The multifaceted planetesimal formation process
Accumulation of dust and ice into planetesimals is an important step in the planet formation process. Planetesimals are the seeds of both terrestrial planets and the solid cores of gas and ice giants. Leftover planetesimals in the form of asteroids, Kuiper belt objects and comets provide a unique record of the state of the solar system during formation, while debris from planetesimal collisions around other stars signposts that the planetesimal formation process, and hence planet formation, is ubiquitous in the Galaxy. Planetesimal formation extends from dust to sizes which can undergo run-away accretion, the latter ranging from 1 km to several thousand km dependent on gravitational torques from the turbulent gas. Preplanetesimals face many barriers during this growth, arising mainly from inefficient sticking, fragmentation and radial drift. Two promising growth pathways are coagulation-fragmentation, where particles grow larger by sweeping up collisional fragments, and fluffy growth, where particle cross sections and sticking are enhanced by a low internal density. A wide range of particle sizes, from mm to m, experience concentration events in the turbulent gas flow. Overdense regions can fragment gravitationally into bound particle clusters with typical masses equivalent to planetesimals of 100 to 1000 km sizes. We propose a hybrid model where dust growth starts unaided by self-gravity but later proceeds inside self-gravitating particle clumps to yield the final initial mass function of planetesimals.