When interstellar comet 3I/ATLAS swung around the Sun in late 2025, NASA's James Webb Space Telescope was watching. In early June 2026, scientists published what they found: methane. A lot of it. More, relative to water, than in any comet we have ever tracked from our own solar system.
That single detection tells a story about where 3I/ATLAS came from, and it is a long way from here. Researchers estimate the comet could be up to 12 billion years old, formed in a region far colder and more carbon-rich than our solar neighbourhood. The methane was buried deep in its icy body, only sublimating when the Sun's heat finally reached the subsurface layers during closest approach.
Webb's MIRI instrument captured two sets of observations as the comet moved back out of our solar system: December 15 to 16, when 3I/ATLAS was about 329 million kilometres from the Sun, and again on December 27 at 379 million kilometres. The results appeared in The Astrophysical Journal Letters in June 2026.
The chemistry is genuinely strange. 3I/ATLAS also releases far more carbon dioxide relative to water than typical solar system comets, and carries deuterium levels ten times higher than local icy bodies. Both point to a formation environment quite unlike anything in our cosmic backyard.
This is only the third interstellar object ever confirmed. 'Oumuamua in 2017 was too faint to characterise properly before it left. Borisov in 2019 was more familiar. 3I/ATLAS is the first caught with a telescope sensitive enough to read its chemistry in this kind of detail.
Key takeaways
- Webb's MIRI detected methane on 3I/ATLAS, the first time on any interstellar visitor
- The comet may be up to 12 billion years old, from a region colder than our solar system
- Deuterium levels are ten times higher than typical solar system comets
- Results published in The Astrophysical Journal Letters, June 2026
The comet is heading back out of the solar system and will not return. But the chemical fingerprint Webb captured will keep researchers busy for years.