Gravitational instability in a planet-forming disk

The canonical theory for planet formation in circumstellar disks proposes that planets are grown from initially much smaller seeds1-5. The long-considered alternative theory proposes that giant protoplanets can be formed directly from collapsing fragments of vast spiral arms6-11 induced by gravitational instability12-14-if the disk is gravitationally unstable. For this to be possible, the disk must be massive compared with the central star: a disk-to-star mass ratio of 1:10 is widely held as the rough threshold for triggering gravitational instability, inciting substantial non-Keplerian dynamics and generating prominent spiral arms15-18. Although estimating disk masses has historically been challenging19-21, the motion of the gas can reveal the presence of gravitational instability through its effect on the disk-velocity structure22-24. Here we present kinematic evidence of gravitational instability in the disk around AB Aurigae, using deep observations of 13CO and C18O line emission with the Atacama Large Millimeter/submillimeter Array (ALMA). The observed kinematic signals strongly resemble predictions from simulations and analytic modelling. From quantitative comparisons, we infer a disk mass of up to a third of the stellar mass enclosed within 1″ to 5″ on the sky.