protoplanets (Wetherill 1990, Chambers & Wetherill 1998). Here we propose, instead, that the terrestrial planets of the Solar System formed earlier by gas-driven convergent migration of protoplanets toward ~1 au. To investigate situations in which convergent migration occurs and determine the thermal structure of the gas and pebble disks in the terrestrial planet zone, we developed a radiation-hydrodynamic model with realistic opacities (Zhu et al. 2012, Semenov et al. 2003). We find that protoplanets grow in the first 10 Myr by mutual collisions and pebble accretion, and gain orbital eccentricities by gravitational scattering and the hot-trail effect (Chrenko et al. 2017, Eklund & Masset 2017). The orbital structure of the inner Solar System is well reproduced in our simulations, including its tight mass concentration at 0.7-1 au and the small sizes of Mercury and Mars. The early-stage protosolar disk temperature exceeds 1500 K inside 0.4 au implying that Mercury grew in a highly reducing environment, next to the evaporation lines of iron and silicates, influencing Mercury’s bulk composition (Hauck et al. 2013). On contrary, a dissipating gas disk is cold, and pebbles drifting from larger heliocentric distances would deliver volatile elements.