Why astronomers reluctantly announced a possible exomoon discovery

By Davide Castelvecchi

The high-profile quest to spot moons orbiting distant planets has been a series of let-downs, with each hint of an ‘exomoon’ fading under closer inspection. So astronomer David Kipping, at Columbia University in New York City, didn’t want to reveal his team’s detection of another possible exomoon, until they could confirm it using the Hubble Space Telescope.

That plan was abandoned a few days ago, after news of the team’s request for Hubble time rocketed around social media. It culminated in the announcement that “exomoon candidate Kepler-1625 b I” had been observed orbiting a planet 4,000 light years (1,230 parsecs) from Earth, in an arXiv preprint1 posted on 27 July. That paper, reporting the results of a 5-year search for exomoons, was hastily amended to include the exomoon claim.

Kepler-1625 b is a candidate planet that Kepler, NASA’s flagship exoplanet mission, had previously observed. Periodic dips in the host star’s brightness indicated that a massive object was crossing the line of sight from the star to Earth; but the dips were lopsided, suggesting that perhaps instead of one object there were two: a Jupiter-sized planet with a Neptune-sized moon in tow. If this were indeed an ‘exomoon’, it would have been a long-awaited discovery. But it was still a big if.

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