Planetesimal Formation in Protoplanetary Disk Rings

ALMA’s iconic image of HL Tau began a new era of planet formation theory, where we know that dust rings and their associated pressure bumps are an ubiquitous feature of protoplanetary disks. These dust rings are a natural site for the formation of planetesimals. Our group has conducted the first full 3D simulations of planetesimal formation by the streaming instability (SI) in a large slice of a disk with a pressure bump of the same scale as those in HL Tau and other disks. We show that for cm-size particles planetesimal formation is extremely robust. In a disk with solar-like metallicity of Z = 0.01 and no a priori dust concentration our models readily form planetesimals for a wide range of bump amplitudes and reinforcement mechanisms. Importantly, a full pressure trap (where particle drift stops) is not needed, and in all these scenarios planetesimal formation remains an efficient process. However, we find that planetesimal formation is not robust to particle size. If the solid mass is dominated by mm-size grains, planetesimal formation is very difficult and unlikely unless the pressure bump can trap particles. We conducted an extremely high resolution long-term simulation with a large pressure bump that never even approached the Roche density. If mm-size grains dominate the disk, then only a full pressure trap can lead to planetesimal formation and in that instance the process is not mediated by the SI. Taken together, our results put important new constraints on the conditions needed for planet formation. Simply put, circumstellar disks that form rocky planets or giant planet cores must either have low alpha or sticky grains that can reach cm-sizes at the midplane, or form very large pressure bumps that can trap smaller particles and completely bypass the streaming instability.