Interstellar comet 3I/ATLAS is up to 12 billion years old and unlike anything found in our solar system, new James Webb Space Telescope (JWST) observations suggest.
Comet 3I/ATLAS became a celestial celebrity last year after the interstellar visitor was discovered hurtling through our cosmic neighborhood. Not long after, online speculation suggested that the space rock could be an alien spacecraft. However, most astronomers are confident that 3I/ATLAS is a comet from an unknown star system.
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Researchers already knew from the comet’s speed and trajectory that it was potentially the oldest comet ever seen. Previous estimates put the comet’s age at somewhere between 3 billion and 11 billion years old. The new findings further narrowed down the comet’s age and origin by looking at isotope measurements taken by JWST when the comet flew past Earth in December 2025.
“They show that 3I/ATLAS isotopic composition is very different from solar system comets and suggest that it likely formed 10-12 billion years ago,” Romain Maggiolo, a research scientist at the Royal Belgian Institute for Space Aeronomy who was not involved in the study, told Live Science in an email. “In other words, 3I/ATLAS formed in a stellar environment different from ours, not only somewhere else in space, but also at a much earlier time in the history of the Milky Way.”
Comet 3I/ATLAS is only the third interstellar object ever recorded in our solar system. The space rock, which Hubble Space Telescope observations suggest is somewhere between 1,400 feet (440 meters) and 3.5 miles (5.6 kilometers) wide, zoomed into our solar system at around 137,000 mph(221,000 km/h) last year before slingshotting around the sun.
After reaching its closest point to our star, known as perihelion, on Oct. 29, 2025, the comet then made its closest approach to Earth on Dec. 19, when it came within about 168 million miles (270 million km) of our planet. JWST made the observations that have been analyzed in the new study a few days later on Dec. 22.
A relic from the ancient universe
Comets heat up as they fly closer to stars, which causes ice on their surfaces to sublimate into gas. By studying the composition of this gas, researchers can start to figure out what they’re made of and the conditions in which they formed.
The authors of the new preprint looked at the ratio of isotopes, or versions of elements, in material outgassed by 3I/ATLAS. They found that the comet’s water is more enriched in deuterium, a heavier hydrogen isotope, than any previously studied comet, while its ratio of carbon isotopes also exceeded levels normally seen in our solar system.
The results offer clues to what conditions may have been like in whatever ancient planetary systems forged the comet in the early years of the Milky Way.
“If 3I/ATLAS is indeed as old as this study suggests, the large amounts of volatile molecules it contains indicate that rich prebiotic chemistry may already have been occurring in star-forming regions very early in the history of our Galaxy,” Maggiolo said.
The results also indicate that the comet formed in a cold environment that was around 30 kelvins (minus 406 degrees Fahrenheit, or minus 243 degrees Celsius), likely in a dense and well-shielded protoplanetary disk, according to the study.
While the study is still in the preprint stage, Maggiolo, who has studied comet 3I/ATLAS as part of his own research, didn’t have any major concerns about it. The new measurements help researchers “better understand this interstellar messenger,” he said.
Josep Trigo-Rodríguez, the research principal investigator of the Asteroids, Comets and Meteorites research group at the Institute of Space Sciences (CSIC/IEEC) in Spain who has previously identified erupting “ice volcanoes” on comet 3I/ATLAS, described the new findings as a good compilation of scientific results, using different techniques from well-recognized experts.
“This manuscript exemplifies that interstellar comets are unique bodies that are able to sample remote regions of our Milky Way galaxy,” Trigo-Rodríguez told Live Science in an email.
There’s a good chance that researchers will never know which star system birthed comet 3I/ATLAS. The comet has likely been traveling through space for billions of years and has come a very long way in that time. Maggiolo’s own research has found evidence that the object is extremely irradiated, with all that time in space exposing it to cosmic rays that could have fundamentally altered its chemical composition, making its origins more difficult to decipher.
“The isotopic composition of the material outgassed by 3I/ATLAS provides a crucial new piece of the puzzle,” Maggiolo said. “But the puzzle is far from being complete!”
Finding those puzzle pieces is a race against time for astronomers, as comet 3I/ATLAS is now hurtling out of the solar system. It’s currently passing Jupiter, where it is expected to make its closest approach on Sunday (March 15). The comet will come within about 33 million miles (54 million km) of the gas giant — much closer than it got to Earth.
The interstellar traveller will then continue its journey away from us, crossing Saturn’s orbit in July, Uranus’ orbit in April 2027 and Neptune’s orbit in March 2028. You can track the comet using NASA’s Eyes on the Solar System simulation of the comet’s trajectory.
