DDP

Abstract

I will describe recent progress on the formation of planetesimals, focusing on dynamics. Self-gravity is a promising mechanism to collect a sea of small solids into a gravitationally bound planetesimal, but stirring by turbulent gas is a formidable obstacle. However several processes can concentrate particles in gas disks --- both despite and because of turbulence. Perhaps the most powerful is the streaming instability, mediated by two-way drag forces between solids and gas. Sufficiently dense particle clumps trigger collapse into planetesimals. While these mechanisms are promising, many outstanding questions remain. How large must particles grow by coagulation until these dynamical processes take over? How does the final collapse proceed, and what is the initial mass function of planetesimals?