Forming Earth-like and low-mass rocky exoplanets through pebble and planetesimal accretion

Context. The theory of planet formation through pebble accretion has gained in popularity over the past decade. Recent studies claim that pebble accretion could potentially explain the mass and orbits of the terrestrial planets in the Solar system, the size and water contents of the planets in the TRAPPIST-1 system, and the formation of super-Earth systems at small orbital radii. However, all these studies start with planetary embryos much larger than those expected from the streaming instability.
Aims. We analyse the formation of terrestrial planets around stars with masses ranging from 0.09 to 1.00 M_⊙ through pebble accretion, starting from small planetesimals with radii between 175 and 450 km.
Methods. 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. We analysed two different prescriptions for the pebble accretion rate.
Results. We find that Earth-like planets are consistently formed around 0.49, 0.70, and 1.00 M_⊙ 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_⊙ stars, challenging the proposed narrative of the formation of the TRAPPIST-1 system.
Conclusions. Although we have the ability to 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.