A pair of astronomers combing through data from the Kepler spacecraft have discovered the first exomoon. The moon is in the Kepler 1625 system about 8,000 light years away, in the constellation Cygnus. It orbits the gas giant Kepler 1625b, and, unlike all the moons in our Solar System, this one is a “gas moon.”

It was only a matter of time before we found an exomoon. We’ve found thousands of exoplanets, thanks mostly to the Kepler spacecraft. And where there are planets, we can expect moons. But even though it seemed inevitable, the first confirmed exomoon is still exciting.

The exomoon is much different than what we see in our own Solar System. It orbits a gas giant several times larger than Jupiter, called Kepler 1625b. Preliminary evidence indicates that the moon itself, which is called Kepler 1625b I, is a gas moon, and is about the size of Neptune. Nothing like this exists in our system.

“One jarring aspect of the system is the sheer scale of it.” – from the paper, Teachey and Kipping, 2018.

Here’s where we caution our readers, though. Strictly speaking, astronomers have discovered evidence of the moon. Its existence still needs to be confirmed. But since scientists are cautious by nature, it’s a safe bet that additional observations will confirm that its there.

“If confirmed, this finding could completely shake up our understanding of how moons are formed and what they can be made of.” – Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate.

Alex Teachey and David Kipping are astronomers at Columbia University. They discovered the moon in Kepler data they were analyzing. They focused on 284 exoplanets discovered by Kepler that were orbiting their stars in wide orbits. A wide orbit is one that’s longer than 30 days, and they’re considered the best places to find potential exomoons. They found one anomaly in those 284 exoplanets that suggested an exomoon was present.

After the planet completed its 19 hour long transit, there was a second dip in the star’s brightness. This dip is consistent with a moon, making it the first exomoon ever observed.

“We saw little deviations and wobbles in the light curve that caught our attention.” – David Kipping, Columbia University.

“This intriguing finding shows how NASA’s missions work together to uncover incredible mysteries in our cosmos,” said Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate at NASA Headquarters, Washington, D.C. “If confirmed, this finding could completely shake up our understanding of how moons are formed and what they can be made of.”

“It was definitely a shocking moment to see that Hubble light curve.” – David Kipping, Columbia University.

After they found the anomaly in the Kepler data, they used the Hubble to take another look. The pair spent 40 hours of Hubble time getting more precise data on the dip in light from the parent star. After observing the planet itself transit in front of the star, they saw the same second dip that Kepler did, 3.5 hours after the planet’s transit.

“We saw little deviations and wobbles in the light curve that caught our attention,” Kipping said.

The pair of astronomers ran out of time before they could observe the moon’s complete transit, but they saw something else. The planet’s transit occurred more than one hour earlier than predicted. This is consistent with the planet and moon orbiting a common center of gravity. This would cause the planet to wobble the same way Earth wobbles as the Moon orbits Earth.

“A companion moon is the simplest and most natural explanation for the second dip in the light curve and the orbit-timing deviation,” Kipping explained. “It was definitely a shocking moment to see that Hubble light curve, my heart started beating a little faster and I just kept looking at that signature. But we knew our job was to keep a level head and essentially assume it was bogus, testing every conceivable way in which the data could be tricking us.”

There’s good reason to be cautious about the moon conclusion. A second undetected planet could be responsible. It’s possible that a second planet is there, but that Kepler is unable to detect it.

Both the planet and its moon are in the star’s habitable zone, meaning liquid water could be present. But unfortunately, the planet is a gas giant and the moon is a gas moon. No life as we know it could exist there.

The pair of astronomers published their paper in the journal Science Advances. They say that not only is the moon a gas moon, which is alien to our Solar System, it’s also comparable in size to Neptune. That’s way larger than any moons in our system. The duo say that the existence of this moon might yield new insights into how planetary systems form, and into how moons form.

The Exomoon’s Origins: A Captured Object?

Astronomers think that moons form out of dust left over from the formation of the planet. (Although Earth’s Moon may have formed differently.) But Kepler 1625b and its moon are both gaseous worlds, so some other formation mechanism must be at work.

The exomoon’s orbit may be tilted by about 45 degrees to the planet’s orbital plane. If that’s the case, then it is similar to Neptune’s moon Triton. But astronomers think Triton is a captured Kuiper Belt Object (KBO) rather than a moon that formed from the same dust as Neptune. It’s possible that Kepler 1625b I is a captured object rather than a classic moon. But in the study the two astronomer’s use caution in reaching that conclusion. It’s all rather preliminary at this point.

Throughout the conclusion of the paper, the two astronomers urge caution. They are satisfied with the accuracy of their data, especially from the follow-up Hubble data. And they’re satisfied that their interpretation is rigorous. The problem is the unlikelihood of the discovery compared to anything we know about. This is an entirely new discovery.

In their conclusion, the two astronomers state that “This is a complicated and involved analysis where a minor effect unaccounted for, or an anomalous artifact, could potentially change our interpretation. In short, it is the unknown unknowns that we cannot quantify. These reservations exist because this would be a first-of-its-kind detection—the first exomoon.” They go on to say that the first exoplanet claims were met with great skepticism because they were so new.

There are nearly 200 moons in our Solar System, and that’s in a system with only 8 major planets. Not all of the planets have moons, but the mathematical average is still 25 moons per planet. So with almost 4000 exoplanets discovered, math is on our side.

Even if this turns out to not be an exomoon, astronomers will keep finding other candidate exomoons. The James Webb telescope will have something to say about the search. It’s powerful observing capabilities will jump start the search for planets and moons around other stars.

It’s only a matter of time before we find one.

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