Rapid planetesimal formation in turbulent circumstellar discs

The initial stages of planet formation in circumstellar gas discs proceed via dust grains that collide and build up larger and larger bodies1. How this process continues from metre-sized boulders to kilometre-scale planetesimals is a major unsolved problem2: boulders stick together poorly3, and spiral into the protostar in a few hundred orbits due to a head wind from the slower rotating gas4. Gravitational collapse of the solid component has been suggested to overcome this barrier1, 5, 6. Even low levels of turbulence, however, inhibit sedimentation of solids to a sufficiently dense midplane layer2, 7, but turbulence must be present to explain observed gas accretion in protostellar discs8. Here we report the discovery of efficient gravitational collapse of boulders in locally overdense regions in the midplane. The boulders concentrate initially in transient high pressures in the turbulent gas9, and these concentra-