Thresholds for Planetesimal Formation by the Streaming Instability

The crucial first step in planet formation is to build planetesimals out of dust particles in gaseous protoplanetary disks. The Streaming Instability (SI) is one of the leading mechanisms to concentrate solids in disks and rapidly produce planetesimals. In this talk, I will present our latest work on quantifying the physical conditions needed by the SI to form planetesimals. Specifically, we focus on two parameters that control the SI behaviors: particle size and metallicity (i.e., solid abundance relative to the gas). Our high-resolution simulation results pinpoint a metallicity threshold as a function of particle size. We find that planetesimal formation can occur even at sub-solar metallicities, consistent with recent disk observations that suggest planet formation is well underway at early evolutionary stages. I will also show how our results can be applied to general turbulent disk models. Finally, I will present our preliminary results on the thresholds for planetesimal formation in self-gravitating disks and discuss the implications on early planet formation.