Astronomers may have discovered the first moon ever found outside our Solar System

A pair of astronomers believes they’ve found a moon orbiting a planet outside our Solar System — something that has never before been confirmed to exist. Though they aren’t totally certain of their discovery yet, the find opens up the possibility that more distant moons are out there. And that could change our understanding of how the Universe is structured.

The astronomy team from Columbia University found this distant satellite, known as an exomoon, using two of NASA’s space telescopes. They first spotted a signal from the object in data collected by the planet-hunting telescope Kepler, and then they followed up with the Hubble Space Telescope, which is in orbit around Earth. Thanks to the observations from these two spacecraft, the team suspect this moon orbits around a Jupiter-sized planet located about 4,000 light-years from Earth. And this planet, dubbed Kepler-1625b, orbits around a star similar to our Sun.

Scientists have strongly believed for decades that moons exist outside our Solar System, but these objects have remained elusive for scientists up until now. There have been just a couple of candidates that astronomers have speculated about in the past, but nothing has been confirmed. That’s because moons are thought to be too small and too faint to pick up from Earth. However, this suspected exomoon, detailed today in the journal Science Advances, is particularly large, about the size of Neptune, making it one of the few targets that our telescopes can detect. “You can make the argument that this is the lowest hanging fruit,” Alex Teachey, an astronomy graduate student at Columbia University and one of the lead authors on the paper, tells The Verge. “Because it is so large, in some ways, this is the first thing we should detect because it is the easiest.”

Teachey argues that finding more moons outside our Solar System will change our understanding of how planetary systems formed thousands of light-years away. Our cosmic neighborhood is filled with moons, and they explain a lot about how our planets came to be. Exomoons could tell similar tales. However, none of our moons come close to the size of this one, which creates a puzzle for astronomers. “Because it is so unusual, or at least has not been anticipated largely by the community, this poses new challenges to explain it,” says Teachey. “How do you get something like this?”

It was only a few decades ago — in the late 1980s and early 1990s — that astronomers confirmed the existence of planets outside our Solar System. Since then, thousands of these distant worlds, known as exoplanets, have been confirmed by spacecraft like Kepler and other telescopes. Perhaps the most popular way to find exoplanets is by staring at stars, waiting for them to flicker. When a planet crosses “in front” of its host star, it dims the stars’ light ever so slightly. These dips in brightness can be used to determine how big a planet is and the kind of orbit it’s on.

It’s this technique that was used to find the exomoon as well. Teachey and his co-discoverer David Kipping have been on the search for exomoons for the last seven years, poring over data collected by Kepler. In 2017, they noticed that one star spotted by the spacecraft had a strange dimming pattern that didn’t quite match a normal exoplanet signature. Normally with an ordinary exoplanet, these objects diminish starlight in a very routine way. That’s because they orbit the same way each time, which helps astronomers know when to expect a star’s dimming to occur. “As the planet goes around, it’s going to transit more or less like clockwork,” Teachey says. “You can basically count on when you’re going to see it come around again.”

But for an exoplanet with a moon, the brightness dips don’t seem to follow an orderly scheme. That’s because the exomoon will also block out some of a star’s light when the planet passes (just not as much as its planet does). And when the moon dims the star, it happens in a different way every time because the moon is orbiting around the planet. “Sometimes from our perspective, the moon will be leading the planet, and sometimes it will be trailing the planet,” says Teachey. “It’s going to show up in a different place every time the planet [crosses].” So, on occasion, astronomers will see a slight dip in light before the main dip when the moon is in front of the planet. Other times, the slight dip will occur afterward when the moon is lagging behind.

That causes the dips to seem more sporadic and less routine, but ultimately, they do provide a pattern that astronomers can use to come up with a model for how big the moon is and how it’s orbiting the planet. Plus, there’s another way to tell if an exomoon is present. Even though they’re smaller than their parent planets, moons do still have a gravitational pull. And they will slightly tug on the planets they orbit around, causing the planets to wobble. So sometimes the planet will cross its star a tad early, and sometimes it will cross a tad late.

Teachey and Kipping noticed both of these phenomena — the weird dips and the odd timing — with the Kepler data, and they tentatively announced what they had found in July last year. However, they only claimed to have found an exomoon candidate and wanted more time to follow up. So they requested time with NASA’s Hubble Space Telescope to spy on the system further. (The observatory is about four times more precise than Kepler.) Over the course of 40 hours, they watched the same Kepler-1625b planet with Hubble to help confirm that the exomoon truly was there.

Thanks to Hubble and Kepler, the potential exomoon has come into focus. Its parent planet is located roughly the same distance away from its star that Earth is from the Sun. And that technically puts this system in the coveted habitable zone — the area around a star where temperatures are just right so that water can be a liquid. However, chances are high that there are no pools on this moon or its planet. Since the planet is the size of Jupiter, it’s most likely a gas giant, and the Neptune-sized moon is most likely the same. However, the two objects are likely much warmer than Jupiter and Neptune are in our system, which does make them unique finds.

Still, this isn’t a done deal yet. “We haven’t confirmed this is an exomoon,” Jennifer Wiseman, the senior project scientist for the Hubble Space Telescope at NASA, tells The Verge. “We didn’t even have a chance to see a full [crossing] of this potential moon, and so we don’t have enough information yet to confirm whether this is truly an exomoon. But this has made us even more excited for looking at this system in the future.” Teachey and Kipping are releasing their results now with hopes that other astronomers will pick apart the data and help confirm whether or not this is real. They say more telescope observations are needed.

Before this, there have only been a few inklings of possible exomoons in the past. Astronomers announced in 2013 that they might have found one around a rogue planet, but there was no way to follow up the detection. Additionally, other astronomers have found planets surrounded by large discs of materials, which have gaping holes in them. And it’s believed that those holes are actually carved out by moons. However, that kind of detection is much more indirect than this one, and no one has been able to confirm that the moons are there.

So if this particular exomoon is real, it’ll be the first detection to be confirmed. And that means more of these exomoons may exist out in the wild. Much more precise telescopes — like the future James Webb Space Telescope, which is scheduled to launch in 2021 — might be able to do more detailed searches of exomoons in the future. These worlds could be important tools to learn how exoplanets form. It’s believed that if a planet travels a great distance during its initial formation, it will lose its moons along the way, according to Teachey. So finding more exomoons might mean that these planets didn’t migrate very far when they were born.

Additionally, exomoons could be interesting for another reason: the moons of our Solar System have recently become tantalizing targets in the search for life off of Earth. Jupiter’s moon Europa and Saturn’s moon Enceladus are both thought to have liquid water oceans underneath their surfaces, which are places that could host organisms. While this Neptune-sized exomoon probably doesn’t host life as we know it, perhaps other moons we haven’t seen yet could be just right. “Now if we know that in other star systems there are not only planets but moons, it gives us other locations to consider for potential habitability,” Wiseman says.

But first, this one exomoon needs to be confirmed. Teachey says he’s pretty confident in what they’ve found. “We think that the moon is the best explanation for the data in hand,” he says.

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