Moon-forming disc detected around another world

For the first time, astronomers have unambiguously spotted a disc of gas and dust around a planet in a distant solar system – which could give us insight into how exomoons form.

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

Benisty led the research, which used the sharp eyes of the Atacama Large Millimetre/submillimetre Array (ALMA) in Chile to observe the disc around the exoplanet PDS 70c. This planet is one of two giant Jupiter-like worlds in orbit around an infant star 400 light-years away. Most exoplanets discovered so far orbit mature stars, but this system is a mere 5.4 million years old, making its two worlds the only known exoplanets that are still in the process of forming.

A newborn solar system contains a disc of gas and dust spinning around the star, leftover from its formation. The material in this disc can collide and stick together into progressively larger bodies, eventually forming planets, which can have their own, smaller circumplanetary discs from which satellites – exomoons – can coalesce.

In 2019, astronomers first reported that ALMA had seen hints of such a moon-forming disc around PDS 70c, though it was difficult to separate the disc from the surrounding environment.

Now, Benisty says, the new 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”.

Specifically, ALMA was able to see the system in light with wavelengths less than a millimetre across, between infrared light and radio waves. This enabled the team to study the cool dust grains making up the disc around PDS 70c.

The study, which appears in The Astrophysical Journal Letters, found that the disc’s diameter is equal to the distance from the Earth to the Sun (about 150 million kilometres), and it has enough material to form three Moon-sized satellites.

These new observations also offer astronomers the chance to explore previously untested theories of planetary formation. Although they’ve got the general gist of how planets and moons form, the details of these processes are still fuzzy.

“In short, it is still unclear when, where and how planets and moons form,” says co-author Stefano Facchini from the European Southern Observatory. “This system therefore offers us a unique opportunity to observe and study the processes of planet and satellite formation.”

The team also found that a sister planet, PDS 70b, doesn’t seem to have a moon-forming disc of its own – suggesting that perhaps PDS 70c gobbled up most of the dusty material for itself.

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