Rapid planetesimal formation in turbulent circumstellar disks

The accretion by a protoplanetary disk of material from its surrounding natal envelope has been observed for the first time in the Class 0 protostar NGC 1333–IRAS 4B. This is a crucial early step in the formation of stars and planetary systems, through which all such systems are thought to go. Observations with the Spitzer Space Telescope reveal a rich emission-line mid-infrared spectrum from water vapour, which indicates an origin in an extremely dense disk surface, heated by a shock from the infalling envelope material. Once a protoplanetary disk has formed, planetesimals are thought to develop as the products of collisions between dust grains form ever larger objects. But current theories fail at the point where metre-sized boulders are formed: theory has them falling into the central protostar too quickly to form kilometre-sized planetesimals. New computer simulations suggest that the interaction of the gas disk with the boulders creates extremely dense regions. There the boulders are so close to each other that their mutual gravity draws them together into solid objects of many kilometres in size, forming directly the planetesimals that serve as building blocks of planets.