ALMA observation could offer key insights into how moons are formed

23 Jul 2021

The PDS system is pictured. The star is at the centre and PDS 70c can be seen to the right of the star. All of the bodies are orange, and the picture is primarily a large, circumstellar disc that surrounds the planets and the star.

The PDS 70 system. Image: ALMA (ESO/NAOJ/NRAO)/Benisty et al

Researchers pictured what could be a moon-forming disc in ‘exquisite resolution’, which might shed light on how planets and moons come to be.

The Atacama Large Millimeter/submillimeter Array (ALMA) has potentially detected a moon-forming disc around a planet outside of our solar system for the first time.

This disc surrounds PDS 70c, one of two giant planets orbiting a star almost 400 light-years away. Astronomers had found evidence of this disc before but couldn’t clearly pick the object out from its surroundings.

“Our work presents a clear detection of a disc in which satellites could be forming,” said Myriam Benisty, who led the new research published yesterday (22 July) in The Astrophysical Journal Letters.

“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.”

How moons are formed wasn’t the only object of interest in the research, however. It is important to note the interconnected nature of moons and planets in space.

Planets form in dusty discs around young stars. By taking up material from these discs, they grow over time. During this process, the planets may acquire a disc of their own and moons can form within that disc. These mechanisms aren’t yet fully understood, however.

“It is still unclear when, where, and how planets and moons form,” said European Southern Observatory (ESO) research fellow Stefano Facchini.

For instance, the team noted that PDS 70b, the other giant planet, doesn’t seem to have a disc, possibly because it was deprived of dust material by PDS 70c.

The PDS system is pictured on the left. The star is at the centre and PDS 70c can be seen to the right of the star. All of the bodies are orange, and the picture is primarily a large, circumstellar disc that surrounds the planets and the star. The right image is a zoomed-in shot of PDS 70c. It is small compared to the surrounding material which takes up much of the right-hand side of the image.

Wide (left) and close-up (right) views of PDS 70c. Image: ALMA (ESO/NAOJ/NRAO)/Benisty et al

The research team also found that the disc’s diameter is approximately the distance from Earth to the sun. They noted it has enough mass to form three of our moons.

Jaehan Bae, a researcher from the Earth and Planets Laboratory of the Carnegie Institution for Science and author on the study, said: “These new observations are also extremely important to prove theories of planet formation that could not be tested until now.”

ALMA is partnered with the ESO and is based in the Chajnantor Plateau in northern Chile. This location was chosen as low frequency waves are better received at high altitudes and in dry climates, which is necessary for accurate observations.

A deeper understanding of the PDS 70 system and how moons are formed will be achieved by the ESO’s Extremely Large Telescope, currently under construction on Cerro Armazones in the Chilean Atacama desert. This telescope will be able to map the system in much greater detail because of its higher resolution.