JWST ruled out finding tiny moons around an exoplanet—that’s great news

The James Webb Space Telescope (JWST) has opened many wonders of the cosmos to scientists since it began science operations a few years ago. But one thing it hasn’t done is find an “exomoon.” These still-theoretical moons orbit exoplanets in other star systems, and JWST was supposed to find a plethora of them.

However, it has found precisely none. The cause appears to be noise in the telescope’s instrumentation or from the star itself. But a new paper, available in preprint form on arXiv, from David Kipping, an astronomer at Columbia and host of the Cool Worlds YouTube channel, shows how JWST can find an exomoon. It likely just has to look for one in the same place repeatedly.

To prove the point, Dr. Kipping analyzed data JWST collected on a planet called LP 890-9 c. This exoplanet orbits an ultracool red dwarf located around 105 light-years away. With an 8.46-day orbital period, saying the planet is close to its star is an understatement, but, at least in theory, it is in the habitable zone of this extremely low-temperature host star. Another advantage was how often and how long JWST observed it—Kipping was able to analyze data from 12 different transits.

Previous efforts to detect exomoons relied on a single transit. For example, JWST watched the gas giant Kepler-167 e, hoping to spot a small moon in orbit around it. But “red noise” confused the computer models analyzing the data, making it impossible to detect any moon much smaller than Earth. “Red noise” is caused by slow, wandering changes in the detector, such as when it is warming up, its pointing drifts slightly or sunspots appear on the surface of the star itself.

The key, according to Kipping, is averaging. Exomoons have to obey the laws of physics, so they have to be in a certain place at a certain time around the planet. They don’t simply pop up where an instrumental glitch happened. And, crucially, instrumental glitches (and sunspots) don’t happen the same way every time during multiple transits.

Over time, those glitches and spots can be smoothed out, leaving a clean view of the starlight. In fact, one of the 12 transits of LP 890-9 c suffered from noticeable “red noise.” But when its data was combined with data from just one other “clean” transit, the sensitivity to detecting an exomoon rose significantly.

Unfortunately, in this particular case, that still led to a null result—the authors were able to confirm, with 95% confidence, that there are no moons larger than 0.1 Earth radii orbiting LP 890-9 c. That means no Ios, Europas or even Enceladuses. But it is by far the most sensitive moon hunt ever conducted outside our solar system.

Kipping didn’t actually expect to succeed. LP 890-9 c orbits 0.04 AU from its star—less than a tenth of the distance between the sun and Mercury. That means the gravitational tides from the star are absolutely brutal. Over the course of billions of years, those tidal forces will either strip a moon away, sending it into deep space, or pull it inward, causing it to turn into rings. Simply put, a moon around LP 890-9 c would not survive for long.

So why did Kipping use that specific planet as an example? Because the paper is meant to define a proof of concept rather than act as an attempt to find the first exomoon. It showed that JWST doesn’t have a “noise floor” that prevents it from detecting relatively small moons, as many astronomers had assumed in the past several years. It is very capable of finding moons the size of our moon—provided astronomers decide to observe multiple transits.

Unfortunately, that is easier said than done. Time on JWST is one of the most precious commodities in astronomy. It is responsible for everything from peering back to the earliest epochs of the universe to trying to track moons much closer to home. But if we hope to find an exomoon for the first time using this setup, it appears feasible. We just need more time.

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Gaby Clark

MA in English, copy editor since 2021 with experience in higher education and health content. Dedicated to trustworthy science news.

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Andrew Zinin

Master’s in physics with research experience. Long-time science news enthusiast. Plays key role in Science X’s editorial success.

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