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It’s a moonshot. A signal has been spotted that might be the first moon detected outside our solar system, and researchers are gearing up to use the Hubble Space Telescope to confirm it.

at Columbia University in New York and his colleagues have been using the Kepler Space Telescope to search for moons around other worlds for years, but they haven’t found any yet. “We’ve had candidates in the past and investigated them, and most of them have evaporated,” says Kipping.

The Kepler Space Telescope finds planets by watching them pass in front of their stars, causing a dip in the stars’ light. The new potential moon was found in the same way – as a moon orbits its planet, it leads to an extra fall in the starlight coming from behind.

Kipping and his colleagues observed these characteristic dips over three orbits of the planet around its sun-sized star, which is called Kepler-1625. Their observations suggested that a moon was there with a statistical confidence of just above 4 sigma. That means if the moon is not real, there’s only around a 1 in 16,000 chance of seeing the exact same signal through a fluke in the data.

“It is consistent with the signal that we might expect from a moon, but it might be consistent with other things as well,” says Kipping. The system is almost 4000 light years away and relatively faint, so more observations are needed to verify that the Kepler signal was really a moon and not just a statistical blip.

Hubble is much more powerful than Kepler, so the group has proposed to point the telescope at Kepler-1625 in October, when the planet is expected to transit its star again, to get a clear observation.

“We anticipate that the proposed measurements would be sufficient to confirm the first unambiguous detection of a moon beyond our Solar System,” the team writes in its .

Giant moon

The team says the moon, if it exists, is probably the size of Neptune, and orbiting a Jupiter-sized planet. Given what we know about how planets are born, it seems unlikely this arrangement could have formed to begin with, but the large moon could have been captured by the planet at a later time.

“If there really is something there, it’s such a faint star that it’d have to be a planet-sized moon for them to have seen it transit,” says at Royal Holloway, University of London. “It would be spectacularly different than anything we see in the solar system.”

Because there are so many diverse moons in our solar system, most astronomers assume that there are lots of moons around more distant planets as well. “I think we’re pretty sure that they’re going to be there,” says Waltham. “It would be pretty odd that there are hundreds of moons in the solar system but none anywhere else.”

If Kipping and his team are able to verify this detection, as well as being the first exomoon we’ve ever seen, it would be a much larger moon than we’ve ever seen before. This indicates that there may be even more types of moon than the many we’ve already observed.

“It would be analogous to the first exoplanet detections, which defied our prejudices from the solar system as well,” says Duncan Forgan at the University of St Andrews in the UK.

We’ll have to wait a few months to find out for sure whether it’s out there.

“It may prove to be nothing, or it may prove to be a really fabulous discovery,” says Waltham. “We won’t know until the Hubble data comes back.”

Journal reference:

Read more: Find exomoons by watching how they warp their planet’s light