In a first, ALMA spies huge moon-forming disc around exoplanet

A wide-angle view of the PDS 70 system, at left, shows a large circumstellar disc of dusty debris surrounding a young star 400 light years from Earth. As the close-up view at right shows, a Jupiter-class exoplanet is forming just inside the circumstellar disc that, in turn, is surrounding by its own dust cloud containing enough raw material to form three satellites the size of Earth’s Moon. Image: ALMA (ESO/NAOJ/NRAO)/Benisty et al.

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have spotted the unambiguous presence of a disc around a Jupiter-like exoplanet 400 light years away that could provide the raw material for up to three satellites the size of Earth’s Moon.

“Our work presents a clear detection of a disc in which satellites could be forming,” said team leader Myriam Benisty, a researcher at the University of Grenoble, France, and at the University of Chile. “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.”

The circumplanetary disc in question surrounds an exoplanet known as PDS 70c, one of two known Jupiter-class planets in the system. There were hints of a moon-forming disc around PDS 70c in earlier observations, but it was far from certain.

The new ALMA observations confirm its presence, showing it to have a diameter similar to the distance between Earth and Sun, or one astronomical unit, while the PDS 70 system as a whole is some 170 astronomical units across. For comparison, the diameter of our solar system out to the orbit of Pluto is about 80 astronomical units.

Planets are believed to form in dusty discs of material around young stars. As the planets pull in material and grow, they can acquire their own circumplanetary discs, which help regulate the amount of material falling in while providing material for the formation of moons.

Interestingly, the other known planet in the PDS 70 system, PDS 70b, does not appear to have a disc, indicating it likely was starved of excess dust in its birth environment by PDS 70c.

“More than 4,000 exoplanets have been found (to date), but all of them were detected in mature systems,” said Miriam Keppler, a researcher at the Max Planck Institute for Astronomy in Germany and co-author of a paper in The Astrophysical Journal Letters.

“PDS 70b and PDS 70c, which form a system reminiscent of the Jupiter-Saturn pair, are the only two exoplanets detected so far that are still in the process of being formed,” she said. “This system therefore offers us a unique opportunity to observe and study the processes of planet and satellite formation.”