In a possible first for astronomy, researchers suspect they’ve found a large moon orbiting a planet near a distant star. Astronomers exploring the outer reaches of space have revealed dozens of candidate exoplanets in recent years, but oddly, no one has found strong evidence of any moons around those planets. So far, the only moons we know about exist within our solar system. But a new Science Advances paper published Wednesday, however, could change that entirely.

In the paper, a pair of astronomers at Columbia University outline evidence supporting the existence of a moon orbiting the exoplanet Kepler-1625b, which is about 4,000 light-years away, using data from the Hubble Space Telescope.

“If confirmed, this is the first true exomoon discovery, one that can be followed up with repeat observations of the system,” Columbia graduate student Alex Teachey, who co-authored the paper with assistant astronomy professor David Kipping, Ph.D., tells Inverse. “Moons stand to tell us a great deal about the dynamical history of these exoplanetary systems, and there are still lots of questions about how planetary systems evolve over their billions of years of existence.”

This artist's rendering shows what it may look like for the planet Kepler-1625b and its moon to pass across the star Kepler-1625. Dan Durda

Why Are Exomoons So Rare?

Given that our solar system has so many moons, it may sound strange that exomoons aren’t plentiful elsewhere. The fact of the matter is, it is pretty strange, and this is why Teachey and his colleagues have been hunting for them. In a 2017 interview with Universe Today, he said:

We see moons in our Solar System, but are they common elsewhere? We tend to think so, but we can’t know for sure until we actually see them. But it’s an important question because, if we find out there aren’t very many moons out there, it suggests maybe something unusual was going on in our Solar System in the early days, and that could have major implications for how life arose on the Earth. In other words, is the history of our Solar System common across the galaxy, or do we have a very unusual origin story? And what does that say about the chances of life arising here? Exomoons stand to offer us clues to answering these questions.

How to Find an Exomoon

In the new study, Kipping and Teachey tracked the light signatures of objects around the star Kepler-1625. Following a hunch based on some promising data from the Kepler Space Telescope, they used the Hubble Space Telescope to gather more data on the star Kepler-1625. This is how they figured out that Kepler-1625b, which is about the size of Jupiter and orbits its star at about the same distance that Earth orbits the sun, may be home to a moon.

When searching for exoplanets, astronomers look for dips in the amount of light emanating from a star. By measuring how a planet blocks its home star’s light as it passes in front of it, astronomers can learn a lot about the size, orbit, and even composition of the planet. As a planet transits past the star over and over, the accumulated data allows astronomers nail down the planet’s orbital period pretty precisely.

When observing light from Kepler-1625 in this way, Teachey and Kipping noticed a slight anomaly in the transit data of the planet Kepler-1625b: Each dip in light was accompanied by another small dip — one that couldn’t be explained by just the presence of a planet. With data on only three transits, however, they knew they needed more. Securing 40 hours on the Hubble Space Telescope, they built a much stronger case for their suspicion: The blip in Kepler-1625b’s transit data appeared to be a moon, sometimes trailing the planet, sometimes leading it.

Because of the unique nature of moons, their research required a slightly different approach than most exoplanet hunts. Transit data is a great way to learn about exoplanets, but since moons have slightly irregular orbiting patterns, they are trickier to identify this way.

“Moons are orbiting the planets, so they show up in a different place every time the planet transits, sometimes before the planetary transit, sometimes after,” says Teachey. “So you don’t see that same kind of periodicity, and you can’t really stack the moon transits in the same way to clean up the signal.”

Fortunately, the extra time on the HST allowed Teachey and Kipping to hone in on Kepler-1625b and estimate that the planet and its moon are about the same relative size to one another as Earth and the moon — except they’re about 11 times larger than our home world and its moon.

Huge, if True

Their findings, while exciting, fall into the “Huge… if true” category. In short, they need to be confirmed by further observations. Teachey and Kipping are very careful to get this point across.

“At every step of this process my co-author David Kipping and I have been anxious to let the data and the analysis speak for itself, and not make claims that that data can’t support,” says Teachey. “We’d of course love to be able to say, ‘this is it, we found it, case closed,’ but we’re stopping a bit short of that because we recognize the uncertainties here that cannot (yet) be eliminated.”

He’s sure that some will call this paper a “discovery,” and cautions against that language. Until it’s confirmed, our solar system’s moons remain unique in the near universe, at least as far as we can tell.

Email the author: peter@inverse.com.