Interstellar comet 3I/ATLAS has captured scientific interest as an unprecedented celestial object. When it passed through the inner Solar System last year, telescopes revealed its unparalleled nature, distinguishing it significantly from previous interstellar objects, 1I/’Oumuamua and 2I/Borisov. Recent high-fidelity observations suggest this comet might be nearly as ancient as the Universe itself.
A team led by Martin Cordiner, a molecular astrophysicist at NASA Goddard Space Flight Center, examined the hydrogen and carbon isotopic ratios of 3I/ATLAS. They found that the comet likely originated in a very cold and primordial environment, potentially making it around 12 billion years old.
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Cordiner expressed the significance of these findings to ScienceAlert, saying, “This was a special moment. Amid all the rumors of extraterrestrial technology, a strong scientific narrative was emerging that 3I/ATLAS appeared to closely resemble our typical Solar System comets.” He further noted, “However, the isotopic ratios we measured with JWST show that it is not only distinct, but also likely much older than our Solar System. Suddenly, we are no longer asking ‘is this a comet?’, but ‘what can this unique object tell us about the history of our galaxy?'”
3I/ATLAS was first detected on July 1, 2025. Despite waning public interest after its closest approach to the Sun in late October that year, scientists have continued to study it enthusiastically. Initial observations indicated its unusual nature, with its composition and emitted gases suggesting a formation environment unlike our Solar System.
Subsequent analyses determined that 3I/ATLAS likely formed in a very cold, distant region, with velocity data estimating its age between 3 and 11 billion years. Cordiner’s team conducted detailed analyses using infrared observations from JWST and radio observations from ALMA, focusing on the comet’s chemical isotopic ratios.
Water in the comet’s coma contained a high concentration of deuterium, an isotope of hydrogen, while its carbon isotopes were unlike those found in Solar System objects. “To get such definitive evidence of a distant origin (in space and time) is enough to turn the scientific narrative around, and show that this object is indeed something scientifically very unique and interesting,” Cordiner stated.
The deuterium findings support earlier analyses showing an unusually high proportion of heavy hydrogen, with a measured deuterium-to-hydrogen ratio of 0.98 percent in the comet’s water—over ten times higher than in Solar System comets. Ice chemistry models suggest such enrichment occurs when water forms at temperatures below 30 kelvin (-243 °C, or -406 °F), preserving signatures from an extremely cold setting.
The carbon isotopes provide another clue to 3I/ATLAS’s history. The comet’s high carbon-12 to carbon-13 ratio indicates it formed from material not yet enriched by successive generations of stars. Heavier elements became more abundant only after several star generations lived and died, scattering these elements into space for incorporation into new formations.
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By comparing the comet’s carbon measurements with models of the Milky Way’s evolving chemistry, researchers estimate that 3I/ATLAS formed roughly 11 to 12 billion years ago, during the galaxy’s youth. However, it is possible the comet formed in a remote area of space not yet dusted by dying stars, making its chemistry appear older.
Cordiner noted, “There are ~200 billion stars in the galaxy, each moving on its own orbit. Calculating the interacting orbits of more than two bodies is a notoriously hard problem in astrophysics, so calculating the orbits of all the stars in the galaxy is completely intractable, especially when you consider the ill-defined densities (gravities) and motions of interstellar clouds, which can exert their own tug. Tracing the orbit of 3I/ATLAS back 10 million years is the present limit of our capabilities, so it seems its precise origin will never be known.”
Despite this uncertainty, the findings offer valuable insights into the Milky Way’s history.
Cordiner explained, “Our own Solar System comets have been largely frozen in time since their birth at the dawn of the solar system. In an astronomical context, you don’t have to get very far from a star to reach the extremely cold temperatures of interstellar space. As soon as you reach that temperature (only 10-20 degrees above absolute zero), there is no possibility of outgassing or thermally-driven chemistry – everything remains solid as a rock.”
Related: 4 Powerful Telescopes Agree: Interstellar Comet 3I/ATLAS Really Is Bizarre
Currently, 3I/ATLAS is beyond Jupiter’s orbit, continuing its journey out of the Solar System to travel across the galaxy. Despite its growing distance, the comet has much to teach us. Cordiner remarked, “3I/ATLAS is now at 8 au from the Sun (approaching the orbit of Saturn), and will pass beyond the orbit of Pluto in 2029, exiting the heliosphere in around 2035. Therefore, there would still be time to catch up with it if we chose to throw the world’s resources at that problem.”
The research findings have been published in Nature Astronomy.
