Princeton University Extrasolar Planet Discussion Group

The assembly of planetesimals, a key step in the formation of planets, remains riddled with questions. In particular, there is ambiguity regarding the environments in which dust accumulations at the mid-planes of protoplanetary disks become unstable to their own self-gravity, and contract to form planetesimals. Moreover, how these specific conditions relate to the planetesimal initial mass function (IMF) is uncertain. In this talk, I investigate these questions. Drawing upon the analogy of underwater sand ripples, I will first introduce a new linear, axisymmetric instability that leads to filament formation in the dusty mid-plane of protoplanetary disks, provided diffusivity decreases sufficiently fast with increasing dust loading — a premise supported by our numerical simulations of protoplanetary disks where dust-gas interactions self-generate turbulent diffusion. The second part of my talk will discuss the IMF of planetesimals that form when these filaments fragment under their own self-gravity. I will present an analytic framework that maps the turbulent conditions in the filament onto the IMF as well as numerical simulations of planetesimal formation. This theory provides characteristic size predictions for initial planetesimals that are of order 100km, an estimate consistent with Asteroid Belt data.