Astronomers confirm the discovery of a moon-forming disk around a small exoplanet

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Artist’s interpretation of the planetary disk. Motion picture: ESO/L. Calzada, Alma (ESO/NAOJ/NRAO) / Benisty et al./Gizmodo

Two years ago, astronomers discovered an apparent planetary disk around a planet outside our solar system, but they weren’t sure. Follow-up observations now show that they were right, in a discovery that could improve our understanding of exomoons and how they form in emerging star systems.

Protoplanet PDS70c has a planetary disk, according to new research published in The Astrophysical Journal Letters. Scientists thought a lot in a paper published two years ago, but they were unable to distinguish the disk from its surroundings, which led to suspicion. Follow-up observations by the Atacama Large Millimeter/ Sub-millimeter Array (ALMA) in Chile were instrumental in confirmation.

“Our ALMA observations have been obtained with such remarkable accuracy that we can determine that the disk is associated with the planet and that we are able to constrain its size for the first time,” astronomer Myriam Benisti explained in a statement. Benesti, who led the study, is a researcher at the University of Grenoble in France and the University of Chile.

In the center, the planetary disk surrounding the giant protoplanet. The larger wide ring dominates the right part of the ALMA image. Photo: ALMA (ESO/NAOJ/NRAO) / Benisty et al.

By observing the system in millimeter light (the wavelength off between the microwave and the infrared portion of the spectrum), the team was able to confirm the presence of copious amounts of dust in orbit around PDS70c. If the disk were in our solar system, it would extend from the center of the sun to a little further than the Earth’s orbit.

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This Jupiter-like protoplanet, along with its giant brother, PDS70b, orbits a 5 million-year-old young star located 370 light-years from Earth. The star itself hosts a stellar disk, within which the two protoplanets absorb material, creating pockets or cavities around them. Both PDS70b and PDS70c are gas giants, both far from their host star, at distances of 22 AU and 34 AU, respectively. By comparison, Neptune is 30 astronomical units from our Sun, with 1 AU being the average distance from Earth to the Sun.

The planetary disk around PDS70c is about 500 times larger than Saturn’s rings, and as the new paper notes, the amount of mass in this disk could make up as many as three outer moons the size of our moon.

PDS 70 as photographed by ALMA. It shows the donut-like disk surrounding the host star, the protoplanet tucked away along the right interior. Photo: ALMA (ESO/NAOJ/NRAO) / Benisty et al.

Interestingly, the new research rules out a planet-wide disk around PDS70b. Scientists say that his greedy brother, PDS70c, is not a good participant, and that he devoured the necessary building materials for himself.

According to Stefano Faccini, co-author of the paper and research fellow at the European Southern Observatory, “This system provides us with a unique opportunity to observe and study the formation processes of planets and satellites.”

Indeed, confirmation of a planetary disk around PDS70c could add new clarity to ongoing theories about exomoons and how they form. In one possible scenario, protoplanets would steal mass from the disk surrounding the surrounding star, and the resulting mass would fall onto the forming planet. But some bits of the matter stay in orbit, and clump together to form natural satellites.

It’s an exciting discovery, but the team isn’t finished with the PDS70 yet. They are already looking forward to ESO’s Very Large Telescope, which is currently being built in the Atacama Desert in Chile. With this instrument, astronomers hope to obtain a high-resolution scan of the system and build 3D views that show the movement of gas through the planet’s disk.

More: Astronomers wonder: Could moons have moons?

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