Webb makes first methane detection on an interstellar visitor
The James Webb Space Telescope has detected methane on the interstellar comet 3I/ATLAS, marking the first direct detection of this molecule on any object from outside our solar system. The discovery was made using Webb's Mid-Infrared Instrument during observations in December 2025, when the comet was between 329 million and 379 million kilometers from the Sun.
Comet 3I/ATLAS is only the third confirmed interstellar object ever detected, after 1I/'Oumuamua and 2I/Borisov. Unlike 'Oumuamua, which appeared inactive, 3I/ATLAS has a bright coma of gas and dust. Its nucleus is about 2.6 kilometers in diameter. The research is published in the Astrophysical Journal Letters.
High methane levels point to an exotic origin
The amount of methane relative to water found on 3I/ATLAS is surprisingly high, with few similar examples in our own solar system. Methane is highly volatile, meaning it easily turns from solid ice into gas. Its delayed appearance in the comet suggests it was buried beneath the surface and only released when heat from the Sun reached deeper icy layers after the comet's closest approach.
The comet also remains rich in carbon dioxide compared to typical solar system comets. Scientists say the combined methane and CO2 abundances suggest the comet formed around a different type of star under conditions unlike anything in our solar system's early history.
Multiple instruments track the comet's chemistry
The discovery builds on earlier observations using ground-based telescopes, the ALMA radio observatory, and NASA's SPHEREx telescope. Previous detections found cyanogen, atomic nickel, methanol, hydrogen cyanide, water, carbon dioxide, and carbon monoxide. After the comet passed closest to the Sun, its gas production dropped sharply, with water showing the most pronounced decline as the surface cooled.