Gravitational instability in a planet-forming disk

• Published in: Nature (London) 2024-09, Vol.633 (8028), p.58-62
• Main Authors: Speedie, Jessica, Dong, Ruobing, Hall, Cassandra, Longarini, Cristiano, Veronesi, Benedetta, Paneque-Carreño, Teresa, Lodato, Giuseppe, Tang, Ya-Wen, Teague, Richard, Hashimoto, Jun
• Format: Article
• Language: English
• Subjects: 639/33/34/4122; 639/33/34/862; Accretion disks; Cooling; Dynamic structural analysis; Gravitation theory; Gravitational instability; Gravity; Humanities and Social Sciences; Instability; Kinematics; Morphology; Motion stability; multidisciplinary; Planet formation; Planets; Protoplanets; Radio telescopes; Science; Science (multidisciplinary); Simulation; Star formation; Stellar mass; Structural stability; Velocity
• ISSN: 0028-0836; 1476-4687; 1476-4687
• DOI: 10.1038/s41586-024-07877-0

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. Observations of gravitational instability in the disk around AB Aurigae using deep observations of 13 CO and C 18 O line emission provide evidence that giant protoplanets can be formed from collapsing fragments of vast spiral arms.