Planetesimals are the precursors to planets, and understanding their formation is an essential step towards developing a complete theory of planet formation, whether it be that of our own solar system or of the many extrasolar planetary systems discovered in recent years. Furthermore, a detailed understanding of planetesimal formation is necessary for explaining the observed properties of asteroids and Kuiper Belt objects.
Traditional theories attempt to explain planetesimal formation from a “bottom-up” approach; small particles (e.g., dust grains) continually grow upward in mass and scale, finally reaching gravitationally bound objects. For these small solid particles to coagulate into planetesimals, however, requires that these particles grow beyond centimeter sizes; with traditional coagulation physics, this is very difficult. The streaming instability, however, generates sufficiently dense clumps of these smaller constituents that the mutual gravity between the particles eventually causes their collapse towards planetesimal mass and size scales. In this talk, I will first describe the streaming instability and how it solves the centimeter growth problem. I will then present a series of high resolution, first principles numerical simulations of protoplanetary disk gas and dust to examine in detail, the formation of planetesimals and their resulting size frequency distribution. Finally, I will discuss the implications of these calculations for the formation of asteroids and Kuiper Belt Objects and for the construction of planetary systems, both our own and beyond.