The Emerging Paradigm of Pebble Accretion

Pebble accretion is the mechanism in which small particles (“pebbles”) accrete onto big bodies (planetesimals or planetary embryos) in gas-rich environments. In pebble accretion , accretion occurs by settling and depends only on the mass of the gravitating body , not its radius. I give the conditions under which pebble accretion operates and show that the collisional cross section can become much larger than in the gas-free, ballistic, limit. In particular, pebble accretion requires the pre-existence of a massive planetesimal seed. When pebbles experience strong orbital decay by drift motions or are stirred by turbulence, the accretion efficiency is low and a great number of pebbles are needed to form Earth-mass cores. Pebble accretion is in many ways a more natural and versatile process than the classical, planetesimal-driven paradigm, opening up avenues to understand planet formation in solar and exoplanetary systems. These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.