2 independent methods. 2 different teams. They both landed on the same impossible answer: this comet is nearly as old as the universe.
image from grok
The interstellar comet 3I/ATLAS, discovered in July 2025, is estimated to be 3-12 billion years old based on two independent analytical methods: kinematic age derived from its trajectory and carbon isotopic composition measured by the James Webb Space Telescope. This makes it potentially older than our entire Solar System (4.6 billion years), raising a fundamental survival puzzle—typical comets in interstellar space persist for only tens of millions of years before being destroyed by stellar encounters, molecular clouds, or galactic tidal forces. Its parent star is unidentifiable after billions of years of galactic wandering, but its existence challenges current models of cometary longevity in the interstellar medium.
There are three known objects that entered our Solar System from interstellar space. Two of them were confirmed within weeks of each other. The third took longer to find. When astronomers spotted it on July 1, 2025, using the ATLAS telescope in Chile, it came with a number that stopped them cold: 3I/ATLAS may be nearly as old as the universe itself.
Not almost. Nearly.
The number is not the story. The story is what it implies about when the Galaxy could build worlds.
Two independent analyses arrive at the same conclusion. Aster Taylor and Darryl Seligman at the University of Michigan and Michigan State University estimated in July 2025 that the comet's kinematic age — derived from its velocity and trajectory — places it between 3 and 11 billion years old. A separate team led by Martin Cordiner, currently in review at Nature, analyzed the comet's isotopic composition using the James Webb Space Telescope and found isotopic signatures indicating it accreted roughly 10 to 12 billion years ago, early in the Galaxy's history. The methods are unrelated. The answers are consistent. ESA FAQ arXiv:2603.06911 arXiv:2507.08111
What this means is not just "old comet." It means volatile-rich planetesimals — the raw material for planetary systems, including the water and carbon chemistry that life requires — were forming in the Milky Way before our Sun existed. The Solar System coalesced 4.6 billion years ago. 3I/ATLAS was already ancient. It is evidence that the conditions for rocky, water-bearing worlds appeared in the Galaxy roughly five to seven billion years earlier than Solar System formation models imply.
That matters for the people type0 covers. Volatile-rich planetesimals forming earlier means the chemistry for planetary systems — and potentially for life-supporting worlds — opened up earlier in the Galaxy's history than our models predicted. If you are building anything related to planetary science, astrobiology, or early-Universe chemistry, this is a data point that restarts the clock.
The survival puzzle
Ages aside, 3I/ATLAS presents a mechanical problem. Comets are fragile aggregates of ice, dust, and rock. Over billions of years, they face lethal hazards: close passages to other stars, the tidal forces of the galactic center, passage through molecular clouds dense enough to strip away material. The dynamical lifetime of a typical comet in interstellar space is measured in tens of millions of years — orders of magnitude less than what 3I/ATLAS appears to have endured. ESA FAQ
Yet here it is.
One possibility: 3I/ATLAS may have originated in the outer reaches of its parent star's system, in what would be analogous to the Oort Cloud — a sphere of icy bodies extending a light-year or more from the central star. Objects at that distance barely feel the star's gravity. If the star later evolved and shed mass during stellar evolution, these distant objects would be gently ejected rather than violently scattered. This is one of the gentlest possible exits from a stellar system. 3I/ATLAS would have been in interstellar space before its star died — it simply kept going.
The alternative — that it was hurled outward from the inner regions of its parent system, or that it has been wandering since the Galaxy was young — requires either extraordinary timing or extraordinary luck. The survival puzzle remains open.
What JWST found
Whatever its history, 3I/ATLAS is not an empty chunk of ice. The JWST data reveals an object rich in volatiles. Carbon dioxide, water, carbon monoxide, and carbonyl sulphide were all detected escaping as the comet approached the Sun and began to sublimate. Water ice was present on the surface. ESA FAQ
The isotopic results go further. The deuterium-to-hydrogen ratio in 3I/ATLAS's water is (0.95 ± 0.06)% — more than ten times higher than most Solar System comets. The carbon isotope ratios in CO2 and CO also fall outside the range seen in Solar System bodies, nearby interstellar clouds, and protoplanetary disks. These extremes indicate formation at temperatures around 30 Kelvin or colder, in a metal-poor environment — conditions that correspond to the early Galaxy, before successive generations of stars had seeded the interstellar medium with heavier elements. arXiv:2603.06911
In the Cordiner team's careful framing — a paper still in review at Nature — "3I/ATLAS thus represents a preserved fragment of an ancient planetary system, and provides direct evidence for active ice chemistry and volatile-rich planetesimal formation in the young Milky Way."
The JUICE observation
The discovery has its own coincidence worth noting. In autumn 2025, as 3I/ATLAS swung through the inner Solar System, ESA's Jupiter Icy Moons Explorer happened to be in the right position to observe it. Juice was already committed to its primary science mission, but the comet fell within view of its navigation camera and some of its spectrometers. The team captured a quarter of a NavCam image before the spacecraft had to resume its planned operations. The rest of the data, stored on board, came down in February 2026. The first science camera image shows a clear tail and structure in the coma. ESA FAQ
We are not going to send a spacecraft to 3I/ATLAS. At roughly 250,000 kilometers per hour at closest approach to the Sun — the highest speed ever recorded for a Solar System visitor — it is already moving too fast and in the wrong direction for any current mission to intercept. All of our observations come from telescopes and spacecraft that happened to be in the right place at the right time. The Juice data is unrepeatable.
Three total
There is a number that puts this in perspective: three. That is how many interstellar comets humanity has ever confirmed. 'Oumuamua in 2017 revealed that such objects were common enough to detect. 2I/Borisov in 2019 showed that at least some are chemically similar to Solar System comets. 3I/ATLAS in 2025 is teaching us that at least one of them carries material from the Galaxy's first billion years.
The comet is currently speeding away from the Sun. It will not return. Within a few years it will be beyond the reach of any telescope we have. What it leaves behind is a data point that reframes when the chemistry for planetary systems opened up in the Milky Way — earlier than our models predicted, earlier than we had evidence for until now.
Article revised after publication
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Research completed — 0 sources registered. Three independent age estimates converge on ancient origin: (1) Taylor & Seligman kinematic method: 3-11 Gyr (ApJL, July 2025); (2) Hopkins et al. thi
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Headline selected: 3I/ATLAS Is Here. It's Ancient. It Has No Star.
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