This Neptune-sized world could be the weirdest moon astronomers have ever seen

Meet Kepler-1708 b and its not-so-little friend (that may or may not exist).

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Illustration of Kepler-1708b i
Helena Valenzuela Widerström

Our Solar System is full of hundreds of moons, with the outer giant planets having dozens each to their name. But so far, we haven’t convincingly detected a moon orbiting a planet outside the Solar System.

Part of the problem is that our telescopes, for the most part, are not sensitive enough to see signs of such small objects so far away. But that hasn’t stopped scientists from combing through data for hints of “exomoons” that might be on the larger end of the moon size spectrum.

“Exomoons are really at the edge of what we are capable of detecting,” Laura Kreidberg of the Max Planck Institute for Astronomy tells Inverse.

But new research may have found a moon around a giant, distant planet — but one unlike anything in our Solar System.

What’s new — In a study published Thursday in Nature Astronomy, a team of researchers describes how it investigated data of 70 giant planets detected by the Kepler space telescope — and found hints of a possible exomoon around one of them, a giant planet named Kepler-1708 b. The team refers to the moon candidate as Kepler-1708 b-i.

The team is cautious in presenting their findings as an exomoon candidate. Based on their false-alarm tests, there’s a 1 percent chance that the blip in data they identify as a potential moon is caused by natural fluctuations in the data, David Kipping of Columbia University, the lead author of the study, tells Inverse.

Kipping and others had previously identified another exomoon candidate around the giant planet Kepler-1625 b. Astronomers haven’t confirmed this candidate either; some follow-up studies saw hints of an exomoon there as well, while others did not. That leaves both the authors and the astronomy community cautious about the newest claim.

“It’s not a slam dunk detection, and they don’t present it that way,” Kreidberg says. But the candidate is “absolutely worthy of follow up in the future,” she adds.

René Heller of the Max Planck Institute for Solar System Research says he isn’t convinced that this candidate will end up being a real moon. “That said,” he tells Inverse, “what I find astounding, really, about this paper is that it demonstrates that we are at the cusp of being able to find these moons, if they exist.”

This montage of Saturn and some of its 82 moons from Voyager 1 data shows the rich diversity of moons out there. Some are nearly planet-sized; others just a fraction of the size of our own Moon.


How they did it — To look for exomoon candidates, Kipping and the team looked at Kepler space telescope observations of stars with “cool giants,” or giant planets that orbit their host stars at a relatively large distance, much like Jupiter or Saturn. Some theoretical research suggests that moons are more likely to form around planets that sit farther out from their stars.

The Kepler data consists of measurements of how bright these stars appeared over time. When planets crossed in front of these stars, their brightnesses dipped. Kipping and the team compared these observed dips to models of what the dips would look like if they were caused by a planet alone or a planet and moon together.

Of the 70 cool giant planets they examined, three seemed to show dips that looked like they might be caused by a planet and moon together instead of a planet alone. After running some other tests, the culprit for one of these three outliers seemed to be a starspot — a relatively dim patch on a star’s surface — while an instrumental effect probably caused another, Kipping says.

For the third planet, Kepler-1708 b, the team hasn’t been able to find an alternative explanation, so they suggest that this planet may have a moon. If confirmed, this moon would be about two and a half times as wide as the Earth — a little smaller than the planet Neptune. This would place it as far larger than our Solar System’s largest moon, Ganymede, which is slightly larger than Mercury.

This illustration shows Kepler-1625b and a purported-companion — one of the first exomoon claims.

NASA, ESA, and L. Hustak (STScI)

Why it matters — Researchers still don’t know a lot about how moons form, and studying planetary systems with moons could teach us not only how moons form, but how their host planets form as well.

“We think that moon formation is probably a natural outcome of how planets form, but that process is still poorly understood,” Kreidberg says. “Seeing the full range of outcomes would really help us understand in more detail how these giant planets form.”

What’s next — To determine whether the candidate exomoon for the planet Kepler-1708 b is actually a moon, researchers need to observe the system for longer to try to catch the planet passing in front of its star a few more times.

Since following up this specific candidate would be time-consuming — Kepler-1708 b only orbits its star once every two years or so — looking for other exomoon candidates might be more worthwhile, Kipping suggests.

One option is the newly-launched James Webb Space Telescope, which would be able to spot moons as small as Jupiter’s moon Europa, Kipping says. Another exciting prospect for an exomoon-hunting survey is the PLAnetary Transits and Oscillations (PLATO) mission, planned for launch in 2026, which will observe brighter stars than Kepler did, making spotting any exomoons much easier.

“PLATO, in my opinion, is the next best bet for a mission to provide us with the data for an exomoon discovery,” Heller says.

Abstract — Exomoons represent a crucial missing puzzle piece in our efforts to understand extrasolar planetary systems. To address this deficiency, we here describe an exomoon survey of 70 cool, giant transiting exoplanet candidates found by Kepler. We identify only one exhibiting a moon-like signal that passes a battery of vetting tests: Kepler-1708 b. We show that Kepler-1708 b is a statistically validated Jupiter-sized planet orbiting a Sun-like quiescent star at 1.6 au. The signal of the exomoon candidate, Kepler-1708 b-i, is a 4.8σ effect and is persistent across different instrumental detrending methods, with a 1% false-positive probability via injection–recovery. Kepler-1708 b-i is ~2.6 Earth radii and is located in an approximately coplanar orbit at ~12 planetary radii from its ~1.6 au Jupiter-sized host. Future observations will be necessary to validate or reject the candidate.

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