The cosmos has a knack for making something out of very little. It molds stars from clouds of dust and gas, planets from the residue left over from the creation of stars, and moons from the scattered fragments around newly glazed planets. The universe spins cosmic matter around, guided by the forces of gravity, shaping the celestial figures that fill the expanse.
Astronomers, with the help of telescopes, have captured glimpses of some of the raw materials before they become something whole—a diffuse glow about to coil into a star, a swirl of particles that could clump into planets. And the silt from which moons could form? That has been the trickiest to detect, but at last some has been found, whirling around a planet many light-years away.
It’s in the picture above. Not the big, glowing ring; that is the material orbiting the star at its center, named PDS 70, some of which has coalesced into planets. The potentially moon-forming stuff is the faint glow around that gleaming speck of a planet. Astronomers call this material a circumplanetary disk, and although it looks like little more than some pixels, this is the first time they’ve detected one with such clarity. Here’s a closer look:
The disk surrounds an exoplanet called PDS 70c, one of two gas giants orbiting a star nearly 400 light-years from Earth. Astronomers can’t see any new moons in this swirling disk of matter, nor can they say with certainty whether any have taken shape, or will. But the materials are there. “The brighter the disk is, the more mass there exists in the disk,” Jaehan Bae, an astronomer at the Carnegie Institution of Washington and one of the authors on the new discovery, told me. And his team’s measurements suggest that there’s enough mass in this glowing disk to produce three moons the size of our own. The moons would form as planets do, through a series of collisions of fast-moving particles. The bits and pieces would stick, and gravity, over time, would smooth the misshapen clumps into spheres.
Based on this image, PDS 70c and its sibling planet, the very creatively named PDS 70b, are young planets, fresh out of their own cosmic kiln. Their detection a few years ago wasn’t so shattering, because astronomers have discovered more than 4,400 exoplanets to date and are working to acquire more data for another 7,600 potential planets. Exomoons, on the other hand, have been far more elusive. Small exoplanets are more challenging to find than bigger ones, which means that spotting small moons is even more difficult. One team announced in 2018 that they had discovered what could be the first known exomoon, an unusual one about the size of Neptune, located in a star system about 8,000 light-years away. But other researchers haven’t yet managed to replicate the original team’s analysis, and the existence of this potential exomoon remains uncertain.
Astronomers know that exomoons are certainly out there, guided by the same logic that indicated exoplanets must have existed before anyone had ever discovered one: Our planetary system and the sun at its center are not special but, judging by all those stars in the sky, one of many in the Milky Way. More than 200 moons reside in our own solar system, mostly around Jupiter and Saturn, its biggest planets. Some are tiny bits of rock, while others are worlds unto themselves, studded with rumbling volcanoes and flowing with subsurface oceans. Moons, I’d argue, are more interesting than planets in at least one sense—maybe the most important sense. Icy moons such as Jupiter’s Europa and Saturn’s Enceladus, for example, might be the best places to look for evidence of extraterrestrial life beyond Earth.
Not every moon comes into existence in the same way. Some are caught by a planet’s gravity as they pass by, becoming part of the permanent collection. According to one popular theory, our moon was likely forged from the rocky pieces that splattered into space after a mysterious Mars-size object smashed into Earth. Jupiter’s biggest moons may have coalesced from a ring of material that surrounded the gas planet in the early solar system—a circumplanetary disk, like the one astronomers detected around PDS 70c.
The discovery of a potential moon-forming disk is a piece of one of these origin stories. “Just like the moons in our solar system, exomoons have a lot to tell us about the formation and evolution of planetary systems, and it all starts with these circumplanetary disks,” Alex Teachey, an astronomer at the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan and one of the discoverers of the maybe-exomoon, told me.
But for astronomers, it’s also just exciting to finally detect one of these circumplanetary disks, rather than approximate it in a computer simulation. “To actually be able to see an image—not a model, not an interpretation, but an image—with high-enough resolution that you can point at this one spot … is really exciting,” Kamber Schwarz, an astronomer at the University of Arizona’s Lunar and Planetary Laboratory who studies planet formation, told me.
Of course, the image of the PDS 70c’s maybe-moon-forming disk is just a snapshot, a zoomed-out view of some place we’ll never actually be able to visit. Perhaps, in the 400 years that it has taken for the light of this star system to reach Earth, Schwarz said, the disk around PDS 70c has dissipated. The disk may have left nothing behind at all, but it is far more fun to consider what this otherworldly region could produce. Because PDS 70c is a giant planet, Bae explained, the environment “would look more like Jupiter and its satellite system instead of the Earth-moon system.” Perhaps this circumplanetary disk, and others like it, could make its own Europa, a marble of ice with water sloshing under its shell. Or Io, with its molten lakes of lava and a surface reminiscent of moldy cheese. Or Ganymede and Callisto, their dark, rocky surfaces speckled with bright craters.
Exomoons are interesting astronomical targets for researchers on Earth. But, if you join me in a hint of dreamy speculation, they might be something else to the planets they orbit. Consider the ways our moon has influenced Earth—not only in the push and pull of the planet’s tides, but in the imagination of its inhabitants. This month marks the anniversary of the first moon landing, the moment a few members of humankind leapt away from the gravity of their own world and fell into that of another. How tantalizing to imagine that perhaps some other civilizations have visited their own moons too.