Pebble Accretion

Pebble accretion refers to the growth of planetary bodies through the accretion of pebble-sized particles. Pebbles are defined in terms of their aerodynamically size $\tau_s$, which describes the level of coupling to the disk gas. Observations confirms the presence of pebble-sized particles in both protoplanetary disks and the early solar system. Pebble accretion proceeds through the settling mechanism, where particles settle to the surface of the planet. This Chapter discusses the key aspects of the pebble accretion framework: the accretion regimes, the planet initiation mass, and the planet isolation masses. The accretion behavior of loosely coupled $\tau_s > 1$ particles, referred to as "large pebbles", is also examined. The pebble accretion probability, $\epsilon$, is shown to be a useful parameter for evaluating the efficiency of the process, though this quantity is not necessarily high. Distinctions between pebble and planetesimal accretion are outlined. Pebble accretion, in particular, can be a highly effective mechanism in dense rings, as witnessed with ALMA.