The James Webb Space Telescope’s MIRI instrument directly detected methane on interstellar comet 3I/ATLAS, marking the first such identification on an object from another star system. Webb and complementary ground and ALMA observations found methane alongside methanol, hydrogen cyanide, cyanogen and atomic nickel. The delayed methane signal suggests subsurface release as solar heating penetrated deeper ices, and the comet’s high CO2-to-H2O ratio implies formation in conditions unlike those of our Solar System.
James Webb Detects Methane on Interstellar Comet 3I/ATLAS — Chemistry Unlike Our Solar System
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The James Webb Space Telescope has for the first time directly detected methane on an interstellar comet, offering a rare chemical window into material formed around another star. Webb's Mid-Infrared Instrument (MIRI) measured the spectral fingerprints of gases escaping from comet 3I/ATLAS, revealing a mix of volatile molecules that differs from typical Solar System comets.
Key observations: Webb observed 3I/ATLAS twice in December 2025. The comet is estimated to be roughly 1.6 miles (2.6 km) long. While ground-based visible spectroscopy and radio observations added molecules such as cyanogen (CN), atomic nickel, methanol (CH3OH) and hydrogen cyanide (HCN) to the inventory, MIRI uniquely detected methane (CH4) in the mid-infrared.
Methane Appeared After A Delay: The methane signature was not immediate; it emerged after a delay between observations. Scientists interpret this as methane trapped beneath the surface ices that was released only after solar heating penetrated deeper layers, a behavior that helps reveal the comet’s layered volatile structure.
Unusual Composition: The team also found an unusually high carbon dioxide (CO2) relative to water (H2O), strengthening the conclusion that 3I/ATLAS formed under chemical and thermal conditions different from those in our own Solar System.
Why it matters: As Caltech astronomer Matthew Belyakov explained, interstellar objects provide targeted glimpses into extrasolar small-body populations and are valuable for comparing planetesimal formation processes across the galaxy.
The results, described in a paper published in the Astrophysical Journal Letters, show how instruments like Webb’s MIRI combined with ground-based and radio facilities such as ALMA can build richer inventories of water, carbon, and other astrobiologically relevant compounds. Mapping these building blocks improves models of how planetary systems form and evolve.
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