Webb Detects Methane in Interstellar Comet 3I/ATLAS

NASA’s James Webb Space Telescope has made the first direct detection of methane on an interstellar object, identifying the simple organic molecule in the comet 3I/ATLAS with mid-infrared observations that give scientists a rare chemical look at material formed around another star.

That matters because 3I/ATLAS is just the third confirmed interstellar object ever observed, after ’Oumuamua and Borisov. Interstellar objects are small bodies that formed in other star systems and were later thrown into interstellar space. Because so few have been seen, each one offers an unusual chance to compare extrasolar material with comets from our own solar system.

NASA said Sunday that Webb captured its first mid-infrared chemical fingerprint of an interstellar object during a revisit to 3I/ATLAS. The result comes from “The Volatile Inventory of 3I/ATLAS as seen with JWST/MIRI,” a paper by Matthew Belyakov of Caltech, Ian Wong of the Space Telescope Science Institute and coauthors, available on arXiv and referenced as published in The Astrophysical Journal Letters in 2026. Webb used its Mid-Infrared Instrument, or MIRI, and its medium-resolution spectrometer to observe the comet after perihelion, the point of closest approach to the sun, during two windows on Dec. 15-16, 2025, and Dec. 27, 2025.

In those spectra, the team identified methane fluorescence features between about 7.50 and 7.65 microns. “We also report the first direct detection of CH4 in an interstellar object,” the authors wrote in the paper. Mid-infrared spectroscopy is especially useful here because molecules absorb and emit light at distinct wavelengths, producing fingerprints that can be hard to confirm at other parts of the spectrum. In methane’s case, its bending mode falls in a wavelength range Webb can resolve.

The same MIRI data also showed water vapor and strong carbon dioxide features. The authors reported that 3I/ATLAS has an unusually high carbon-dioxide-to-water ratio, along with a methane-to-water ratio enriched relative to typical solar system comets. Over the roughly 12 days between the two observing periods, the comet’s methane production rate fell by about 50%, while overall gas production also declined. The drop in water was especially steep.

The researchers said that pattern points away from methane coming mainly from the comet’s irradiated outer surface. Instead, they interpreted the delayed post-perihelion appearance of methane as a sign that subsurface methane ice was exposed or activated as solar heating reached deeper into the nucleus. That interpretation is not presented as settled fact, but as the team’s best explanation for the timing of the signal.

Earlier observations had already shown that 3I/ATLAS was rich in carbon dioxide. The new Webb result adds a direct methane identification in the mid-infrared, expanding the chemical inventory of a comet that lead author Belyakov called “a very interesting object” in a Caltech press release in April. 3I/ATLAS was discovered in July 2025 by the ATLAS survey in Chile, and follow-up observations confirmed its interstellar origin, making it the third confirmed visitor from beyond the solar system.