James Webb’s MIRI instrument detected methane from the interstellar object 3I/ATLAS in mid- and late-December, about two months after its perihelion. The observations — the first chemical fingerprint of an object from another star system — also mapped carbon dioxide and water. A high methane-to-water ratio and abundant CO2 suggest 3I/ATLAS formed in a different chemical environment than most Solar System comets.
James Webb Detects Methane From Interstellar Visitor 3I/ATLAS — First Chemical Fingerprint of an Object From Another Star System
Illustration by Tag Hartman-Simkins / Futurism. Source: Shutterstock
The James Webb Space Telescope's Mid-Infrared Instrument (MIRI) has detected methane coming from the interstellar object 3I/ATLAS, marking the first-ever chemical fingerprint of a body from beyond our Solar System. Observations were taken in mid- and late-December, roughly two months after the object passed perihelion (its closest approach to the Sun).
What Webb Saw
Analyses of the MIRI data, published in The Astrophysical Journal Letters, show not only methane but also carbon dioxide and water distributed around the object. Because the gas was detected post-perihelion — when comets typically release the most material as they warm — researchers infer the methane was likely trapped beneath a thick, icy crust and only escaped when heating eroded that layer.
Why This Matters
Methane is highly volatile and sublimates from ice into gas at relatively low temperatures, so finding it on 3I/ATLAS is surprising. The methane-to-water ratio is higher than in most Solar System comets, and the object appears unusually rich in carbon dioxide. Together, these chemical signatures point to a formation environment and chemistry that differ from the majority of comets that formed around our Sun.
“Both these findings point to a very different formation environment and chemistry than the vast majority of comets that formed within our Solar System,” the European Space Agency wrote in response to the findings.
These results deepen our understanding of small bodies from other star systems and give astronomers rare direct evidence of the chemical diversity of material circulating in the galaxy. Continued analysis of archival and follow-up observations will help refine models of where and how such interstellar objects form.
Reference: Observations and analysis reported in a paper in The Astrophysical Journal Letters, based on MIRI data from NASA’s James Webb Space Telescope.
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