JWST Hasn't Found Any Moons Around Other Planets — But a New Study Shows How It Could

The James Webb Space Telescope (JWST) has transformed astronomers' view of the cosmos since it began operating, but it has yet to find a single "exomoon" — a moon orbiting a planet in another star system — despite expectations that it would find many. A new study, posted as a preprint on arXiv by Columbia University astronomer David Kipping, shows how JWST could still find one: by observing the same system's transits repeatedly and averaging the results.

Kipping analyzed JWST data on LP 890-9 c, a rocky exoplanet orbiting an ultracool red dwarf star about 105 light-years away with an 8.46-day orbital period, placing it in the star's habitable zone despite the star's very low temperature. Unlike earlier exomoon searches, which relied on a single transit, Kipping used data from 12 separate transits of the planet.

Previous single-transit searches, such as one targeting the gas giant Kepler-167 e, were confounded by "red noise" — slow, wandering instrument changes such as the telescope warming up, its pointing drifting, or sunspots appearing on the star — that masked the small signal of a possible moon. Because exomoons must obey the laws of physics and appear in a consistent location relative to their planet, while instrumental glitches and sunspots don't recur the same way across transits, averaging many transits together can smooth out that noise. Combining just one of the noisier transits with a single clean one already substantially raised the sensitivity of the search.

The result there was still a null one: the analysis confirmed, with 95% confidence, that no moons larger than 0.1 Earth radii orbit the planet — the most sensitive exomoon search ever conducted. Kipping did not expect to find a moon there, since the planet's extreme closeness to its star, less than a tenth of the distance between the Sun and Mercury, would subject any moon to tidal forces strong enough to destroy it over time. The study's goal was instead to show that JWST has no fundamental noise floor preventing it from detecting moons the size of Earth's, given enough observing time on the telescope.