And then there were two—maybe. Astronomers say they have found a second plausible candidate for a moon beyond our solar system, an exomoon, orbiting a world nearly 6,000 light-years from Earth. Called Kepler-1708 b-i, the moon appears to be a gas-dominated object, slightly smaller than Neptune, orbiting a Jupiter-sized planet around a sunlike star—an unusual but not wholly unprecedented planet-moon configuration. The findings appear in Nature Astronomy. Confirming or refuting the result may not be immediately possible, but given the expected abundance of moons in our galaxy and beyond, it could further herald the tentative beginnings of an exciting new era of extrasolar astronomy—one focused not on alien planets but on the natural satellites that orbit them and the possibilities of life therein.
There are more than 200 moons in our solar system, and they have an impressive array of variations. Saturn’s moon Titan possesses a thick atmosphere and frigid hydrocarbon seas on its surface, possibly an analogue of early Earth. Icy moons such as Jupiter’s Europa are frozen balls that hide subsurface oceans, and they may be prime habitats for life to arise. Others still, such as our own moon, are apparently barren wastelands but could have water ice in their shadowed craters and mazelike networks of tunnels running underground. An important shared trait among these worlds, however, is their mere existence: six of the eight major planets of our solar system have moons. Logic would suggest the same should be true elsewhere. “Moons are common,” says Jessie Christiansen of the California Institute of Technology. “In our solar system, almost everything has a moon. I am very confident that moons are everywhere in the galaxy.”
The only problem is finding them. We can look for exoplanets in a number of ways, such as spying the dip in light they produce as they move in front of their star, an occurrence known as a transit, or getting a telltale glimpse of their gravitational tug on their host star. Finding exomoons, which are by nature decidedly smaller than the planets they orbit, is much more difficult, however. “They’re just so small,” Christiansen says. To date, only one truly plausible candidate has been found: Kepler-1625 b-i, a supposed Neptune-sized world orbiting a Jupiter-sized exoplanet about 8,000 light-years from Earth that was reported in October 2018. But even the existence of this more behemothic world has been called into question by subsequent analysis.
Kepler-1708 b-i’s existence was first hinted at in 2018 during an examination of archival data by David Kipping of Columbia University, one of the discoverers of Kepler-1625 b-i, and his colleagues. The team analyzed transit data from NASA’s Kepler space telescope for 70 so-called cool giants—gas giants, such as Jupiter and Saturn, that orbit relatively far from their stars, with years consisting of more than 400 Earth days. The team looked for signs of transiting exomoons orbiting these worlds, seeking additional dips in light from any shadowy lunar companions. Then the researchers spent the next few years killing their darlings, vetting one potential exomoon candidate after another and finding each better explained by other phenomena—with a single exception: Kepler-1708 b-i. “It’s a moon candidate we can’t kill,” Kipping says. “For four years we’ve tried to prove this thing was bogus. It passed every test we can imagine.”
The magnitude of the relevant smaller, additional dip in light points to the existence of a moon about 2.6 times the size of Earth. The nature of the transit method means that only the radius of worlds can be directly gleaned, not their mass. But this one’s size suggests a gas giant of some sort. “It’s probably in the ‘mini-Neptune’ category,” Kipping says, referring to a type of world that, despite not existing in our solar system, is present in abundance around other stars. The planet this putative mini-Neptune moon orbits, the Jupiter-sized Kepler-1708 b, completes an orbit of its star every 737 days at a distance 1.6 times that between Earth and the sun. Presuming the candidate is genuinely a moon, it would orbit the planet once every 4.6 Earth days, at a distance of more than 740,000 kilometers—nearly twice the distance of our own moon’s orbit around Earth. The fact that only this single candidate emerged from the analysis of 70 cool giants could suggest that large gaseous moons are “not super common” in the cosmos, Christiansen says.
