Possible Exomoon discovery

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.

Another astronomer noticed that Kipping’s team had requested time on the Hubble Space Telescope in October and shared the news on Twitter. Kipping is one of the most prominent astronomers in the hunt for exomoons. So if he wanted to use the Hubble, there was only one possible reason.

Kipping says he does not blame the colleague for tweeting about something that was in the public record. But then, on 25 July, his phone started to ring: journalists were wondering whether Kipping had made the big discovery he had focused his whole career on.

So, to pre-empt speculation, the team decided to be transparent about what it had found — and about what it hadn’t. “We figured the only option we had was to get ahead of the story,” says Kipping. As news reports appeared, one of the authors of the arXiv preprint, Alex Teachey, wrote a guest-blog post for Scientific American to explain his team’s decision.

“Let’s be clear: we’re not just trying to save ourselves from embarrassment,” Teachey wrote. “The announcement and subsequent retraction of potentially ground-breaking results has the effect of eroding public trust in science over time, and we are chiefly concerned with not contributing to that problem.”

Jean Schneider, an exomoon hunter at the Paris Observatory, says that the authors were right to make this candidate public. Now, he says, “other people can re-analyse the Kepler data for Kepler-1625 b and make their own opinion”.

Arxiv – HEK VI: On the Dearth of Galilean Analogs in Kepler and the Exomoon Candidate Kepler-1625b I

Exomoons represent an outstanding challenge in modern astronomy, with the potential to provide rich insights into planet formation theory and habitability. In this work, we stack the phase-folded transits of 284 viable moon hosting Kepler planetary candidates, in order to search for satellites. These planets range from Earth-to-Jupiter sized and from ~0.1 to 1.0 AU in separation – so-called “warm” planets. Our data processing includes two-pass harmonic detrending, transit timing variations, model selection and careful data quality vetting to produce a grand light curve with a r.m.s. of 5.1 ppm. We find that the occurrence rate of Galilean-analog moon systems can be constrained to 95% confidence for the 284 KOIs considered. A single-moon model of variable size and separation locates a slight preference for a population of Super-Ios, ~0.5 R_Earth moons orbiting at 5-10 planetary radii. However, we stress that the low Bayes factor of just 2 in this region means it should be treated as no more than a hint at this time. Splitting our data into various physically-motivated subsets reveals no strong signal. The dearth of Galilean-analogs around warm planets places the first strong constraint on exomoon formation models to date. Finally, we report evidence for an exomoon candidate Kepler-1625b I, which we briefly describe ahead of scheduled observations of the target with the Hubble Space Telescope.