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



Author: E. Chiang UC Berkeley, A. Youdin CITA

Source: https://arxiv.org/







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