天文学：新しい系外衛星の有望な候補天体

A new candidate for an exomoon — a moon that orbits planets beyond the realm of our Solar System — is presented in a paper published in Nature Astronomy. If its exomoon status is confirmed, Kepler-1708 b-i — which is 2.6 times larger than Earth — could represent a missing piece in the puzzle to understand the formation and evolution of extrasolar planetary systems.

Moons are ubiquitous in our Solar System, however, we have yet to confirm any that orbit exoplanets— although previous candidates have been suggested, such as Kepler-1625 b-i in 2018. The cool giant planets that orbit at some distance from their star, such as Jupiter or Saturn, are a favoured location for moon formation. However, such planets are challenging to detect with the transit technique— the most widespread method to find exoplanets that observes the small dip in brightness a planet­­­–moon system creates when it passes in front of its star.

David Kipping and colleagues surveyed the exoplanets discovered by the Kepler space telescope via transit to look for traces of exomoons. They focused on 70 gas giant planets with cool temperatures (less than 300 kelvin, roughly equivalent to 27 °C), which orbit their respective stars at a distance further than that between the Sun and Earth, namely with an orbiting period longer than a year. After rigorous vetting, the authors found only one signal, around a Jupiter-sized exoplanet called Kepler-1708 b. This signal can be best explained by the existence of an exomoon around Kepler-1708 b, named Kepler-1708 b-i, although there is a 1% probability of this signal being an artefact.

The authors caution that further evidence will be required to confirm the reality of the Kepler-1708 b-i signal and its subsequent status as a possible exomoon. Understanding the origins of such large moons, they state, will, however, represent a challenge for planetary formation theories.