Can the Streaming Instability Form the First Planetesimals in Globally Turbulent Protoplanetary Disks?
Understanding the formation of the first planetesimals remains key to deciphering the history of planet formation within our own solar system and beyond. Evidence from the vast meteorite record [e.g., 1] as well as observations [e.g., 2] strongly suggest that the first planetesimals, and perhaps giant planet core accretion occurred well within the first million years of disk evolution. Moreover, chemical and lithological mixing as well as observations of line-broadening in protoplanetary disks (PPDs) [e.g., 3-6] suggest that the solar system nebula in this epoch was weakly-to-moderately turbulent in the regions where particle growth is of the greatest interest [7-8]. It is a well-known though that global hydrodynamic turbulence complicates particle growth due to a slew of barriers that can slow or even stall particle or aggregate growth at pebble sizes (with corresponding small particle Stokes numbers St) that can lead to loss to the central star via radial drift before planetesimals can ever form [9,10], requiring that some other mechanism come into play that collects growth-frustrated pebbles into gravitationally bound multi-km bodies – objects that are “born big” [11]. The current leading candidate for such a “leap-frog” mechanism is the so-called Streaming Instability (SI), a gas-drag mediated momentum exchange resonance in which the relative velocity between the particle component and a rotating gaseous fluid can lead to high densities in the particle field [12], which has been invoked in a number of recent PPD models that include a turbulent intensity α [e.g., 13-14] as the defacto mechanism for planetesimal formation if conditions for the SI (depending on particle St and the solids-to-gas mass ratio) are satisfied. However, these previous works use conditions established from occurrence studies for the onset of SI in laminar disks [15-17] in which the only source of turbulence is that self-generated by the settling particle layer, and not externally driven global turbulence. Recent analytical theories of the SI subject to global turbulence predict much more stringent conditions for the effectiveness of the SI than the laminar case [18-19]. Thus, whether the efficient operation of the SI can be attained in realistic models of the solar nebula have yet to be established. In this work, we ask whether the conditions under which the SI can produce gravitationally bound particle overdensities can actually be met in the first million years of evolution in globally turbulent PPDs.