3I/ATLAS observations reveal methane and carbon dioxide in unusual abundance relative to water, providing a glimpse into material formed outside the solar system
The James Webb Space Telescope has provided a rare chemical glimpse of a body outside our solar system. In new observations of the interstellar comet 3I/ATLAS, Webb directly detected methane gas—the first time methane has been measured in this way in an interstellar object. The observations also confirmed that the comet is unusually rich in carbon dioxide relative to water, a feature that sets it apart from most comets that have formed in the solar system. (NASA Science)
The data was collected using Webb's MIRI instrument, the Mid-Infrared Spectrometer. Webb observed the comet on two occasions after it passed close to the Sun and began to move away from the inner solar system again. The first observation was made on December 15–16, 2025, when the comet was about 329 million kilometers from the Sun. The second observation was made on December 27, when the distance had increased to about 379 million kilometers.
Temporary guest from another planetary system
3I/ATLAS is not another long-period comet from the edge of the solar system. It is an interstellar body: an object formed around another star, ejected from its system, and now briefly passing through our neighborhood. NASA notes that the comet was discovered in the summer of 2025, and that it poses no danger to Earth. Precisely because it is only passing here, observations of it are a short window of time for material formed under conditions different from those that gave birth to the planets, asteroids, and local comets.
Comets are of interest to researchers because they retain volatile, frozen materials from their formation. When a comet approaches the Sun, the ice within it heats up, some of it evaporates and turns into gas, and a halo forms around the nucleus. In the case of 3I/ATLAS, this halo serves as a kind of “vapor sample” of a body formed far away.
Why did methane appear late?
Methane is a particularly volatile substance. It easily changes from a solid to a gas, so it could be expected to be emitted at an earlier stage in the comet's heating. The fact that Webb detected it after the comet's close approach to the sun suggests, according to the researchers, that it was buried beneath the comet's outer layer. Only when the heat penetrated deeper into the icy subsurface did the methane break free and appear in the spectrum.
This is an important detail. It tells us not only what materials are present in the comet, but also how they are arranged within it. The outer layers may have been processed along its interstellar journey, while deeper material is preserved in an earlier state. If so, 3I/ATLAS serves not only as a messenger from another system, but also as a layered archive of the processes it has undergone since its formation.
Carbon dioxide, water, and a different environment
The other notable finding is the unusual abundance of carbon dioxide relative to water. Webb confirmed that 3I/ATLAS emits much more carbon dioxide relative to water than most local comets. The amount of methane relative to water is also considered high, and very few comets in the solar system are comparable in this respect.
NASA notes that the two findings together — the methane and the abundance of carbon dioxide — indicate a different chemical environment than that in which most comets in the solar system formed. The comet may have formed in an especially cold region in a protoplanetary disk around another star, or under conditions in which the ratio of ice types was different. At this stage, this is not a detection of life, nor is it a direct hint of life. This is a chemical finding, but such a finding is important for understanding the processes by which planets and comets form in other systems.
Race against time
As 3I/ATLAS moves away from the Sun, its activity wanes. Webb has already detected a sharp drop in gas emissions between the two observations, with water emissions declining most noticeably. This behavior is expected: as the comet receives less heat, less ice evaporates. Water, which is less volatile than methane and carbon dioxide, “shuts down” faster in terms of emission rate.
That’s why every observation of an interstellar object is important. 3I/ATLAS won’t stay here. It will continue its orbit outside the solar system, and the opportunity to measure its composition up close will be gone. For researchers of comets, planetary chemistry, and astrobiology, it’s a brief visit from a messenger from another system—and one of the few ways to learn what the building blocks of worlds other than ours look like.
Short FAQ:
Does the detection of methane indicate life? No. This is a chemical measurement of gases emitted from the comet, not a detection of life.
Why is 3I/ATLAS important? Because it was formed outside the solar system, its composition therefore allows comparisons between the chemistry of different planetary systems.
What is special about carbon dioxide? The comet emits much more carbon dioxide relative to water than most local comets.
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