In the last fifteen or so years, we've discovered thousands of planets around other stars, confirming astronomers' long-held belief that our Sun isn't unique in hosting a solar system. But, our solar system is made up of more than just planets. Taking a step down in size, we reach the 200 or so moons which orbit our eight planets. Smaller and fainter than planets, these are an even bigger challenge to detect. But, just maybe, we've finally found one!
There are a bunch of ways we detect exoplanets and, in theory, most of them could also be used to detect these so-called exomoons. The reality, though, is much murkier. Scientists continue to search through data from the Kepler Space Telescope in search of the signal of a moon, but have thus far come up empty. This new discovery utilizes an older, chancier technique.
Back when every new exoplanet was still a big deal, a method called gravitational micro-lensing was a popular way to discover them. The technique utilizes a unique property of Einstein's theory of general relativity: gravity bends light. If an object like a star or planet happens to pass in front of a background star in just the right alignment, its gravity can bend more of that background star's light towards us, making it momentarily appear brighter. If that star or planet is orbited by something, the star can be lensed twice. This is where things get tricky.
By measuring the relative brightening of these two events, astronomers can estimate the ratio of the masses of the two objects. But the ratio can't tell us everything. In this instance, for example, the pair could be a dim star and a Neptune-sized planet or it could be a Jupiter-sized planet with an Earth-sized moon.
The only way to discriminate between these two possibilities is to measure the distance to the pair. If the objects are close by, it's probably a planet and a moon. Otherwise, the star and planet are more likely. Unfortunately, this hits at one of the reasons the gravitational micro-lensing technique isn't too popular: it's nearly impossible to make any sort of follow up observations. The chances of another lucky alignment are astronomically low and whatever these objects are, they seem too faint to detect otherwise.
This means we'll likely never know whether this marked our first ever encounter with an alien moon. What we can know for sure, however, is that eventually we'll find one. Then two. And, eventually, our view of exomoons will likely be similar to today's view of their planetary hosts: that just like in our solar system, moons are everywhere!