Moons are abundant in our solar system: Earth has one, Mars has two, Jupiter has 79 and counting, not to mention the other gas giants. Even objects as small as asteroids can have moons. But astronomers have never confirmed an exomoon—a moon that orbits a world outside of our solar system.



Until maybe now. And here's the really weird part: The tentative exomoon seems to be as big as Neptune.

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A Faint Signal of Something Huge

In research published this week in Science Advances, Columbia University professor David Kipping and graduate student Alex Teachey reported using the Hubble Space Telescope to spot something trailing the planet Kepler-1625b. They're just still trying to confirm that the something in question is what it appears to be: the first exomoon.

The object, which is 7,800 light years away, appears to orbit a Jupiter-sized planet called Kepler-1625b, which was discovered by the famous exoplanet-hunting Kepler Space Telescope. Kepler finds worlds via the transit method, when they pass in front of their star from our point of view and dim its light.



Three times while studying the planet, the Kepler telescope also picked up something else: the faint signal of something trailing Kepler-1625b. Kipping and Teachey announced those initial results last year, calling it an exomoon candidate, but needed Hubble confirmation of their findings to bolster the case, as Hubble is the only instrument capable of seeing such a distant, faint star and making any sense of the data.

"I think it certainly seems reasonable, given our own solar system, that moons should be common because we only have two planets that don’t have them," Kipping says in a press conference in advance of the release of the paper. Indeed, previous studies have found at least 100 good systems to look for exomoons, and efforts are under way to find them at Beta Pictoris b, a nearby gas giant.

Mega-Moon

If confirmed, this exomoon would be something of a mega-moon, as it is close in size to the gas giant planet Neptune.

Mammoth moons are not uncommon. For example, two moons in our solar system, Jupiter's Ganymede and Saturn's Titan, are larger than Mercury, the current smallest planet. Seven solar system moons are larger than Pluto, the once-and-maybe-future planet. What's weird about the potential exomoon is that its gargantuan expanse suggests a small gas giant orbiting a much larger one—if indeed it is a moon and not something else causing this weird transit trail.

"As much as we might like for this thing to be real, we’ve tried to be hard nosed about it every step of the way," Teachey said in the same press conference. So far, though, no explanation but exomoon has stood up to scientific scrutiny.

Kipping and Teachey have rejected the idea that the two objects could be binary planets, in which two planets have a barycenter—or common point of gravity—in space rather than inside either planet, meaning one's gravity doesn't dominate the other. But considering the 1.8 million miles from the possible exomoon to its home planet and the likely mass ratio between the worlds, Kipping and Teachey think they're seeing a planet-moon system. Planet-moon systems have a barycenter within the planet, the likely scenario in this system.



A few more tests are needed before the candidate can become a real moon. One smoking gun would be to witness a transit in which the moon is on the other side of the planet, which would indicate it's in orbit. The next transit opportunity comes later this month.

"We’re confident in the work we’ve done, and we’re confident, ultimately, that the scientific community will get to the bottom of this, one way or another," Teachey says.

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