An artist’s impression of the potential exomoon (blue) orbiting its planet.Credit: Dan Durda

Astronomers have spotted what could be the first known moon to orbit an exoplanet.

Researchers have detected hints of this object before. Last year, a team led by Alex Teachey, an astronomer at Columbia University in New York City, reported its initial suspicion1 that a moon orbits the planet Kepler-1625b, which lies 2.4 kiloparsecs (about 8,000 light years) from Earth in the constellation Cygnus.

But now, Teachey and his colleagues have new and better observations from the Hubble Space Telescope — and they are much more confident that the exomoon is real. “We’re not cracking open champagne bottles just yet,” says Teachey. But “things look exciting, tantalizing, maybe compelling”. The work2 appears on 3 October in Science Advances.

"I'm about 75% thinking it's a moon," says Sarah Ballard, an astronomer at the Massachusetts Institute of Technology in Cambridge. "That encapsulates my scepticism and my confusion and my hope."

The first confirmed detection of an exomoon would mark a milestone in exploring planetary systems throughout the Galaxy. It would, among other things, allow scientists to test ideas of moon formation using examples from beyond the Solar System.

Teachey’s proposed exomoon is already throwing up some surprises. Evidence suggests the moon is about the size of Neptune, orbiting a planet roughly the size of Jupiter. That would make it unlike anything in the Solar System, where most moons are much smaller than the planets they orbit. “It’s raising new questions about the dynamical processes that create planets and moons,” Teachey says.

Follow the leader

He and his colleagues first spotted the possible exomoon in data from NASA’s Kepler space telescope. The instrument finds planets by watching the light of a star dim as an orbiting planet passes across its face, temporarily eclipsing some of the starlight. Spotting moons is even harder than looking for planets because they’re so small and block only a tiny fraction of the light from a star. Team member David Kipping at Columbia has led a project searching for signs of exomoons in Kepler data for years.

Over the course of four years, Kepler recorded the planet moving across the face of its star three separate times. Each of those passages also showed faint, additional dimmings that hinted at the presence of a moon. In October 2017, Teachey and Kipping used the much more powerful Hubble Space Telescope to confirm their suspicions.

Two things jumped out at the scientists from the Hubble data. First, the planet started moving across the star 78 minutes earlier than expected — an observation suggesting that the gravity from some other celestial body, such as a moon, was tugging on the planet and temporarily speeding it up. Second, after the planet finished moving across the star, its light didn’t brighten immediately as much as expected. It was as if something, such as a moon, was trailing after the planet as it crossed the face of the star.

The researchers need more observations to confirm the existence of an exomoon. So Teachey has put in a proposal to use Hubble to collect more data when the planet next crosses its star, in May 2019.

Reserving judgement

“It’s a very, very exciting object,” says René Heller, an astrophysicist at the Max Planck Institute for Solar System Research in Göttingen, Germany. But he remains sceptical.

Heller questions how the Kepler-1625b exomoon could have formed, because it is too big compared to its planet to be explained easily by most theories3. He also notes that the star itself is faint, making it harder to extract a meaningful signal from the data. “It’s just tricky,” Heller says.

"This is not a smoking-gun detection of an exomoon," adds Rodrigo Luger, an astronomer at the Center for Computational Astrophysics in New York City. He and Ballard both note that Teachey and Kipping did a very thorough job in analyzing all their data. But there's a chance that a second, previously unknown planet orbiting the star could create the signals being interpreted as a moon around Kepler-1625b.

Future missions — such as the James Webb Space Telescope, slated to launch in 2021 — might be able to observe the star well enough, and for long enough, to help nail down whether the proposed exomoon really exists.

But Kepler — which has far exceeded its planned four-year lifespan — won’t be part of the hunt. The spacecraft is almost entirely out of fuel, which it needs to orient itself properly. Because it can’t do that any more, it can’t point reliably at celestial targets. On 26 September, mission controllers put Kepler into sleep mode, which does not require fuel. They hope to be able to download the last of its science data starting on 10 October.