First Possible Exomoon Is the Size of Neptune

You read that right. Astronomers have a good lead on what appears to be a moon in a distant star system that’s as big as the planet Neptune.

In a way, a candidate exomoon would have to be massive—the bigger the body, the easier it is for our telescopes to observe it. In this case, they spotted it some 4,000 light years away in data gathered by the Kepler space telescope, which detects exo-bodies by watching for the regular, repeat dimming of a star, which indicates a planet (or moon) passing in front of it.

An artist's impression of an exomoon orbiting a gas giant.

Astronomers noticed the curious signal after carefully analyzing readings of 284 planets that looked like Jupiter and were most likely to host a moon.

Here’s Michael Greshko, reporting for National Geographic:

Some of the planets are Jupiter-size but are snuggled close to their stars. Astronomers think these so-called hot Jupiters formed in the chillier outskirts of their star systems but then migrated inward—raising questions about what would happen to their moons.

“They’re looking at planets that are much closer to their suns than Jupiter is to our own,” says Leiden Observatory astronomer Matthew Kenworthy, who wasn’t involved with the study. “So the question is, during this process of migration, do big fat gas giants lose their moons?”

One, of course, stood out. That it’s a moon is not confirmed yet—astronomers have booked time on the Hubble space telescope in October to get a better look—and it is only one data point. But if scientists can get an answer to this one, and maybe find several more, they might have a better idea about what happened to all those moons once held by gas giants.

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