Webb finds methane on interstellar comet 3I/ATLAS

NASA’s James Webb Space Telescope has captured something remarkably rare: a detailed chemical fingerprint from an object that came from beyond our Solar System. In new observations of interstellar comet 3I/ATLAS, Webb directly detected methane gas for the first time in an interstellar visitor, adding a striking new piece to the puzzle of how this alien comet formed.

The result, announced by NASA on 1 June 2026 and published in The Astrophysical Journal Letters, matters because 3I/ATLAS is not simply another icy wanderer. It is passing through once and then disappearing back into interstellar space. That gives astronomers a narrow window to ask a profound question: what does leftover building material from another planetary system actually look like?

Webb’s data suggest that 3I/ATLAS is chemically unusual compared with most comets born in our own cosmic neighbourhood. Alongside methane, the comet was confirmed to be especially rich in carbon dioxide relative to water, pointing to a formation history unlike that of the vast majority of Solar System comets.

What Webb detected on comet 3I/ATLAS

The observations were made with Webb’s Mid-Infrared Instrument, or MIRI, using its Medium Resolution Spectrometer. This instrument splits infrared light into fine wavelength bands, allowing researchers to identify gases escaping from the comet and map how those gases were distributed around its nucleus.

Webb observed 3I/ATLAS on two occasions after the comet had already swung around the Sun. The first set of observations took place on 15 to 16 December, when the comet was about 205 million miles from the Sun. A second observation followed on 27 December, when it had receded to about 236 million miles.

That timing turned out to be revealing. Methane is a highly volatile substance, so it turns from ice to gas very readily. Yet on 3I/ATLAS it appeared late, suggesting the methane had been hidden beneath the comet’s outer layer. Only after the close solar pass did heat penetrate far enough into the subsurface to release it. In other words, Webb may have caught the comet exposing deeper, more pristine material. How often do we get a glimpse inside an interstellar relic like that?

Observation detail	What NASA reported
Telescope and instrument	James Webb Space Telescope, MIRI Medium Resolution Spectrometer
Observation dates	15–16 December and 27 December, post-perihelion
Distance from the Sun	About 205 million miles, then 236 million miles
Key gases identified	Methane, water and carbon dioxide
Headline finding	First direct methane detection in an interstellar object

Why the methane discovery is scientifically unusual

The methane itself is only part of the story. NASA said the amount of methane relative to water was surprisingly high, with very few close parallels among comets in our own Solar System. Webb also showed that 3I/ATLAS continued to release far more carbon dioxide relative to water than typical Solar System comets do.

Taken together, those measurements point towards a different chemical environment during the comet’s birth. NASA’s interpretation is cautious but clear: 3I/ATLAS likely formed under conditions unlike those that produced most local comets. That does not tell us which star system it came from, nor does it justify broad claims about all extrasolar comets. Even so, it sharply widens the range of comet chemistry astronomers now have to consider.

The behaviour of the outgassing also fit expectations as the comet moved away from the Sun. Webb recorded a steep decline in gas production, especially for water. That makes physical sense. Water is less volatile than methane or carbon dioxide, so once sunlight weakens and the comet cools, water vapour production shuts down faster.

The spatial pattern of those gases added another layer of detail. According to NASA, water vapour spread far beyond the nucleus because much of it was being released from icy grains in the coma, while methane and carbon dioxide remained more concentrated near the nucleus itself.

A fleeting visitor, and a bigger picture

NASA has been using multiple missions to study 3I/ATLAS since its discovery in the summer of 2025, precisely because interstellar objects do not linger. The comet poses no threat to Earth, but scientifically it is a gift that cannot be revisited once it fades back into the dark between the stars.

That is what makes Webb’s new result more than a technical triumph. Infrared spectroscopy has turned 3I/ATLAS from a moving point of light into a chemically readable worldlet, one carrying material forged around another star. The finding does not settle every mystery surrounding the comet, but it does show that interstellar small bodies can preserve volatile combinations rarely seen among our own comets.

For years, interstellar visitors have teased astronomers with brief appearances and limited data. With 3I/ATLAS, the view is becoming richer. Not complete, certainly, but rich enough to suggest that the galaxy may be filled with icy remnants shaped by planetary nurseries very different from our own.