Astronomers Detect Potential Moon-forming Disk around an Exoplanet

For the first time, astronomers have clearly detected a dusty disk around a young giant planet that may go on to form moons. The results appear in the July 20th Astrophysical Journal Letters.

Disks of gas and dust left over from stellar formation can create circumstellar disks, shrouding a newborn star in planet-making potential. Planets that form carve their way through the dust to trace out rings and other structures, perhaps gathering their own personal dusty disk, called a circumplanetary disk, in the process.

Astronomers had previously found a circumstellar disk around PDS 70, a young star nearly 400 light-years away in the constellation Centaurus, the Centaur. Last year, they also confirmed the presence of two planets — a Jupiter-Saturn pair, dubbed PDS 70b and c. These baby planets dwell in a cavity between two rings of dust, one close to the star, the other farther out. Observers thought they saw hints of a smaller disk around just PDS 70c, but they couldn’t distinguish it from the brighter stellar disk ring nearby.

Now, a team led by Myriam Benisty (University of Chile and University of Grenoble, France) presents high-resolution data from the Atacama Large Millimeter/submillimeter Array (ALMA) that clearly shows PDS 70c has a disk of its own, separate from the larger, encompassing circumstellar disk. This giant world is the first exoplanet to have a directly detected circumplanetary disk, making this system “the textbook example of planet formation,” according to Sebastiaan Haffert (University of Arizona), who wasn’t involved with the study. Such a discovery confirms astronomers’ theories about how moons and planets form.

A Disk Around PDS 70c

Circumplanetary disks are vital to the formation of an exoplanet, since they control how much material the growing planet accumulates. Planetary disks also set the budget for satellite formation, determining how much material will be left over for moons to coalesce from.

Both PDS 70b and c are still gathering mass, albeit slowly, but only PDS 70c unambiguously hosts its own disk. This planet, a few Jupiter masses at most, resides 34 times farther from its star than Earth does from the Sun (1 astronomical unit, or a.u.) – that’s a little farther than Neptune’s orbit. Even though the circumplanetary disk is distinct from the stellar disk’s outer ring in the data, ALMA could not resolve its structure, so Benisty’s team is still unclear just how far it extends or how massive it is. At its largest, PDS 70c’s disk would be 1.2 a.u. in diameter, holding roughly 3 lunar masses (about 3% Earth’s mass) of material.

Characteristics like mass are hard to pin down in part because the size of dust particles — which is hard to determine — influences many aspects of a circumplanetary disk, like how moon formation could happen.

PDS 70c’s circumplanetary disk could make moons in different ways. Small dust particles can get trapped in the disk, creating conditions just right so that these particles stick together, forming pebbles, then rocks, then moons. On the other hand, if larger particles get trapped, they can spontaneously congregate into clumps, causing them to collapse into the beginnings of a satellite.

Of course, these are only a couple of ways to build a moon. “We know so little about satellite formation,” says Jason Wang (California Institute of Technology), who has independently studied the PDS 70 system. He adds that while this discovery does not solve all moon-related mysteries, “it is certainly a stepping stone in understanding when and how efficiently satellites form around planets.”

Why Doesn’t PDS 70b Have a Disk?

The other planet, PDS 70b, shows signs of tenuous dust near its orbit, but nothing actually encircling the planet. Maybe the dust is trapped at one of the gravitationally stable points along 70b’s orbit, or maybe it’s part of a stream of material, linking each planet to the inner disk surrounding the star. Either way, PDS 70b doesn’t have the circumplanetary disk that its sibling shows off.

“The largest surprise is the non-detection of a circumplanetary disk around PDS 70b,” says Haffert, who has also studied this system. “This probably means that it already has completed its potential satellite system.” He adds that even though the process has obviously halted for 70b, any moons that do form around these planets will sadly not be detectable with current technology.

Benisty’s team also has a few theories for why one planet is hogging all the goods. Since PDS 70b is closer to the star than its sibling, perhaps its realm of influence is much smaller, meaning that any dust that would orbit PDS 70b is instead pulled towards the star. Another explanation is that PDS 70c starves b of dust: Circumplanetary dust must come from the outer, cooler ring of the stellar disk, and since PDS 70c is closer to that region, it may catch what it can and allow only a trickle of tiny dust particles to drift towards 70b.

Whatever the reason, Haffert and Wang agree with Benisty’s collaboration that this study presents the most compelling evidence yet for a bona fide circumplanetary disk. With this new real-life laboratory to study growing planets and moons, astronomers can construct a more complete picture of planet formation.

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