Starting Small in Planet Formation

Abstract

Recent advances in theory and simulations reveal a promising mechanism for the formation of solids larger than a kilometer, i.e. the planetesimals that are a necessary ingredient for the production of planetary systems and their debris disks. Rocks and boulders several to tens of cm in size start to decouple from the primordial gas disk. In the midplane these rocks are clumped by turbulent fluctuations and streaming instabilities until self-gravity induces collapse into planetesimals. Since collisional growth of the original population of rocks remains uncertain, I consider whether even smaller pebbles, below cm-sizes, can also undergo collapse into planetesimals. I will describe recent numerical work, and present analytic calculations which show that smaller particles undergo a slow mode of gravitational collapse. This process lasts many orbital times, and is consistent with observed T Tauri disk lifetimes and Solar System constraints from asteroids and meteorites.