Turbulence May Make it Difficult for Planetesimals to Form

Planetesimals are 1-100 km-scale building blocks of planets. Yet, how planetesimals form has remained one of the major open questions in all of planet formation. Recent years has seen the emergence of a promising mechanism to forming these bodies: the streaming instability (SI), which aerodynamically concentrates dust grains to the point of gravitational collapse, producing planetesimals. Previous work has shown that the SI is quite robust and can even work in regions of the disk where the dust-to-gas ratio is well below that inherited by the ISM (1%), depending on the size of the grains that participate in the SI. Despite the success of the SI model, there remains one physical effect that requires a broader study over what has been done to date: disk turbulence. In this work, we present a large parameter survey using high-resolution simulations of planetesimal formation via the SI under the influence of externally driven (i.e., not driven by the SI itself) turbulence. We find that even modest levels of turbulence may make it much harder for the SI to produce planetesimals. More quantitatively, for mm size grains at ~50 AU in a typical planet-forming disk, the SI requires a factor of ~10 enhancement in the dust-to-gas ratio (above the ISM value) if the turbulent α ~ 1e-3. These results put strong constraints on the environments in which planetesimals can form via the SI. In particular, if the disk is modestly turbulent, it may be that only regions such as dust rings or vortices, where grains can be highly concentrated, are favorable to planetesimal formation via the SI.