It’s the First Confirmation of a Moon-Forming Disk Around an Exoplanet!

This image, taken with the Atacama Large Millimeter/submillimeter Array (ALMA), shows the PDS 70 system located nearly 400 light-years away and still in the process of being formed. The system features a star at its center and at least two planets orbiting it, PDS 70b (not visible in the image) and PDS 70c, surrounded by a circumplanetary disc (the dot to the right of the star). Credit: ALMA (ESO/NAOJ/NRAO)/Benisty et al.

There are well over 4,000 currently-known exoplanets orbiting stars in our galaxy — 4,438 to be exact! (as of July 22, 2021) — but still to date no confirmed* direct observation of any exomoons, despite we know they’re likely present around most planets just as they are in our own Solar System. Now, astronomers have gotten one step closer with the confirmation of a moon-forming disk of material found surrounding a still-forming, Jupiter-like exoplanet, called PDS 70c, which orbits a dwarf star about 400 light-years away in the southern constellation Centaurus.

Although no individual moons have yet been identified in the approximately 1-AU-wide circumplanetary disk around PDS 70c, it’s exactly the type of environment scientists think moons form within.

“Our work presents a clear detection of a disc in which satellites could be forming. Our ALMA observations were obtained at such exquisite resolution that we could clearly identify that the disc is associated with the planet and we are able to constrain its size for the first time.”
— Myriam Benisty, a researcher at the University of Grenoble, France, and at the University of Chile, who led the new research

The image above shows the dwarf star PDS 70 surrounded by a ring of protoplanetary material. Near the three o’clock position is the exoplanet PDS 70c, surrounded by the glow from its own moon-forming disk.

This disk is as wide across as the distance from Earth to the Sun and contains enough mass to form up to three of our Moons.

This image, taken with the Atacama Large Millimeter/submillimeter Array (ALMA), in which ESO is a partner, shows a close-up view on the moon-forming disc surrounding PDS 70c. Credit: ALMA (ESO/NAOJ/NRAO)/Benisty et al.

Another gas giant exoplanet, PDS 70b, is known to be in the process of formation within the same system but it does not exhibit a similar ring—possibly because its sibling has already taken much of the available material in the area for itself. Greedy!

Learn more about this observation in the video below from ESO:

*A preliminary exomoon discovery was announced in October 2018 around the exoplanet Kepler 1625b but later fell into question when further observation data indicated it may not actually exist. Read more here and here.