The Origin of Planetesimals
Andrew Youdin

I will summarize recent progress on a longstanding problem in cosmogony, the formation of planetesimals. The first generation of solid bodies wider than an kilometer, planetesimals are the crucial building blocks for terrestrial planets. They are also of great interest for the Solar System's asteroid and Kuiper belts, for the debris disks they generate in extrasolar planetary systems and for the volatiles they can deliver to habitable zones. The great debate in planetesimal formation focuses on the relative role of collisional sticking and collective gravitational collapse. I will describe the Streaming Instability, a promising mechanism that spontaneously triggers the clumping of heavy solids as they settle into the disk midplane. The large planetesimals, > 100 km in scale, engendered by the Streaming Instability was a surprising theoretical prediction. This surprise is causing profound reconsideration of Solar System planetesimals and of planet formation in general. These theoretical breakthroughs are subject to multifaceted empirical constraints --- which arise from the microscopic study of meteorites, collisional experiments in microgravity, the statistical ensemble of exoplanets, and the mapping of protoplanetary disks (currently being revolutionized by ALMA). Planetesimal formation thus remains a rich and timely problem.