Challenge of forming planetesimals from mm grains in dust rings

The outer disk is a challenging region for planetesimal formation theory. Small particle sizes and fast radial drift make it difficult to collect enough solids to trigger planetesimal formation. I will present evidence that it is unlikely that the streaming instability (SI) can form planetesimals from mm grains inside the axisymmetric pressure bumps responsible for the dust rings observed by ALMA. We conducted the largest simulation of the SI so far (7 million CPU hours), consisting of a large slice of the disk with mm grains, a solar-like dust-to-gas ratio (Z = 0.01), and the largest pressure bump that does not cause gravitational instability (GI) in the particle layer. We used a high resolution of 1000/H to resolve as many SI unstable modes as possible. The simulation produced a large and long-lived particle over-density, but no planetesimals, or even dust filaments normally associated with the SI. The likely reason is that the time it takes particles to cross the high-density region is shorter than the growth timescale of the SI. This result restricts the pathways for planet formation: Either protoplanetary disks form grains larger than 1mm at 50 AU, or planetesimals do not form in the observed dust rings. Since bumps large enough to induce gravitational instability without the SI are likely to be Rossby Wave unstable and form vortices, we propose that planetesimals in the outer disk may form in vortices.