3I/ATLAS, an object from another system, carries an unexpected chemical trace in its material.
This comet was discovered in July 2025, then quickly passed through our Solar System. Unlike comets that remain around the Sun, this one comes from interstellar space.
In December 2025, the James Webb Space Telescope observed the gas escaping from 3I/ATLAS as the comet warmed up. Under the Sun's influence, some of its ices transformed directly into gas. This cloud could be analyzed remotely.
Observations of comet 3I/ATLAS with the Gemini South Observatory.
Credit: International Gemini Observatory/NOIRLab/NSF/AURA/Shadow the Scientist. Image processing: J. Miller & M. Rodriguez, T.A. Rector, M. Zamani
The researchers focused on methane, a gas also found on Earth. They discovered an unusual proportion of deuterium, a heavier form of hydrogen. Its nucleus contains one additional particle compared with ordinary hydrogen.
The comparison is striking: the methane in 3I/ATLAS contains approximately 13 times more deuterium, proportionally, than that of comet 67P. The latter, from our own Solar System, was studied by the European Rosetta probe. Measurements of both comets are rare, because this type of gas is difficult to analyze with such precision.
Why this difference? In extreme cold, certain chemical reactions favor the incorporation of deuterium into molecules. Some of this signature can then remain trapped in ice for a very long time. It thus becomes an indication of the conditions under which the material formed.
According to the researchers, the ices of 3I/ATLAS may have originated in an especially cold cloud of gas and dust. This environment may have existed even before the birth of the star around which the comet formed. Radiation from space may also have played a role.
One might think that this discovery reveals the comet's exact age. That is not the case. Other analyses of its water and carbon suggest a very ancient origin, possibly even predating the birth of our Sun.
The data collected by James Webb are now accessible to researchers. They can be compared with measurements of future comets. For now, 3I/ATLAS and 67P remain the only two examples of comets in which this form of methane has been analyzed with certainty.