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The first exomoon may be a Neptune-size world that orbits a Jupiter-size planet.

Dan Durda

First moon outside the solar system could be as big as Neptune

With help from the Hubble Space Telescope, astronomers say they have found compelling evidence for the first known moon outside the solar system. Just as the first alien planets were unexpected “hot Jupiters” that revolved close to their stars, this first reported “exomoon” is also strange: a Neptune-size megamoon, some 8000 light-years away, that looms over a giant planet, twice as large in the sky as Earth’s moon.

Both the study team and critics alike say that further confirmation is still needed, however, adding an element of caution to what would be a landmark discovery. “It’s exciting to see the hunt for the first exomoon continue, and with what would be a shockingly large moon,” wrote Sara Seager, an astrophysicist at the Massachusetts Institute of Technology in Cambridge who was not involved with the work, in an email.

Most theoretical models of planet formation struggle to produce such a hefty satellite. However, searches are biased toward the largest moons that might be out there, because bigger things are easier to detect. That makes it plausible that the new exomoon is just as off-the-menu as the “hot Jupiters” that surprised early exoplanet hunters, says Stephen Kane, an astrophysicist at the University of California, Riverside, who was not part of the research team. “That doesn’t mean it’s normal, it just means that might be all we’re sensitive to,” Kane says.

The first hints for the exomoon came from archival data from the Kepler probe, a NASA planet-hunting spacecraft, which looks for dips in brightness caused by unseen planets transiting in front of their suns. Alex Teachey and David Kipping, both of Columbia University, found that three dips, attributed to the planet Kepler-1625b, might actually be caused by a planet and a moon. The dimmings seemed to show additional dips in brightness, as if another body was tagging along as the planet crossed the star. They hoped Hubble, trained on another transit in October 2017, would clinch the case. They went public with their suspicion after Twitter sleuths pointed out that they had won time on the space telescope.

In the meantime, a new release of Kepler data smoothed away many of those bumps, weakening the original case for a moon search. “That’s kind of ironic,” Teachey says. Had they used the newer data, “we might not have asked for that [Hubble] time.” But the results of those Hubble observations, published today in Science Advances, bring the putative moon back in play; the team finds two new lines of evidence for a moon.

First, the transit occurred about 75 minutes earlier than expected. Gravitational tugs from other, unknown planets in the system—or a moon—can produce shifts in the exact timing of transits. Second, 3.5 hours after the dip caused by Kepler-1625b, another Neptune-size dip began—suggesting a moon, trailing after the planet. But the Hubble observations ended before the second dip was complete.

Teachey and Kipping are hoping to try again, and have requested time on Hubble for the next transit in May 2019. Until then, they will look for additional exomoon candidates in the Kepler catalog. Their work shows the most promising host planets would have distant orbits and long years, perhaps because gravitational interactions closer to stars can strip away moons.

For now, though, the team stands by its decision to publish its qualified probably-maybe discovery. “Science can’t operate by teams such as ourselves refusing to publish our results and hiding behind closed doors,” Kipping says in an online outreach video posted concurrently with the paper. “If refuted, then we have lost nothing, and the search goes on.”