Webb Watched a Rocky Planet 105 Light-Years Away Cross Its Star 12 Times and Found No Moon Bigger Than a Tenth of Earth. That Is the Good News.

Astronomers have found more than 6,000 planets orbiting other stars and not one confirmed moon. A new analysis of James Webb Space Telescope data does not change that count, but it changes what the null result means. David Kipping of Columbia University combined 12 separate transits of the rocky exoplanet LP 890-9c and found no moon down to a tenth of Earth's radius, with 95% confidence, anywhere in the region where a moon could survive. It is by far the most sensitive search for an exomoon ever carried out, and it shows that Webb can detect bodies that small if astronomers give it enough transits.

LP 890-9c orbits an ultracool red dwarf about 105 light-years from Earth, circling it every 8.46 days at a distance of only 0.04 astronomical units, roughly a tenth of Mercury's distance from the Sun. Because the star is so faint and cool, that tight orbit leaves the planet temperate rather than scorched, and its rocky, roughly Earth-sized profile has made it a target for atmospheric studies with Webb. Kipping used those observations for a different purpose: to look for the small, additional dips in starlight, or the shifts in transit timing, that a moon would produce.

The obstacle in exomoon searches has never been the size of the telescope so much as "red noise," slow drifts in the signal caused by the detector, tiny changes in the telescope's pointing and spots on the star's surface. A single transit can hide a moon's signature inside that noise. Kipping's approach was to average across all 12 transits so that the random drifts cancel while any real, repeating signal survives. The result was clean enough to exclude moons larger than 0.1 Earth radii throughout the planet's Hill sphere, the zone where its gravity dominates the star's.

That limit rules out analogs of Europa, Rhea and Umbriel, and of course our own Moon, which is 0.27 Earth radii. The paper was posted to the arXiv on September 4 and has been accepted for publication in the Astronomical Journal.

Kipping, who also hosts the Cool Worlds channel on YouTube, argues the non-detection is what makes the result useful. Many astronomers had assumed Webb had an effective noise floor that would stop it from ever seeing moons this small, and had planned accordingly. The LP 890-9c data show there is no such floor. The telescope "is very capable of finding moons the size of our moon, provided astronomers decide to observe multiple transits," he wrote. Even a single high-quality transit, the paper finds, can produce a meaningful constraint.

The absence of a moon around this particular planet is not itself surprising. At 0.04 AU, tides from the star would strip a large moon from LP 890-9c over time, so the planet was never a strong candidate to host one. The better targets are planets on wider orbits, where moons can persist for billions of years, and those are also the planets whose transits are rarer and harder to stack. Kipping's point is that the trade is now worth making. A dozen transits of the right planet, observed with Webb, could turn the first exomoon from a decades-old ambition into a measurement.