The apparently large size of this exomoon, compared with its host planet, is “surprising,” Kipping says, but not wholly unexpected: Kepler-1625 b, the planet that the previous exomoon candidate Kepler-1625 b-i purportedly orbits, appears to have a similar, if slightly larger, configuration. If both these moons truly exist, that could be telling us something very interesting about possible planet-moon configurations in the galaxy, namely, that giant worlds could host equally giant moons. That in itself raises questions about the genesis of such worlds. It is unlikely that such a large moon could directly form in orbit around a planet, with the planet more likely to sweep up any potential satellite-birthing material, suggesting another origin story is more probable.
“One scenario is this moon got captured by the planet as the planetary system was forming,” Christiansen says. “Early planetary systems are quite violent, chaotic places. We see examples of capture in our own solar system: for instance, Triton, one of Neptune’s moons. We think that was captured. So we know that this can happen; we just hadn’t scaled it up to the idea that a Jupiter-sized planet could capture a Neptune-sized moon.”
Not everyone is sold on this moon’s supposed existence, however. René Heller of the Max Planck Institute for Solar System Research in Göttingen, Germany, says he is not sure that the transit signal the team has seen is the result of a moon. “It doesn’t convince me,” he says. Instead, Heller adds, the dip in light could simply be the result of natural variations on the star, such as sunspots we see on our own sun, passing across its surface at the same time as the planetary transit. Kipping and his team, for their part, say they have ruled out such a possibility because the dip supposedly caused by the moon began before the planet started to pass in front of the star.
Laura Kreidberg of the Max Planck Institute for Astronomy in Heidelberg, Germany, says she “wouldn’t call it a slam dunk yet,” but the result is “absolutely worth following up” to try to see another transit from the purported moon. We will not be able to do so immediately, however. Given the long orbit of the planet, it and its possible moon will not transit again until 2023, Kipping says, meaning we would have to wait until then to try to spy the exomoon again. If the exomoon really is there, the James Webb Space Telescope (JWST), launched in December 2021, should be able to nearly instantly confirm or refute its existence. “It would be a piece of cake for Webb,” Kipping says. “It could find moons smaller than Europa around Jupiter. It’s a ridiculously powerful telescope.”
That in itself raises an exciting possibility: JWST could be used to perform some sort of survey to look for exomoons. In the way its predecessor, the Hubble Space Telescope, has made huge strides in exoplanet science, JWST might prove to be defined by its contribution to exomoons. “My team right now is planning out what an exomoon survey strategically would look like for Webb,” Kipping says. “It will be the first time in human history that will be possible. I’m really excited about the future.”
The reasons for doing so are severalfold. Once we start finding exomoons in abundance, we will start to get a true handle on their variability and importance. Tides from our own moon, for example, may have played a role in Earth’s habitability, leading to the evolution of life in tidal pools. The study of exomoons may tell us more about the planet-formation process, too. “If we want to have a comprehensive understanding of how planet formation works, we need to understand moons,” Kreidberg says. And there is another, more simplistic reason to study them: “Moons are cool.”
Exomoons themselves may also represent prime targets in the hunt for life. Given they can seemingly range in size from small to Earth-sized and beyond, it is reasonable to assume that some rocky exomoons may orbit gas giant planets within their stars’ habitable zone, where liquid water can exist. “This is one of those cases where science fiction might precede science fact,” Christiansen says. “You have the example of the movie Avatar of a habitable moon around a gas giant. In Star Wars, you have habitable moons around gas giants. You can technically create a rock around a gas giant that has the average radiation from the sun such that it could have liquid water on the surface.”
There are complications, though. A moon around a giant planet would experience a considerable gravitational push and pull from that larger world, which, in extreme circumstances—such as those of Jupiter’s moon Io—can result in intense volcanic activity. The radiation from gas giants such as Jupiter can be deadly, too. And such systems can have peculiar characteristics. “If you’re lined up right, you’d have your day and night from your rotation but an additional day-night cycle from going behind the planet,” Christiansen says. “There are almost certainly rocks of the right temperature around gas giants. Whether or not they’re habitable is an open question and something a lot of people are excited about.”