Planetary core formation via multispecies pebble accretion

Abstract

In the general classical picture of pebble-based core growth, planetary cores grow by accretion of single pebble species. The growing planet may reach the so-called pebble isolation mass, at which it induces a pressure bump that blocks inward drifting pebbles exterior to its orbit, thereby stalling core growth by pebble accretion. In recent hydrodynamic simulations, pebble filtration by the pressure bump depends on several parameters including core mass, disc structure, turbulent viscosity and pebble size. We have investigated how accretion of multiple, instead of single, pebble species affects core growth rates, and how the dependence of pebble isolation mass on turbulent viscosity and pebble size sets the final core masses. We performed numerical simulations in a viscous one-dimensional disc, where maximal grain sizes were regulated by grain growth, fragmentation and drift limits. We confirm that core growth rates and final core masses are sensitive to three key parameters: the threshold velocity at which pebbles fragment on collision, the turbulent viscosity and the distribution of pebble species, which yield a diversity of planetary cores. With accretion of multiple pebble species, planetary cores can grow very fast, reaching over 30–40 ME in mass. Potential cores of cold gas giants were able to form from embryos initially implanted as far as 50 au. Our results suggest that accretion of multispecies pebbles could explain: the estimated 25–45 ME heavy element abundance inside Jupiter’s core; the massive cores of extrasolar planets; the disc rings and gaps at wider orbits; and the early and rapid formation of planetary bodies.