Astronomers spot ‘moon-forming disc’ around an exoplanet for the first ever time

Cose-up view of the moon-forming disk surrounding PDS 70c, a young Jupiter-like planet nearly 400 light-years away (ALMA (ESO/NAOJ/NRAO)/Benisty et al)

Astronomers have spotted a moon-forming disc around a planet outside of our solar system.

It is the first time that scientists can be sure they have seen such a disc – which could be forming new satellites around its host world – around an exoplanet.

The new findings could help show how moons and planets are able to form in solar systems when they are still young. That process still remains largely mysterious to scientists.

Astronomers will now be able to watch the disc through its life and hope to see those moons as they form.

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

The new breakthrough used the Atacama Large Millimetre/submillimeter Array, or ALMA, to gather pictures of the phenomenon in precise detail.

“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,” she adds.

The region is strictly known as a circumplanetary disk, and is made up of gas and dust that wraps around the planet.

The newly discovered example is around a planet known as PDS 70c. That is one of two huge, Jupiter-like planets that orbit around a star almost 400 light years away.

Scientists had previously suspected the planet had its own disc around it, but it was too difficult to tell it apart from the environment around it. The new discovery marks the first time scientists can be sure.

The detailed images allowed the team to measure the disc’s size and its mass – finding that it is roughly as big as the distance between the Sun and the Earth, and has nough material to form up to three Moon-sized satellites.

“We used the millimeter emission from cool dust grains to estimate how much mass is in the disk and therefore, the potential reservoir for forming a satellite system around PDS 70c,” said Sean Andrews, a study co-author and astronomer at the Center for Astrophysics (CfA).

Planets are formed out of discs of dusts that wrap around young stars, gathering up clumps of material and sticking them together to form worlds like those in our solar system.

But as they do, planets can also acquire their own disc. They are useful to the planet because they help keep a check on how much material falls onto it.

They can also grow their own moons, as pieces of material collide with each other repeatedly.

But that process still remains mysterious. “In short, it is still unclear when, where, and how planets and moons form,” said European Southern Observatory research fellow Stefano Facchini, also involved in the research.

Astronomers have found more than 4,000 exoplanets, usually by spotting them as they pass in front of their star. But they are almost always in mature systems – the system at the centre of the new study is unique among them in that they are still being formed.

Scientists hope that they can see such systems in even more detail soon, with the Extremely Large Telescope that is currently being built in the Chilean desert.

“The ELT will be key for this research since, with its much higher resolution, we will be able to map the system in great detail,” says co-author Richard Teague, a co-author and Submillimeter Array (SMA) fellow at the CfA.