The canonical theory for planet formation in circumstellar disks proposes that planets are grown from initially much smaller seeds^1,2,3,4,5. The long-considered alternative theory proposes that giant protoplanets can be formed directly from collapsing fragments of vast spiral arms^{6,7,8,9,10,11} induced by gravitational instability^12,13,14—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^15,16,17,18. Although estimating disk masses has historically been challenging^19,20,21, the motion of the gas can reveal the presence of gravitational instability through its effect on the disk-velocity structure^22,23,24. 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.

星周盘中行星形成的经典理论提出，行星最初是由较小的种子生长而成的^1,2,3,4,5 。长期考虑的替代理论提出，如果圆盘在引力上不稳定，那么巨大的原行星可以直接由引力不稳定引起的巨大旋臂^{6、7、8、9、10、11}的塌缩碎片形成^12、13、14 。为了实现这一点，圆盘与中心恒星相比必须是巨大的：圆盘与恒星的质量比为 1:10，被广泛认为是触发引力不稳定、激发大量非开普勒动力学并产生显着影响的粗略阈值。旋臂^{15、16、17、18} 。尽管估计盘质量历来具有挑战性^19,20,21 ，但气体的运动可以通过其对盘速度结构的影响揭示重力不稳定的存在^22,23,24 。在这里，我们利用阿塔卡马大型毫米/亚毫米阵列 (ALMA) 对¹³ CO 和 C ¹⁸ O 线发射的深度观测，提出了 AB Aurigae 周围盘中重力不稳定的运动学证据。观察到的运动学信号与模拟和分析模型的预测非常相似。通过定量比较，我们推断出圆盘质量高达天空中 1 英寸到 5 英寸范围内恒星质量的三分之一。