A Circumplanetary Disk Candidate Spotted with ALMA

Astronomers may have found a gaseous disk around a planet orbiting the young star AS 209. This is one of just four circumplanetary disk candidates discovered so far, and these observations enable the first mass estimate of a circumplanetary disk’s gas.

Views of AS 209 in emission from (clockwise from top left) CO, HCN, C2H, HC3N, and H2CO. Click to enlarge. [Adapted from Öberg et al. 2021]

Disks within Disks

The disks of gas and dust that collect around young stars are the sites of planet formation. Thanks to high-resolution radio observations from instruments like the Atacama Large Millimeter/submillimeter Array (ALMA), we can study these disks in incredible detail and attempt to catch planet formation in the act.

Recently, researchers used ALMA to observe five nearby circumstellar disks at high resolution as part of the Molecules with ALMA at Planet-forming Scales (MAPS) observing program. Using MAPS data, a team led by Jaehan Bae (University of Florida) finds evidence that the 1.6-million-year-old star AS 209 hosts a disk within a disk — a circumplanetary disk around a hidden planet orbiting far from the central star.

A Planet, Perhaps?

Bae and collaborators analyzed new ALMA observations of radio emissions from three forms of carbon monoxide gas and archival observations of 1.25-millimeter dust emission. Several pieces of evidence suggest that the team has detected carbon monoxide gas belonging to a circumplanetary disk within the larger disk surrounding AS 209:

  1. The location of the circumplanetary disk (CPD) candidate in the 13CO observations. [Adapted from Bae et al. 2022]

    The carbon monoxide (12CO) observations reveal a 78-au-wide gap in the gas of AS 209’s disk at a distance of 200 au from the central star, in the same location as a gap seen in scattered-light images of the disk. This suggests that a young planet is carving a gap in the disk.
  2. The 12CO data show localized changes in the velocity of the gas near this gap, which can result from an embedded planet disturbing the gas of the disk.
  3. The 13CO observations show a point source 206 au from the central star — right in the middle of the 12CO gap and close to the velocity perturbations. Given ALMA’s resolving power, the point-like nature of the object indicates that it is no larger than 14 au in diameter.

Left: Comparison of the location of the gap (a) and the circumplanetary disk candidate (b). Middle: 12CO emission with the location of the gap marked with the gray dashed lines. Right: A zoomed-in version of the previous panel with the 13CO emission contours placed on top. Click to enlarge. [Bae et al. 2022]

Point-Source Possibilities

Based on these observations, Bae and collaborators estimated that the circumplanetary disk contains roughly 30 Earth masses of gas and just 2.2 Moon masses of dust, suggesting a low dust-to-gas ratio of 0.0009. At a temperature of 35K, the gas is 13K warmer than expected given the distance from the central star. This likely means that there is an additional heat source in the vicinity of the circumplanetary disk, such as accretion by the planet or turbulence within the disk.

AS 209 hosts just the fourth circumplanetary disk candidate ever found, and this work marks the first time that researchers have detected the gas within a circumplanetary disk and estimated its mass. Future observations with ALMA and JWST should help answer lingering questions about the structure of the circumplanetary material, the mass and age of the young planet, and how the planet formed at such a large distance from its parent star.


“Molecules with ALMA at Planet-forming Scales (MAPS): A Circumplanetary Disk Candidate in Molecular-line Emission in the AS 209 Disk,” Jaehan Bae et al 2022 ApJL 934 L20. doi:10.3847/2041-8213/ac7fa3