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1st moon-forming disk around an exoplanet

Bright dot with fainter smudge around it, and larger brighter regions on either side.
Closer view of the young giant planet PDS 70c (bright dot), with the circumplanetary disk of dust and rocky debris around it. There is enough material in the disk for three moons the size of our own moon. Image via ALMA (ESO/ NAOJ/ NRAO)/ Benisty et al.

There are hundreds of moons in our solar system, from the size of asteroids to larger than Mercury. What about distant solar systems? Do their planets have moons, too? Astronomers have discovered thousands of exoplanets, or worlds orbiting distant stars. And it would seem logical that some should have moons – exomoons – also. So far, though, astronomers haven’t confirmed any exomoon. Such moons would tend to be smaller than their planets, and exoplanets themselves aren’t easy to find. But now, astronomers have found the first clear evidence of a moon-forming disk around a young giant exoplanet. The European Southern Observatory announced the exciting finding on July 22, 2021.

The star is PDS 70, a very young, newly formed T Tauri star (variable) star – in the direction to the constellation Centaurus, located nearly 400 light-years away. The planet is called PDS 70c. It’s one of two giant, Jupiter-like planets orbiting this young star.

The researchers’ paper was published in peer-reviewed The Astrophysical Journal Letters on July 22, 2021.

Moon-forming disk around an exoplanet

The researchers made the discovery using the ALMA telescope in Chile. Myriam Benisty, a researcher at the Institute of Planetology and Astrophysics of Grenoble in France, who led the new research, said in a statement:

Our work presents a clear detection of a disk in which satellites could be forming. Our ALMA observations were obtained at such exquisite resolution that we could clearly identify that the disk is associated with the planet and we are able to constrain its size for the first time.

The discovery isn’t quite that of exomoons themselves yet. Instead, it’s the circumplanetary disk in which exomoons might ultimately form. There were previous hints that the large exoplanet PDS 70c had a disk. But astronomers weren’t able to confirm the disk until now. The other known planet in the system is PDS 70b. As explained in the paper:

These new observations provide the most compelling evidence of the presence of a circumplanetary disk around an accreting planet to date.

Circumplanetary disks are similar to cirumstellar disks, which are disks of rocky and dusty debris surrounding stars. Moons form in circumplanetary disks via the same processes that planets form in circumstellar disks. That is, they form as solid materials in the disk that collide and stick together. Ultimately, as the moon or planet grows, it becomes massive enough that its own gravity takes over, sweeping up more solid bits and gradually forming round worlds in space.

Enough material for 3 moons

The researchers found that the disk around PDS 70c is quite substantial. It has enough mass to create three moons the size of Earth’s moon. That could mean that this planet will have multiple moons in the future, just as the giant planets in our own solar system do.

Chart showing confirmed and provisional moons for each planet in our solar system.
Confirmed moons and provisional moons around the planets in our solar system. Astronomers rely on the foundational belief that planets in distant solar systems form moons via the same processes as our own solar system’s planets. Image via NASA.

Probing moon and planet formation

According to Benisty and her colleagues, these results provide valuable new clues as to how both moons and planets form. Co-author Jaehan Bae at the Earth and Planets Laboratory of the Carnegie Institution for Science commented:

These new observations are also extremely important to prove theories of planet formation that could not be tested until now.

Scientists know that planets form in huge disks of dust that surround young stars, but there are still a lot of questions as to exactly how that happens. As planets form within a circumstellar disk, they can then attract enough dusty debris to create their own circumplanetary disks. It is in those disks that moons are born.

Smiling woman with long hair over one shoulder.
Myriam Benisty at the Institute of Planetology and Astrophysics of Grenoble (IPAG) in France led the new research that discovered the first circumplanetary moon-forming disk around an exoplanet. Image via M. Benisty.

As Stefano Facchini at the European Southern Observatory (ESO) said:

In short, it is still unclear when, where, and how planets and moons form.

This system therefore offers us a unique opportunity to observe and study the processes of planet and satellite formation.

Moon-forming-disk around an exoplanet.
Left, the circumstellar disk around the star PDS 70. Right, the inset showing the circumplanetary disk – a moon-forming disk – around the young giant planet PDS 70 c (bright dot with material around it). Image via ALMA (ESO/ NAOJ/ NRAO)/ Benisty et al.

Young planets … and young moons?

Both of the giant planets in the PDS 70 system are young and still forming, reminiscent of when the planets in our solar system were still forming billions of years ago. Another co-author, Miriam Keppler, a researcher at the Max Planck Institute for Astronomy in Germany, noted this, saying:

More than 4,000 exoplanets have been found until now, but all of them were detected in mature systems. PDS 70b and PDS 70c, which form a system reminiscent of the Jupiter-Saturn pair, are the only two exoplanets detected so far that are still in the process of being formed.

Both planets, PDS 70b and PDS 70c, have been known about for some time. Astronomers discovered them in 2018 and 2019 respectively, using ESO’s Very Large Telescope (VLT). The moon-forming disk, however, wasn’t found until it showed up in these new ALMA observations.

Bright star in upper right, with thousands of more distant stars in background.
This image shows the sky around the faint orange dwarf star PDS 70 (in the middle of the image). The bright blue star to the right is Chi Centauri. Image via ESO/ Digitized Sky Survey 2/ D. De Martin.

When will we find the 1st exomoon?

Interestingly, while the giant planet PDS 70c has a circumplanetary disk, it appears that the other known planet in the system, PDS 70b, does not. The researchers suggest there might not have been enough material available from the circumstellar disk to create moon-forming disks for both planets. Perhaps PDS 70b will end up forming without moons, while PDS 70c might have several. In our solar system, all of the giant planets have many moons each. It would be interesting to find out what the differences were in the PDS 70 system.

Although PDS 70c now has the first confirmed moon-forming disk, the disk’s existence suggests many more such disks out there, yet to be found. It’s perhaps worth noting that astronomers found planet-forming disks, too, before they found the first exoplanets. And so, as technology advances, it seems we’re getting closer to finding the first verified exomoons as well. That’ll be an exciting discovery, too!

Bottom line: Astronomers have found the first solid evidence for a moon-forming disk around a distant exoplanet, providing new clues about how both moons and planets form.

Source: A Circumplanetary Disk Around PDS70c



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