The system showed up in a study using micro lensing, which looks for the bending of starlight due to the gravitational pull of an unseen object between a star and Earth. In this case the massive object might well be a planet and a moon. But the signal is not very clear, the researchers acknowledge, and could instead represent a dim star and a lightweight planet. “An alternate star-plus-planet model fits the data almost as well” as the planet-plus-moon explanation, the scientists reported in a paper that was posted this week on the preprint site arXiv. The study has not yet been peer-reviewed.

"I was excited by this paper," says astronomer Jean Schneider of the Paris Observatory, who was not involved in the research. Exomoons have "become fashionable these days," he adds, and are one of his personal "holy grails." Schneider wrote a paper in 1999 on how to detect exomoons using an alternative method, called transiting. (The transit technique looks for the dimming of a star's light caused when a planet or moon passes in front of the star from Earth's perspective).