The James Webb Space Telescope (JWST) was used to search for moons around the rocky, temperate exoplanet LP 890-9c. Using twelve JWST transits, the team found no evidence for a moon. They can rule out moons as small as 0.1 Earth radii with 95% confidence across the planet’s full Hill region, the zone where a planet can hold on to satellites.

Previous JWST attempts to find exomoons have used only a single transit and were less sensitive. The authors argue that fitting a combined planet-plus-moon model to a single event leaves a lot of flexibility in the fit, and that unexpected instrumental or observational noise can hide or mimic a moon. By using twelve transits the researchers reduced that ambiguity and searched more deeply for small signals.

Reaching a limit of 0.1 Earth radii is significant because it rules out moons similar in size to many real Solar System satellites. The study explicitly excludes analogs to moons such as Europa, Rhea and Umbriel around this planet. The authors describe this result as the most sensitive exomoon search so far and show that JWST can probe down to remarkably small moon sizes.

There are important caveats. LP 890-9c orbits fairly close to its star at about 0.04 astronomical units. The authors note that tidal forces at that distance would make it unlikely for moons larger than roughly 0.1 Earth radii to survive for billions of years. Also, one of the twelve observed transits was affected by red noise, a kind of correlated systematic error, which degraded the sensitivity for that epoch. The team found that pairing a noisy epoch with a cleaner one could restore sensitivity, however.

Overall, the study shows that repeated JWST transits can set strong limits on small exomoons and that a small number of clean observations can be nearly as powerful as many. The JWST data used in the work are archived at the Mikulski Archive for Space Telescopes, and the approach offers a clear path for more sensitive exomoon searches in the future.