I investigate the formation process of planetesimals through the collapse of pebble clouds in protoplanetary disks. Massive clouds collapse fast and have fragmenting pebble-pebble collisions. The resulting planetesimal consists of both pebbles and dust, allowing the interior to be compact. Low-mass clouds collapse more slowly, only have bouncing collisions and result in a porous pebble-pile.
Planet formation occurs in protoplanetary disks of gas, dust and ice which orbit around newborn stars. Pebbles are thought to form through coagulation of dust grains in these disks. Such particles drift toward the star on short timescales because of gas drag. It has been proposed that hydrodynamical effects can form pebble clouds and that planetesimals form from the collapse of such clouds.