An exomoon is a moon orbiting an alien planet outside our solar system, and a team led by David Kipping of Columbia University may have just found the first one yet.

Credit: Pixabay, LoganArt.

NASA’s Kepler Space Telescope mission has so far found in excess of 1,935 confirmed exoplanets, more than 100 of which are Earth-sized. Earlier this year, the American space agency announced one of its most exciting discoveries: seven Earth-sized planets orbiting a star called TRAPPIST-1, which lies just 39 light-years from Earth. Three of these planets orbit their parent star in the so-called Goldilocks Zone where it’s not so hot, not so cold — just enough for liquid water to be stable and, possibly, good enough for life to exist.

From what we know about our own solar system where we can find 173 known moons (182 if you count the moons of Pluto), scientists reckon out of all the exoplanets we’ve found thus far, many of them must be populated by natural satellites. But finding or confirming such a claim is far more difficult than it sounds.

It’s remarkable in itself that we’re able, using our clever instruments, to discover alien planets many billions of miles away from home. There are already some very impressive techniques to achieve this but by far the most common one is the ‘transit method’. Using this technique, astronomers are able to infer not only the presence of an exoplanet but also some very important qualities like mass, density, composition, and even whether or not it has an atmosphere. For the method to work, the exoplanet’s orbiting plane needs to be aligned with our observational plane so instruments can sense a dip in the parent’s star brightness. Depending on how often the star blips, we can infer a planet’s orbiting period or whether or not there is more than one planet in the system. The light wavelengths that reach us then tell us about the exoplanet’s properties.

Exomoons can be found using the same method but seeing how moons are far smaller than planets, they’re also much harder to spot. In fact, we’ve yet to confirm such a sighting, though Professor Kipping’s team may have found the most promising candidate yet. And yes, this must be a very big moon — about the size of Neptune judging from the signal, which must mean its parent planet is huge, likely the size of Jupiter.

Using the transit method, they spotted what looks like an exomoon as its supposed parent planet made three trips around its star. There are some significant limitations, though. The star is some 4,000 years away from Earth which makes the light that ultimately reaches us quite dim to begin with.

The researchers’ statistical confidence in their findings is above 4 sigma. Sigma (which represents the standard deviation) is a measure of the significance with which scientists believe a result to be real as opposed to a random fluctuation in the background signal. A 4-sigma result means that the chance the signal is real is more than 99%, but in science, there are no certainties. Kipping will have the chance to confirm his results once the Hubble Space Telescope directs its lens on the star in question. For now, Kipping, who has been on the lookout for exomoon for most of his career, advises caution.

“We would merely describe it at this point as something consistent with a moon, but, who knows, it could be something else,” he told the BBC. 

“We’re excited about it… statistically, formally, it’s a very high probability. But do we really trust the statistics? That’s something unquantifiable. Until we get the measurements from Hubble, it may as well be 50-50 in my mind,” he added.

The findings were uploaded, for now, in the arXiv preprint server.