The dust and gas in protoplanetary disks

Planets form as the dust grains in a protoplanetary disk grow into pebbles and then finally into planets. Because small dust grains interact with gas (via the drag it imparts), the gas in protoplanetary disks influences the distribution of small grains and hence the growth of planets. Astronomers trying to unravel how dust-gas interactions affect planet development are particularly interested in studying the disk thickness (its "vertical height") versus the distance from the star; the disk flares outward in most cases where the central star dominates the mass of the system. By independently measuring the heights of the gas and the small dust grains, astronomers can study fundamental disk characteristics such as the gas-to-dust mass ratio and turbulence in the disk.