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 seeds 1-5 . The long-considered alternative theory proposes that giant protoplanets can be formed directly from collapsing fragments of vast spiral arms 6-11 induced by gravitational instability 12-14 -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 arms 15-18 . Although estimating disk masses has historically been challenging 19-21 , the motion of the gas can reveal the presence of gravitational instability through its effect on the disk-velocity structure 22-24 . Here we present kinematic evidence of gravitational instability in the disk around AB Aurigae, using deep observations of 13 CO and C 18 O 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.