Astronomers may have spotted a distant alien moon being born

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PDS 70 star system

This image shows the star PDS 70 (centre) and the planet PDS 70 c, surrounded by a circumplanetary disc (the dot to the right of the star). The whole system is surrounded by a circumstellar disc

ALMA (ESO/NAOJ/NRAO)/Benisty et al.

We may be watching the birth of a moon for the first time. Astronomers have spotted a disc of debris around a distant planet called PDS 70 c, and it is massive enough that the young exoplanet might be in the process of forming exomoons.

When a new stellar system is forming, the planets coalesce out of a cloud of debris called a circumstellar disc. Then, the planets can suck gas and dust from that cloud to form their own circumplanetary discs, which feed the planets’ growth and provide the material for moons to form.

The star PDS 70, which is about 370 light-years from Earth, has provided researchers with a unique laboratory to study this process. Its two giant planets, PDS 70 b and c, are the only two that have been observed while still embedded in their circumstellar disc. Now Myriam Benisty at the Université Grenoble Alpes in France and her colleagues have confirmed that PDS 70 c – and maybe PDS 70 b as well – also has a circumplanetary disc.

“We know lots of planets, but those are done planets, and we have to use models to try to understand how planets form by looking at the final product,” says Alessandro Morbidelli at Côte d’Azur Observatory in France, who wasn’t involved in this work. “With these two, we are directly seeing how giant planets and their moons form, so these planets are exceptional.”

The researchers spotted this disc using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. There had been hints of a circumplanetary disc there before, but never anything conclusive.

They found that, depending on the size of the dust grains in the disc around PDS 70 c, it probably contains a total dust mass that is about 0.7 to 3.1 per cent the mass of Earth. “We cannot identify any moons that are being formed, but there is enough material to form them, and it is very likely that satellites are forming there,” says Benisty. The planet is a few times more massive than Jupiter, so it may eventually form many moons just like Jupiter has, she says.

“Looking outward from the planet, it would be similar to how the Milky Way looks on a really dark night, this shining stripe across the night sky, but it would be much, much broader,” says team member Richard Teague at the Harvard-Smithsonian Center for Astrophysics in Massachusetts. PDS 70 b probably also has a disc, but it isn’t as bright, which could mean that it is made of smaller dust grains or just gas, says Benisty.

The researchers also found streams of dust flowing from the outer circumstellar disc towards the star, into the area where, if this stellar system is like our own, smaller rocky planets could form. “The streamers are bringing material from the outer disc to the inner disc, and that is not only important for the formation of Earth-like planets, but also the star is still a baby star so it’s still accreting matter to grow,” says Benisty.

This system provides us with a window to study the formation of planets and moons generally, but with its two giant planets mirroring Jupiter and Saturn it is also reminiscent of our own solar system, albeit larger. It could help us understand how the planets and moons in our solar system formed and evolved.

Journal reference: The Astrophysical Journal Letters, in print

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