Interstellar comet 3I/ATLAS is up to 12 billion years old and unlike anything found in our solar system, new The James Webb Space Telescope (JWST) observations suggest.
Comet 3I/ATLAS became a celestial celebrity last year after the interstellar visitor was discovered whizzing through our cosmic neighborhood. Not long after, online speculation suggested that the space shuttle might be one alien spacecraft. However, most astronomers are confident that 3I/ATLAS is a comet from one unknown star system.
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Scientists already knew from the comet’s speed and trajectory that it was potential 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 constrained the comet’s age and origin by looking at isotope measurements taken by JWST as the comet flew past Earth in December 2025.
“They show that the isotopic composition of 3I/ATLAS is very different from that of Solar System comets and suggest that it probably formed 10-12 billion years ago.” Romain Maggioloa researcher at the Royal Belgian Institute for Space Aeronomy who was not involved in the study told LiveScience 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. Space rock, like Observations from the Hubble Space Telescope proposed to be somewhere between 1,400 feet (440 meters) and 3.5 miles (5.6 kilometers) wide, zoomed into our solar system at about 137,000 mph (221,000 km/h) last year before darting around the sun.
After reaching its closest point to our star, known as perihelion, on October 29, 2025, the comet made its closest approach to Earth on 19 December, when it came within approx. 168 million miles (270 million km) from our planet. JWST made the observations analyzed in the new study a few days later on December 22.
A relic from the old universe
Comets heat up as they fly closer to stars, causing surface ice to sublimate into gas. By studying the composition of this gas, scientists can begin to figure out what they are made of and the conditions in which they were formed.
The authors of the new preprint looked at the relationship between isotopesor versions of elementsin material degassed by 3I/ATLAS. They found that the comet’s water is more enriched in deuterium, a heavier isotope of hydrogen, than any previously studied comet, while the ratio of carbon isotopes also exceeded levels normally seen in our solar system.
The results provide clues to what conditions may have been like in the ancient planetary systems that 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 have already occurred 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 kelvin (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 been studying comet 3I/ATLAS as part of his own research, had no major concerns about it. The new measurements help scientists “better understand this interstellar messenger,” he said.
Josep Trigo-Rodríguezthe principal researcher for the research group Asteroids, Comets and Meteorates at the Institute of Space Sciences (CSIC/IEEC) in Spain who previously identified eruption of “ice volcanoes” on comet 3I/ATLASdescribed the new findings as a good compilation of scientific results, using different techniques from recognized experts.
“This manuscript exemplifies that interstellar comets are unique bodies capable of sampling distant regions of our Milky Way,” Trigo-Rodríguez told LiveScience in an email.
There’s a good chance scientists will never know the star system in which Comet 3I/ATLAS was born. The comet has probably traveled through space for billions of years and has come a long way in that time. Maggiolos own research have found evidence that the object is extremely irradiatedwith all that time in space exposing it to cosmic rays that could have fundamentally changed 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 complete!”
Finding these puzzle pieces is one race against time for astronomersas Comet 3I/ATLAS is now hurtling out of the Solar System. It is currently transiting Jupiter, where it is expected to come closest on Sunday (March 15). The comet will come within about 33 million miles (54 million km) of the gas giant – much closer than it came to Earth.
The interstellar traveler will then continue its journey away from us, crossing the orbit of Saturn in July, the orbit of Uranus in April 2027 and the orbit of Neptune in March 2028. You can track the comet using NASA’s orbiter Eyes on the Solar System simulation of the comet’s orbit.
