Up until the last few decades, our picture of what might reside around distant stars was shaped entirely by the planets, moons, asteroids, and other bodies in our own Solar System. But the discovery of thousands of exoplanets has dramatically improved our picture of what's out there in terms of large bodies. Comets and asteroids, by contrast, are well below our ability to image for the indefinite future.

Moons, however, are awkwardly in between. It should be possible to image them indirectly, as their gravitational influence will alter the timing with which their planets orbit the star. And we might get a more direct indication of their presence as they will sometimes add to the shadow cast as transiting planets pass in front of their host star. We've searched for these effects, but they'll be subtle, so it could be that it will take years of observations for them to rise above the noise.

But now scientists are suggesting that we've observed an exomoon in the making. By looking at some planets forming around a young star, they think they've spotted a disk around one of the planets that may ultimately condense into moons. And, as a bonus, they found an odd, diffuse structure around a second planet that they can't explain.

Moon formation, in theory

The formation of moons is thought to be tied up in the formation of planets themselves. Young stars are surrounded by a disk of material, including gas, dust, and ice at various distances from the star. Once planets begin to condense from this material and grow above a critical mass, they'll clear out space around their orbit within the disk. There are a number of cases where disks with gaps have been imaged, providing evidence that this is an accurate understanding of planet formation.

But the disk is dynamic, so some of the material within this gap will eventually flow into it, falling under the planet's gravitational influence. (Technically, the planet grabs anything passing through its Hill radius.) This will form a disk similar to the one orbiting the star. Some of its material will flow into the planet, increasing its mass. But some of the material will stay in orbit, where it can form moons. In the words of the new paper, "In addition to regulating the final mass of giant planets, [circumplanetary disks] are the birthplace of satellites such as the main moons of Jupiter and Saturn."

A small international team of astronomers decided to look into a potential example of this, the PDS 70 system. It's centered on a young star that's somewhat smaller than the Sun, with a large disk that includes a suspicious gap. Two likely planets are present in the gap, with one being the first exoplanet ever to have been imaged directly. They're large, with both being several times the mass of Jupiter.

This combination of features—planets that have created a gap in the star's planet-forming disk—makes PDS 70 a great candidate for hosting a circumplanetary disk from which moons could form. Conveniently, the researchers behind the new work didn't even have to do any new observations to look. It turned out there were published observations done with the ALMA telescope. The researchers used an updated calibration of the instrument that had been published since to improve the quality of the earlier observations.

Rings and things

The ALMA images show a large disk orbiting the star but stopping well short of it. There's also a faint inner disk that the star itself is embedded in. ALMA also picks up the two known bodies in the gap, termed PDS 70b and PDS 70c. The latter of the two is right near the inner edge of the disk and difficult to resolve from it. The authors estimate that there's probably a third planet lurking closer to the star, which would account for the large gap between PDS 70b and the faint disk that the star is embedded in.

Things get a bit weird, however, when you compare those images with infrared data on the same system. These also show the dense outer disk and hint that the faint inner disk is there. But they also suggest the outer planet is pulling material out of the nearby disk and spreading it into the gap. Meanwhile, at PDS 70b (the inner planet), there's a blob of dust nearby—but it doesn't seem to be centered on the planet.

The authors interpret these images as showing that PDS 70c is forming a proto-lunar disk as it's funneling additional materials into the planet itself. That disk is embedded in a larger thread of material that extends all the way to the star itself, a feature that requires further study before we can make suggestions about how it forms.

Meanwhile, things at PDS 70b are even stranger, given that there appears to be something that would be a circumplanetary disk—but it's not located where the planet is. "We do not currently have any robust interpretation," the authors acknowledge. It could be that the images from different telescopes haven't been aligned exactly enough, or it's possible that some of the dust has piled up at an orbital location called a Lagrange point. We just can't tell at this stage.

All of this provides evidence that our models of moon formation are probably on the right track. And, if they do form around these planets, it's going to be most impressive. Due to its large mass and current heat content, PDS 70c is estimated to be about three times the size of Jupiter; the disk that surrounds it is estimated to extend up to three times the distance between the Earth and the Sun. Even if it does form a large collection of moons, however, they'll remain difficult to image, as the planet's orbital plane would prevent using either of the two methods we know for spotting them.

Astrophysical Journal Letters, 2019. DOI: 10.3847/2041-8213/ab2a12 (About DOIs).