No Moon Around a Temperate Rocky Planet, After Twelve Looks From Webb

A search for a moon around a rocky planet in another star system has come up empty. Astronomer David Kipping stacked 12 James Webb Space Telescope observations of LP 890-9c, a rocky planet in the temperate zone of its star, and found no sign of a moon.

Kipping reports the search in a paper posted Sept. 4 and accepted by The Astronomical Journal, which has not yet published it. Until now it had not been possible to test for moons as small as our own, even with Webb, and the new search establishes that the telescope can reach that far down.

The transits are the small dips in starlight each time the planet crosses the face of its star. Stacked together, they rule out moons larger than a tenth of Earth's radius at 95% confidence, anywhere around the planet where a moon could hold a stable orbit. That range covers analogs of Europa, Rhea and Umbriel. Kipping calls it by far the most sensitive search of its kind to date.

LP 890-9c orbits close in, at 0.04 astronomical units, a small fraction of Earth's distance from the sun, and tides at that separation would make a moon that big unlikely to last for billions of years.

Only two searches for moons around planets circling other stars had been attempted with Webb before this one, and both relied on a single transit. Kipping attributes their weaker-than-expected sensitivity partly to how much freedom a combined planet-and-moon model has when it is fitted to one event, and partly to noise nobody had anticipated.

A single transit can still deliver useful limits. One of the observations was spoiled by slow-drifting noise, but pairing it with a cleaner one brought the sensitivity back to the level of any other pair, which he says bodes well for a possible second Webb transit of Kepler-167e.