Planetesimal Accretion onto Growing Proto–Gas Giant Planets

Abstract

The solar and extrasolar gas giants appear to have diverse internal structure and metallicities. We examine a potential cause for these dispersions in the context of the conventional sequential accretion formation scenario. In principle, gas accretion onto cores with masses below several times that of the Earth is suppressed by the energy released from the bombardment of residual planetesimals. Due to their aerodynamical and tidal interaction with the nascent gas disk, planetesimals on eccentric orbits undergo slow orbital decay. We show that these planetesimals generally cannot pass through the mean motion resonances of the cores, and the suppression of planetesimal bombardment rate enables the cores to accrete gas with little interruption, thus shortening the timescale of gas giant formation. During growth from the cores to protoplanets, resonances overlap with each other, which strongly enhances the eccentricity excitation of the trapped planetesimals. Subsequent gas drag induces the planetesimals to migrate to the proximity of the protoplanets and collide with them. This process leads to the resumption and a surge of planetesimal bombardment during the advanced stage of the protoplanet growth. Intruder planetesimals with different masses can either be resolved in the envelope or reach the core of the protoplanets. This mechanism may account for the diversity of the core-envelope structure between Jupiter, Saturn, and the metallicity dispersion inferred from the transiting extrasolar planets. During the final formation stage of the proto-gas giants, gap opening in gas disk leads to the accumulation of planetesimals outside the feeding zone of the protoplanets. The surface density enhancement promotes the subsequent buildup of cores for secondary gas giant planets outside the orbit of the first-born protoplanets and the formation of eccentric multiple planet systems.