From Grains to Planetesimals: Les Houches Lecture

TL;DR

This review discusses the complex processes and challenges involved in transforming dust grains into planetesimals within protoplanetary disks, highlighting recent advances and unresolved issues.

Contribution

It provides a comprehensive overview of current theories, mechanisms, and numerical simulations related to planetesimal formation, emphasizing recent progress and remaining challenges.

Findings

Meter-sized solids infall within a hundred years is a key constraint.

Turbulent gas interactions can lead to particle clumping and gravitational collapse.

Various mechanisms like vortices and drag instabilities can concentrate particles.

Abstract

This pedagogical review covers an unsolved problem in the theory of protoplanetary disks: the growth of dust grains into planetesimals, solids at least a kilometer in size. I summarize timescale constraints imposed on planetesimal formation by circumstellar disk observations, analysis of meteorites, and aerodynamic radial migration. The infall of ~meter-sized solids in a hundred years is the most stringent constraint. I review proposed mechanisms for planetesimal formation. Collisional coagulation models are informed by laboratory studies of microgravity collisions. The gravitational collapse (or Safronov-Goldreich-Ward) hypothesis involves detailed study of the interaction between solid particles and turbulent gas. I cover the basics of aerodynamic drag in protoplanetary disks, including radial drift and vertical sedimentation. I describe various mechanisms for particle concentration in gas disks -- including turbulent pressure maxima, drag instabilities and long-lived anticylonic vortices. I derive a general result for the minimum size for a vortex to trap particles in a sub-Keplerian disk. Recent numerical simulations demonstrate that particle clumping in turbulent protoplanetary disks can trigger gravitational collapse. I discuss several outstanding issues in the field.