An image made by the Very Large Telescope in Chile shows a forming planet, the bright spot at right. The overpowering light of the host star is blocked out by a coronagraph inside the telescope. (ESO/A. Müller et al.)

Astronomers have many theories about how planets are formed within the gas, dust, pebbles and gradually rocks of the circumstellar disks that encircle a star after it has been born.  While the general outlines of this remarkable process are pretty well established, many questions large and small remain unanswered.

One is how and when exomoons are formed around these planets, with the assumption that the process that forms planets must also give birth moons.  But the potential moons have been far too small for the current generation of space and ground telescopes to identify.

Now astronomers have detected something almost as significant:  a circumplanetary disk surrounding a young planet that appears to be in the process of making moons.  The moon itself has not been detected, but a forming planet has been found with a ring of dust and gas that surrounds it.  And within that circumplanetary disk, astronomers infer, a moon is possibly being formed.

“Our work presents a clear detection of a disk in which satellites could be forming,” said Dr. Myriam Benisty, an astronomer at the University of Grenoble and the University of Chile.

“The new … observations were obtained at such exquisite resolution that we could clearly identify that the disk is associated with {the exoplanet} and we are able to constrain its size for the first time,” she said in a release.

While the first detection of the planet was made via the European Southern Observatory’s Very Large Telescope in Chile, the more granular observation of the forming planet and its moon-forming disk was made with the Atacama Large Millimeter/submillimeter Array (ALMA), also in Chile.

This ALMA image shows the young PDS 70 planetary system. The system features a star at its center and at least two planets orbiting it, PDS 70b (not visible in the image) and PDS 70c, surrounded by a circumplanetary disk (the dot to the right of the star). Image credit: ALMA / ESO / NAOJ / NRAO / Benisty et al.)

The finding, published in the Astrophysical Journal Letters, came via direct imaging — in effect through extremely high power photography rather through the indirect methods much more common in exoplanet astronomy. The system is named PDS 70 and is nearly 400 light-years from Earth.

And the nature of the system “offers us a unique opportunity to observe and study the processes of planet and satellite formation,” said Stefano Faccnini of the European Southern Observatory and a co-author on the paper.

The reason why:  It has two exoplanets detected so far and they appear to still be growing.

“More than 4,000 exoplanets have been found until now, but all of them were detected in mature system,” said Dr. Miriam Keppler, an astronomer at the Max Planck Institute for Astronomy and another co-author.

“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.”

The inner planet, PDS 70b, is located within the disk gap at a distance of about 22 astronomical units from the star, similar to the orbit of Uranus in our Solar System. (An astronomical unit is the distance between the sun and Earth.)

The outer planet, PDS 70c, is located near the outer edge of the disk gap at 34 AU from the star, similar to Neptune’s distance from our sun.

Benisty and colleagues then found the circumplanetary disk around PDS 70c,  which has about the same diameter as the distance from our sun to the Earth and enough mass to form up to three satellites the size of the moon.

This ALMA image shows a close-up view on the circumplanetary disk surrounding PDS 70c. (ALMA / ESO / NAOJ / NRAO / Benisty et al.)

Stepping back a bit:  Our solar system, about 4.6 billion years old, is somewhat middle-aged for the galaxy.  The PDS 70 system is much younger, with the star forming  only six millions years ago out of the gravitational collapse of a molecular cloud, leading to the initiation of fusion reactions.

This star-formation process produces disks of gas and dust that encircles the star.  New planets are slowly pieced together within this circumstellar disk, with the gas and dust coming together to form into progressively larger bodies (protoplanets) through multiple collisions. Astronomers have observed aspects of this process in the form of cavities carved out of these disks as the gravitational forces of the planets pull material onto them.

In a similar way, many moons are inferred to form in the disks around newly formed planets.  (Our moon, of course, followed a different path, forming after the collision between Earth and a Mars-sized object given the name Theia.)

But astronomers do not yet fully understand the details of these planet and moon formation processes, with Facchini, the astronomer at ESO and co-author, saying that  “In short, it is still unclear when, where, and how planets and moons form.”

Some progress towards understanding, however, has now been made.

“These new observations are extremely important to prove theories of planet formation that could not be tested until now,” said co-author Jaehan Bae, an astronomer in the Earth and Planets Laboratory at the Carnegie Institution for Science.

A team of astronomers have unambiguously detected a moon-forming disc around a distant planet for the first time. The planet is a Jupiter-like gas giant, hosted in a system still in the process of being formed. This European Southern Observatory video visualizes the discovery.

There are several determinative processes under way as a planet forms, the first being the accretion of material in the circumstellar disk into a protoplanet and the rate at which it proceeds. The second is differentiation, as the protoplanet separates into layers ranging from a metallic core to a silicate shell and, finally, an atmospheric envelope.

The manner in these processes play out, and the inventory of chemicals in the disk of a particular young star, have been studied as factors in whether or not the planet, or moon, might evolve into a habitable place.

The Very Large Telescope in Chile’s Atacama Desert was used to find both exoplanets, with PDS 70b’s discovery announced in 2018, and PDS 70c’s in 2019.

A month later, scientists using ALMA, a collection of 66 dishes and antenna for radio astronomy in the foothills of the Andes mountains of Chile,  reported that radio waves emitted by fine dust were emanating from around PDS 70c — promising evidence that a moon-making debris disk surrounds it, and may have already produced a moon.