One of the most important things we've learned from the Kepler mission is that, in many ways, our Solar System isn't unique. Lots of stars have planets, many have multiple planets, and the list of planets includes many with sizes and densities similar to our eight planets. But there are lots of details of our own planets, like the composition and presence of atmospheres, that are much harder to examine at these distances.

One of the features we haven't gotten a grip on is the presence of moons. Most of our Solar System's planets have them, and they seem to form by a variety of mechanisms. We'd expect them to be common in exosolar systems, too, but so far we haven't yet spotted any.

A new paper, which goes into extensive detail about the calculations needed to look for an exomoon, makes it clear why: we simply don't have enough observation time to pick one up in most cases. But the paper also suggests there may be an exception, as the data hints at a Neptune-sized exomoon, though the statistics aren't yet conclusive.

Find me a moon

Why do we care if there are other moons out there? For one, a lot of our Solar System's most interesting geology happens on its moons, which have geysers, atmospheres, and volcanoes, plus oceans that provide a possible habitat for life. For another, their formation tells us something about the Solar System's history. Jupiter's probably formed from a disk of material much like the Solar System itself formed, while the Earth's moon formed from the debris of a later collision. Neptune's moon Triton seems to have formed elsewhere and then ended up captured by the large planet's gravity. Saturn's rings appear to be condensing into a number of small moons that feed back and help shape the rings.

These provide a lot of reasons to look for moons elsewhere, but most of our methods of looking for planets won't find any directly. Moons are too small for direct imaging, and the planet-moon system will be indistinguishable from a single object when it comes to its gravitational influence on its host star.

That leaves only two methods, both of which involve watching the light blocked as the planet and moon transit between their host star and Earth. The first is looking for variations in the timing of this transit, caused as the moon's orbit either has it in front of the planet's orbit, pulling it faster, or behind, which slows it down. These transit timing variations, however, will be pretty small, and they can also be caused by other planets in the system.

The other way is to watch the amount of light blocked out by a planet as it transits in front of its host star. If the moon is eclipsing the planet or vice versa, then the signal will be indistinguishable from the planet itself. But at other times, the moon will also block some of the star's light. Depending on where it is in the orbit, this may lead to a small dip in the star's light before or after the planet creates a larger drop, or it could increase the amount of light that's blocked as the planet transits.

The problem here is that the signal won't be the same on any two consecutive orbits, so you need a lot of transits to confirm that the effect is real. Estimates are that data from at least a dozen transits of the planet in front of its host star would be needed to have any chance of detecting the planet's moon.

Or not

Recently, a draft of a paper on identifying exomoons was posted to the arXiv. While it hasn't been through peer review yet, it gives some sense of just how hard it is to spot a moon given the data we have. That's because it lays out the entire analytical and computational pipeline that the team used to look for exomoons.

While a number of exoplanets are close enough to their host star that we've observed a reasonable number of transits, they're also unlikely to have moons. That's because the gravitational interactions at these distances are complicated enough that any moons are likely to have been ejected, especially if there's more than one planet around. Farther from the star, we have data from fewer orbits—not enough to detect the influence of a moon.

To deal with this, the researchers combined data from multiple planets by centering all their transits on a common point. While the process of combining them will eliminate any clear signal of individual moons, it could provide a hint as to whether something unusual is happening somewhere in the population.

But the analysis pipeline was rather involved—a single step in it took more than 100,000 CPU hours. Because of these constraints, the team could only analyze two model exomoon systems: one with a single moon, and one with a Jupiter-style system of four large moons. There were also a number of assumptions built in, such as inferring the likely mass of an exoplanet based only on the amount of light it blocks.

Overall, this part of the analysis shows very little of interest. There are some classes of planets—habitable zone occupants, those around cold stars, etc.—that show slightly higher probabilities of hosting a moon than others. But there was no clear evidence that any exomoons are out there. In fact, as part of the calculations, the team showed that it would be impossible to detect Jupiter's outermost large moon, Callisto, doing this sort of analysis. So it's hard to say whether moons are rare in the inner portions of exosolar systems (which, to an extent, we'd expect) or whether we just don't have enough data and computer time to find them yet.

Maybe a Nept-moon?

Through the computational approach described above, the authors also ended up with all the transits from each individual planet centered and stacked. So they ran these through the same analysis pipeline. Even though there weren't many transits, there was a chance that something unusual would stand out—and something did.

That something was the data from KEPLER-1625B, a gas giant that orbits in the habitable zone. A visual examination of its transits reveals that something odd's going on, with unusual dips in the star's light adjacent to the transit of the planet, and a transit that, instead of having a flat bottom, seems to have some deeper portions. The authors test a variety of models of the planet alone and a planet + moon combination, and they find that the presence of a moon is heavily favored. Its statistical significance, however, is only just above four sigma—a bit shy of the five sigma required to announce a discovery.

The best fit for the data suggests that KEPLER-1625B is a super-Jupiter, at 10 times the mass of Jupiter. Its moon, meanwhile, is Neptune-sized (Neptune's mass is only about five percent that of Jupiter's). That is, by lunar standards, enormous. Models of moon formation indicate there's simply not enough material present in the disks that form moons to make something that large, which indicates that this was originally a planet that got caught up in the super-Jupiter's gravity. That could have happened as Kepler-1625B migrated inward to reach its current orbit.

The team wasn't planning on saying anything about the moon's possible discovery until after October, when it had reserved time on the Hubble Space Telescope to watch its next transit in front of the host star. But that time turned out to be the team's undoing, as someone who tracked applications for time on the Hubble noticed the team's request, figured out something must be up, and shared it on social media.

Of course, it'll take the team some time to analyze the Hubble data, so we won't know for sure until next year. For now, the data's pretty suggestive.

The arXiv. Abstract number: 1707.08563 (About the arXiv).