Forming Planetesimals in Solar and Extrasolar Nebulae

TL;DR

This paper reviews how planetesimals, the building blocks of planets, can form through gravitational collapse of small particles in turbulent disks, emphasizing the roles of aerodynamic and streaming instabilities.

Contribution

It synthesizes current understanding of planetesimal formation via self-gravity and instabilities, highlighting recent simulation results and future research directions.

Findings

Solids 10-100 cm can gravitationally collapse in turbulent disks.

Aerodynamic and streaming instabilities promote particle concentration.

Self-gravity may also act on smaller objects, suggesting multiple formation pathways.

Abstract

Planets are built from planetesimals: solids larger than a kilometer which grow by colliding pairwise. Planetesimals themselves are unlikely to form by two-body collisions; sub-km objects have gravitational fields individually too weak, and electrostatic attraction is too feeble for growth beyond a few cm. We review the possibility that planetesimals form when self-gravity brings together vast ensembles of small particles. Even when self-gravity is weak, aerodynamic processes can accumulate solids relative to gas, paving the way for gravitational collapse. Particles pile up as they drift radially inward. Gas turbulence stirs particles, but can also seed collapse by clumping them. While the feedback of solids on gas triggers vertical shear instabilities that obstruct self-gravity, this same feedback triggers streaming instabilities that strongly concentrate particles. Numerical simulations find that solids 10-100 cm in size gravitationally collapse in turbulent disks. We outline areas for progress, including the possibility that still smaller objects self-gravitate.