Interstellar comet 3I/ATLAS, the third known object from beyond our Solar System, has been wandering the galaxy for 10 to 12 billion years, making it almost as old as the universe itself and one of the most ancient objects ever studied up close by human instruments.
The finding, published June 22 in Nature (DOI: 10.1038/s41586-026-10771-6), comes from James Webb Space Telescope observations that analyzed the comet’s isotopic composition. The results paint a picture of an object that formed at a time when the universe was only about 13 percent of its current age, long before the Sun or Earth existed.
“This was a unique opportunity to study an ancient object from the distant Galaxy, probably pre-dating our Sun and Solar System,” said Martin Cordiner, a planetary scientist at NASA’s Goddard Space Flight Center and lead author of the study.
Two isotopic clocks
The age determination relied on two independent isotopic measurements performed by Webb’s Near-Infrared Spectrograph (NIRSpec) in December 2025, shortly after the comet passed its closest point to the Sun.
The first was the ratio of deuterium to hydrogen. 3I/ATLAS contains roughly 30 times more deuterium than any comet in our Solar System. Deuterium, or heavy hydrogen, was more abundant in the early universe before being diluted by subsequent stellar fusion. Such a high ratio means the comet’s water ice has remained deeply frozen and unprocessed since its formation, preserving a direct chemical fingerprint of its birth environment.
The second clock was the ratio of carbon-13 to carbon-12. The comet showed only trace amounts of carbon-13, the heavier isotope. Carbon-13 builds up over successive generations of stars, so a very low ratio points to an origin early in cosmic history, before many stars had lived and died to enrich the interstellar medium with heavier isotopes.
Together, these measurements point to a formation epoch during the universe’s cosmic noon, the peak period of star formation roughly 10 to 12 billion years ago.
A comet unlike any other
3I/ATLAS, discovered in July 2025 by the ATLAS survey in Chile, has surprised astronomers at every turn. It is traveling at roughly 61 kilometers per second (137,000 miles per hour) inbound, accelerating to about 153,000 miles per hour at perihelion.
Webb’s observations have revealed an object with a CO2-dominated coma, showing a carbon dioxide to water ratio of roughly 8 to 1, among the highest ever measured in any comet. Webb also detected methane for the first time on an interstellar object, using its Mid-Infrared Instrument (MIRI). The methane, which was buried below the surface and only released after the comet warmed near the Sun, arrived with an unexpectedly high methane-to-water ratio.
The comet’s nucleus is estimated at roughly 2.6 kilometers (1.6 miles) in diameter, with Hubble Space Telescope observations providing the size constraints.
“Our James Webb Space Telescope observations tell us that the planet-forming environment of 3I/ATLAS’s host system was distinct from our own Solar System,” Cordiner said. “It was likely colder, and less metal rich, while being more heavily irradiated by UV and cosmic rays.”
What it means for the search for life
Despite its cold, distant origin, 3I/ATLAS is rich in organic molecules: carbon, hydrogen, nitrogen, oxygen, and sulfur, the basic building blocks of life as we know it. The comet contains complex molecules such as carbon monoxide, carbonyl sulfide, and water ice in addition to the methane and CO2.
“It shows that despite a cold and distant origin, the volatile elements for life as we know it were abundant in this distant planet-forming disk,” Cordiner said.
Stefanie Milam, a co-author at NASA Goddard, framed the discovery in the broader context of astrobiology. “So far, we know of only one place in the vast cosmos where chemical ingredients led to life: our Solar System, our Earth. Analysis of these interstellar objects is a major step towards learning how common, or uncommon, the conditions for the evolution of life are in the universe.”
The results also suggest that our own Solar System may be unusual in the galactic context. The extreme deuterium enrichment implies that the comets we have studied for centuries are not representative of comets elsewhere in the galaxy.
A long journey
Because 3I/ATLAS is so ancient, and the Solar System has migrated around the galaxy in the billions of years since, it is impossible to determine exactly where the comet formed. One possibility is that it originated in the Magellanic Clouds, the nearest dwarf galaxies to the Milky Way, which would imply a travel time as short as 1 billion years.
The comet is currently approaching Saturn’s orbit. It will pass beyond Pluto’s orbit in 2029 and is expected to exit the Solar System’s outer boundary around 2035, vanishing into the darkness of interstellar space forever.
The Vera C. Rubin Observatory in Chile, which began its decade-long sky survey this year, is expected to find many more interstellar objects in the coming years. “We hope they will be as exciting as 3I/ATLAS,” said Cyrielle Opitom of the University of Edinburgh, who led a complementary VLT study on the comet’s carbon and nitrogen isotopes.
The Nature paper is co-authored by a large international team including researchers from NASA Goddard, the University of Maryland, Auburn University, the SETI Institute, and the Southwest Research Institute.