Evidence for a second supermoon beyond our solar system

Hinting at the possibility of more exomoon discoveries to come.


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In a new study, astronomers reported discovering an exomoon, a super-sized moon orbiting a Jupiter-sized planet beyond our solar system. They made this discovery after analyzing archival data, indicating the possibility of more exomoon discoveries to come.

David Kipping and his Cool Worlds Lab at Columbia University said, “Astronomers have found more than 10,000 exoplanet candidates so far, but exomoons are far more challenging. They are terra incognita.”

If confirmed, it could mean that exomoons are as common in the universe as exoplanets. The first exomoon was spotted four years ago. That first-ever sighting is still awaiting confirmation. Similarly, the second-sighting could be a long wait.

The team spotted the giant exomoon candidate orbiting the planet Kepler 1708b. The planet is located 5,500 light-years from Earth in the Cygnus and Lyra constellations. It is about a third smaller than the Neptune-sized moon when it comes to size.

Kipping said, “Both supermoon candidates are likely made of gas that has piled up under the gravitational pull caused by their enormous size. If one astronomer’s hypothesis is correct, the moons may have even started life as planets, only to be pulled into the orbit of an even bigger planet.”

Both exomoons are located far from their host star. Hence, there is less gravity to pull at planets and strip off their moons.

While searching for an exomoon, scientists found giant gas planets on wide orbits, precisely because the analog in our solar system, Jupiter and Saturn, have more than a hundred moons between them.

Kipping said, “If other moons are out there, they will likely be less monstrous, but also harder to spot. The first detections in any survey will generally be the weirdos. The big ones that are easiest to detect with our limited sensitivity.”

The team looked at the sample of the coldest gas giant planets captured by NASA’s planet-hunting spacecraft, Kepler. They scanned 70 planets in-depth and found just one candidate- Kepler 1708b- with a moon-like signal.

Kipping said, “It’s a stubborn signal. We threw the kitchen sink at this thing, but it just won’t go away.”

Eric Agol, an astronomy professor at the University of Washington, said, “he’s doubtful that this latest signal will turn out to be real. It might just be a fluctuation in the data, either due to the star or instrumental noise.”

Michael Hippke, an independent astronomer in Germany, said, “Others were more optimistic. This is science at its best. We find an intriguing object, make a prediction, and either confirm the exomoon candidate or rule it out with future observations.”

“I am very excited to see a second exomoon candidate, although it is unfortunate that only two transits have been observed. More data would be very cool.”

Kipping said“But the search is worth it, as he recalled how the existence of exoplanets was greeted with the same skepticism as exomoons are today. Those planets are alien compared to our home system. But they have revolutionized our understanding of how planetary systems form.”

Journal Reference:

  1. Kipping, D., Bryson, S., Burke, C. et al. An exomoon survey of 70 cool giant exoplanets and the new candidate Kepler-1708 b-i. Nat Astron (2022). DOI: 10.1038/s41550-021-01539-1