The interstellar comet 3I/ATLAS, discovered on July 1, 2025, by a survey telescope in Chile, is now believed by astronomers at Oxford to be roughly 7 billion years old — meaning it was already drifting through the Milky Way for nearly 3 billion years before our solar system even existed — having formed in a part of the galaxy called the thick disk, where the stars are older than almost everything else humanity has ever observed

Seven billion years is the kind of number that stops behaving like time. Our solar system is about 4.6 billion years old. Earth, the Sun and every familiar comet formed long after the earliest date now being discussed for 3I/ATLAS.

I found myself thinking about this in the ordinary way parents measure time. My daughter is one, so a month can still remake the person in front of me. At the scale of 3I/ATLAS, three billion years can be compressed into the phrase “before the Sun existed.”

But there is a correction to make before the wonder takes over. Astronomers have not directly measured a seven-billionth birthday for this comet. The Oxford estimate came from a statistical model of its motion through the Milky Way, and the published paper gives a wide range. It also stepped back from the early claim that 3I/ATLAS came from the thick disk.

The object may actually be older than the headline suggests. What we know, what we infer and what remains unknowable are three different things.

What the Chilean telescope actually found

The firm part of the story begins on July 1, 2025. The NASA-funded Asteroid Terrestrial-impact Last Alert System telescope at Rio Hurtado, Chile, reported a fast-moving object to the Minor Planet Center. Archived images later showed that other instruments had recorded it before the official discovery.

As NASA’s 3I/ATLAS overview explains, the shape of its orbit is hyperbolic. It was moving too fast to be permanently bound to the Sun, and tracing its path backward placed its origin outside our solar system. The “3I” designation marks it as only the third confirmed interstellar object, after 1I/‘Oumuamua and 2I/Borisov.

That part is not an age estimate. It is an orbital conclusion. The comet arrived from interstellar space, passed through the solar system and is heading out again. Its incoming speed and direction then gave researchers a different question: what kind of Galactic population normally moves like this?

How a velocity became an age range

Matthew Hopkins and his colleagues used the Ōtautahi–Oxford interstellar-object population model, which combines data from the European Space Agency’s Gaia mission with models of Galactic dynamics and planet formation. Their paper in The Astrophysical Journal Letters focused in part on 3I/ATLAS’s unusually large motion perpendicular to the plane of the Milky Way.

Older populations of objects tend to have a wider spread of velocities. Over enormous spans of time, interactions with spiral structure and giant molecular clouds can change their Galactic orbits. This age-velocity relationship does not act like the rings in a tree. It gives a probability distribution.

Using the comet’s vertical velocity, the Oxford-led team calculated a 68 percent confidence interval of 7.6 to 14 billion years. That is much more careful than saying it is roughly seven billion years old. The paper says the interval is broad because the relationship between one object’s velocity and its age is weak.

In fact, the authors found that their model’s age probability remained above zero for almost the full range they tested. Their strongest statement was that the probability approached zero below one billion years. The ancient interpretation is favored, but this is not a stopwatch reading.

The thick-disk description did not survive unchanged

The Oxford announcement in July 2025 said 3I/ATLAS most likely came from a star in the Milky Way’s thick disk. This is the origin of the wording repeated in the headline.

The final published paper is more precise. A note says that an early arXiv version explicitly identified 3I with the thick disk, but that there is no clear boundary between the thin and thick disks and the nature of the thick disk is itself debated. Under the definition the authors adopted for publication, 3I/ATLAS is not included in the thick disk.

What remained was a less dramatic and more defensible finding. Its velocity is shared by older, lower-metallicity stars on Galactic orbits that swing far above and below the Milky Way’s plane. A separate backward-orbit study released in 2025 even found its motion consistent with a thin-disk population. The labels are model-dependent, while the measured trajectory is not.

So it is fair to say that the comet appears associated with an old Galactic environment. It is no longer fair to present “formed in the thick disk” as a settled result from the Oxford paper.

Formation time is not the same as wandering time

There is another tempting leap in the story. If 3I/ATLAS formed more than 7.6 billion years ago, and the solar system formed about 4.6 billion years ago, then the material in the comet existed for roughly three billion years before the Sun. That arithmetic is reasonable.

It does not follow that the comet spent all of those years drifting freely between stars.

A comet forms in a planetary system. It must then be ejected, perhaps through an encounter with a planet, a companion star or another gravitational disruption. We do not know when that happened to 3I/ATLAS. It could have left its home system early or remained there for a long time first.

Reconstructing the journey to a specific star is effectively impossible across billions of years. The positions and motions of stars accumulate uncertainty, and the comet’s path is altered as it travels through the Galaxy. The Oxford paper says an interstellar object would need to be younger than about 10 million years for a parent-star trace to be realistic. The 2025 backward-orbit study found no convincing source encounter within the past 10 million years.

Webb later found a separate clue to an even older origin

The story changed again after the comet passed the Sun. In December 2025, the James Webb Space Telescope measured the chemistry of gas released from 3I/ATLAS. Those observations produced an age argument based on composition rather than motion.

A 2026 analysis led by Martin Cordiner reported an unusually high proportion of deuterium in the comet’s water and unusually low amounts of carbon-13 relative to carbon-12. Interpreted through models of Galactic chemical evolution, the carbon ratios suggested that the material accreted roughly 10 to 12 billion years ago in a cold, relatively metal-poor environment.

NASA’s account of the Webb result says the study was published in Nature in June 2026. This is independent support for an ancient origin, although it is still an inference through models, not a direct date stamped into the ice.

The claim that 3I/ATLAS is older than almost everything humanity has observed goes too far. Astronomers observe stars and galaxies whose light comes from much earlier in cosmic history. The narrower comparison is still extraordinary: the Oxford team’s model makes 3I/ATLAS older than known solar system objects and likely older than the two previous interstellar visitors.

The accurate version is fascinating enough. A telescope in Chile found the third known interstellar comet. Its motion produced a broad estimated age of 7.6 to 14 billion years. Its chemistry later pointed toward formation 10 to 12 billion years ago. We cannot name its parent star, fix its birthplace to one Galactic component or say how long it spent in open interstellar space.

For a brief period, though, material from a planetary system older than our own passed close enough for our instruments to read it.