JWST Spots Signs of Exomoon Birth in Alien Planet’s Disk
December 2, 2025
2 min reading
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James Webb Space Telescope spots swirling cradle for exomoons
Scientists have found evidence of the formation of a distant planet’s lunar system
An illustrated disk forming the moon surrounds an alien planet.
NASA, ESA, CSA, STScI, Gabriele Cugno/University of Zurich, NCCR PlanetS, Sierra Grant/Carnegie Science, Joseph Olmsted/STScI, Leah Hustak/STScI
For the first time, scientists have directly detected molecules in a frisbee of gas and dust swirling around an alien gas giant planet. “I didn’t think this was possible,” says astronomer Sierra Grant of Carnegie Science in Washington, DC. Typically, such a weak signal would be invisible in the glare of a star. Grant and his co-author Gabriele Cugno of the University of Zurich, who recently published the results in the Letters from astrophysical journalsbelieve that the carbon-rich disk is a lunar nursery and are already planning to observe several others; Researchers might eventually be able to detect gaps carved into these disks by nascent moons.
Grant and Cugno used the James Webb Space Telescope (JWST) to detect the infrared glow from the disk of gas and dust surrounding a Goliath world called CT Cha b. Spotting the light cast by a planet – let alone a disk around it – is like seeing a firefly in front of a spotlight. This is easier if the firefly is huge and far from the light. CT Cha b weighs between 14 and 24 Jupiter masses and orbits its star about 17 times farther than Neptune does around the sun.
Previous observations had suggested that CT Cha b was gobbling up material from an as-yet-unseen disk, and Cugno aimed to disentangle that disk’s infrared heat glow from the star’s bright glow. Grant was skeptical that it could be done, but Cugno wanted to test the limits of JWST. “It was almost a game,” he says.
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Cugno eventually extracted from the data a “magnificent” light spectrum of the disk, in which Grant spotted clear chemical fingerprints of carbon-rich compounds such as hydrogen cyanide, acetylene and even molecules as complex as six-carbon benzene rings, substances absent from the material swirling directly around CT’s star Cha b. The disk could be a lunar construction zone around the planet.
“This could give us an idea of the material available for the formation of exomoons,” says astrophysicist Danny Gasman of the Max Planck Institute for Astronomy in Germany, who was not involved in the study. She cautions, however, that while CT Cha b’s size and extreme distance from its star make it an ideal target, it also means it could be more like a failed star than a typical gas giant.
Even in our own solar system, the origins of moons remain mysterious. Discs such as those from CT Cha b offer a chance to understand not only the moons of extraterrestrial systems, but also the moons of our own. “It’s really hard to go back 4.5 billion years and imagine how they were created,” says Cugno. “Now we can actually see this process.”
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