Most super-Earths formed by dry pebble accretion are less massive than 5 Earth masses

Abstract

Aims. The goal of this work is to study the formation of rocky planets by dry pebble accretion from self-consistent dust-growth models. In particular, we aim to compute the maximum core mass of a rocky planet that can sustain a thin H-He atmosphere to account for thesecond peak of the Kepler size distribution. Methods. We simulate planetary growth by pebble accretion inside the ice line. The pebble flux is computed self-consistently fromdust growth by solving the advection diffusion equation for a representative dust size. Dust coagulation, drift, fragmentation, andsublimation at the water ice line are included. The disc evolution is computed solving the vertical and radial structure for standard-discs with photoevaporation from the central star. The planets grow from a moon-mass embryo by silicate pebble accretion and gasaccretion. We perform a parameter study to analyse the effect of a different initial disc mass,-viscosity, disc metallicity, and embryolocation. We also test the effect of considering migration versus an in situ scenario. Finally, we compute atmospheric mass loss due toevaporation over 5 Gyr of evolution. Results. We find that inside the ice line, the fragmentation barrier determines the size of pebbles, which leads to different planetarygrowth patterns for different disc viscosities. We also find that in this inner disc region, the pebble isolation mass typically decays tovalues below 5 M within the first million years of disc evolution, limiting the core masses to that value. After computing atmosphericmass loss, we find that planets with cores below 4 M become completely stripped of their atmospheres, and a few 4 5 M coresretain a thin atmosphere that places them in the "gap" or second peak of the Kepler size distribution. In addition, a few rare objects thatform in extremely low-viscosity discs accrete a core of 7 M and equal envelope mass, which is reduced to 3 5 M after evaporation. These objects end up with radii of 6 7 R.Conclusions. Overall, we find that rocky planets form only in low-viscosity discs (. 104). When 103, rocky objects do notgrow beyond 1 Mars mass. For the successful low-viscosity cases, the most typical outcome of dry pebble accretion is terrestrial planetswith masses spanning from that of Mars to 4 M.