Webb detects methane on interstellar comet for first time

Webb Detects Methane on Interstellar Comet 3I/ATLAS

NASA’s James Webb Space Telescope has made the first direct detection of methane on an interstellar object, capturing mid-infrared observations of comet 3I/ATLAS as it traveled away from the Sun late last year. The findings, published in The Astrophysical Journal Letters, reveal a chemical composition unlike anything seen in solar system comets.nasa

A Chemical Fingerprint From Beyond

Webb’s Mid-Infrared Instrument captured spectra of 3I/ATLAS on two dates: December 15–16, 2025, when the comet was about 205 million miles from the Sun, and December 27, when it had moved to roughly 236 million miles away. Both observations came after the comet’s closest approach to the Sun, as it was already heading back toward interstellar space.nasa

The methane detection marks a first for any interstellar visitor. Scientists believe the gas had been buried beneath the comet’s surface layer and was protected from sublimation until solar heat penetrated deeper into the icy subsurface during its passage through the inner solar system. The amount of methane relative to water was “surprisingly high, with few similar analogs in our own solar system,” according to NASA.skyatnightmagazine

An Alien Chemistry

Webb also confirmed that 3I/ATLAS is unusually rich in carbon dioxide, releasing far more of it relative to water than typical solar system comets. These characteristics point to a formation environment markedly different from the conditions that produced comets in our own neighborhood of the galaxy.skyatnightmagazine

The telescope observed a sharp decline in gas production as the comet moved farther from the Sun, with water vapor dropping most quickly — expected behavior, since water is less volatile than methane or carbon dioxide and stops sublimating sooner as temperatures fall.nasa

A Rare Glimpse at Interstellar Material

Comet 3I/ATLAS is only the third interstellar object ever detected passing through our solar system, following 1I/’Oumuamua in 2017 and 2I/Borisov in 2019. It was first spotted on July 1, 2025, by the ATLAS survey and may be as old as 10 billion years, potentially making it the oldest comet ever studied. The comet is now on its way out of the solar system and will never return, making these Webb observations among the last detailed looks scientists will get at this ancient messenger from another star system.phys