The First Kinematic Evidence of Gravitational Instability in Planet Formation Found

The evidence confirms Dr. Cassandra Hall's prediction made in 2020.

In 2020, UGA Assistant Professor of Computational Astrophysics Cassandra Hall and her team used hydrodynamics simulations coupled with radiative transfer calculations to show that a planet-forming disk undergoing gravitational instability has clear kinematic signatures in molecular line observations. Detecting these signatures would provide the clearest evidence for the occurrence of gravitational instability in planet-forming disks, and provide a crucial way to measure disk masses.

Her prediction was confirmed this year with new observations of the young stellar object Elias 2-27 using the Atacama Large Millimeter/submillimeter Array (ALMA). During the observations, scientists confirmed that the Elias 2-27 star system—a young star located less than 400 light-years away from Earth in the constellation Ophiuchus—was exhibiting evidence of gravitational instabilities which occur when planet-forming disks carry a large fraction of the system’s stellar mass.

In a National Radio Astronomy Observatory news release, Dr. Hall, who is a co-author on the research, added that the confirmation of both vertical asymmetry and velocity perturbations—the first large-scale perturbations linked to spiral structure in a protoplanetary disk—could have significant implications for planet formation theory. “This could be a ‘smoking gun’ of gravitational instability, which may accelerate some of the earliest stages of planet formation. We first predicted this signature in 2020, and from a computational astrophysics point of view, it’s exciting to be right.”

One of the barriers to understanding planet formation was the lack of direct measurement of the mass of planet-forming disks, a problem addressed in the new research. The high sensitivity of ALMA Band 6, paired with Bands 3 and 7, allowed the team to more closely study the dynamical processes, density, and even the mass of the disk. Knowing the amount of mass present in planet-forming disks allows researchers to determine the amount of material available for the formation of planetary systems, and to better understand the process by which they form.

Resources

Spiral Arms and a Massive Dust Disk with non-Keplerian Kinematics: Possible Evidence for Gravitational Instability in the Disk of Elias 2-27, Paneque-Carreño et al. ApJ, preview [https://arxiv.org/pdf/2103.14048.pdf]

A Dynamical Measurement of the Disk Mass in Elias 2-27, Veronesi et al. ApJ, preview [https://arxiv.org/pdf/2104.09530.pdf]

About ALMA

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the Ministry of Science and Technology (MOST) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.