SCIENTISTS HAVE IDENTIFIED THOUSANDS OF EXOPLANETS ORBITING OTHER STARS. But spying moons orbiting those planets has been a struggle. On October 1, 2018, researchers announced they may have just found the first of these so-called exomoons.

The Kepler space telescope looked for tiny dips in a star’s brightness, which can signal a planet passing in front of the star. Columbia University astronomer David Kipping and graduate student Alex Teachey looked at dips from 284 stars. In the signal from one, Kepler-1625, they saw an intriguing second, smaller dip — possibly an exomoon. Over 40 hours in October 2017, the Hubble Space Telescope (HST) collected light from Kepler-1625. Teachey and Kipping carefully combined the data with the Kepler observations, taking into account differences between the instruments and other sources of uncertainty. “We’ve tried our best to rule out other possibilities such as spacecraft anomalies, other planets in the system, or stellar activity, but we’re unable to find any other single hypothesis [that] can explain all of the data we have” aside from an exomoon, said Kipping in a press conference. Not everyone is convinced just yet, and Kipping and Teachey acknowledge that more observations are needed. They hope to use HST again in May 2019. The researchers also hope more scientists will probe the results. “Some people will be convinced, and other people will be skeptical,” says Teachey, “and that’s all part of the process.” Indeed, it’s the scientific method at work.