JWST method rules out moons larger than 0.1 Earth radii around exoplanet LP 890-9 c

By the Everything Space news desk · 13 Sep 2026 · written from the sources below

Image: Phys.org

A new analysis technique shows JWST can detect small exomoons by combining multiple transits, opening a path to future discoveries even though this search came up empty.

A new study demonstrates that the James Webb Space Telescope is capable of detecting exomoons much smaller than previously thought possible, even though it hasn't found any yet. Astronomer David Kipping analyzed JWST observations of the exoplanet LP 890-9 c, an exoplanet orbiting a cold red dwarf star about 105 light-years away, and found no moons larger than roughly a tenth of Earth's radius orbiting it.

Earlier attempts to find exomoons relied on single transit observations, which were often overwhelmed by what astronomers call red noise, gradual drifts in the telescope's instruments or subtle changes on the star's surface. This noise made it nearly impossible to spot anything smaller than Earth, and some researchers had begun to suspect JWST had a hard limit on how small a moon it could ever detect.

Kipping's approach instead combined data from 12 separate transits of LP 890-9 c. Because moons must obey consistent orbital physics, their signals stay put across observations, while instrumental noise and stellar disturbances shift unpredictably from one transit to the next. Averaging multiple transits together allowed the random noise to cancel out, sharpening the ability to detect a real moon-sized signal.

Even with one especially noisy transit factored in, combining it with just one cleaner transit measurably improved detection sensitivity. The final result, based on all 12 transits, excluded moons larger than about 0.1 Earth radii with 95% confidence, making it the most sensitive exomoon search ever conducted beyond the solar system.

The null result was expected. LP 890-9 c orbits extremely close to its star, at a distance far tighter than Mercury's orbit around the sun, where intense gravitational tides would either fling a moon away or eventually grind it into a ring system over time. Kipping chose this system specifically to test the method rather than to find a moon.

The main takeaway is that JWST does not have an inherent noise floor preventing it from spotting moons roughly the size of our own moon around other planets. The obstacle going forward is not technology but time, since securing repeated transit observations of the same exoplanet requires a substantial commitment of JWST's heavily oversubscribed observing schedule.

Written in Everything Space’s own words by our news-desk model from the sources above, and checked against them in code: every number in this report appears in its sources, and no sentence is copied from them.