Forming Earth-like and Low-Mass Rocky Exoplanets Through Pebble and Planetesimal Accretion

Abstract

The theory of planet formation through pebble accretion (PA) has gained in popularity over the past decade. Most PA studies start with planetary embryos much larger than those expected from the streaming instability. In this study, we analyse the formation of terrestrial planets around stars with masses ranging from 0.09 to 1.00 M$_\odot$ through pebble accretion, starting from small planetesimals with radii between 175 and 450 km. We performed numerical simulations using a modified version of the N-body simulator SyMBA, which includes pebble accretion, type I and II migration, and eccentricity and inclination damping. Two different prescriptions for the PA rate were analysed. We find that Earth-like planets are consistently formed around 0.49, 0.70, and 1.00 M$_\odot$ stars, irrespective of the pebble accretion model that is used. However, Earth-like planets seldom remain in the habitable zone, for they rapidly migrate to the inner edge of the disc. Furthermore, we find that pebble accretion onto small planetesimals cannot produce Earth-mass planets around 0.09 and 0.20 M$_\odot$ stars, challenging the proposed narrative of the formation of the TRAPPIST-1 system. Although our models can explain the formation of Earth-mass planets around Sun-like stars, we find a low likelihood of Earth-like planets remaining in the habitable zone. Further research is needed to determine if models with a lower pebble mass flux or with additional migration traps could produce more Solar System-like planetary systems.