Try, for a moment, to hold the actual scale of what a 7-billion-year-old comet means.
Seven billion years ago, the Earth did not exist. The Sun did not exist. The planets of our solar system did not exist. The nebula of gas and dust from which they would eventually form was still, at that point, uncondensed material scattered somewhere in a corner of the Milky Way — not yet gathered, not yet ignited into anything a modern human would recognise as a star system.
Nearly three billion years would pass before that changed. Long enough for whole generations of stars to burn out. Long enough for the galaxy to shift, wind, evolve. Long enough, in cosmic terms, for a small dark ice-bound object drifting through interstellar space to travel, essentially without incident, across an appreciable fraction of the Milky Way — passing near stars, out into voids, through the thin edges of nebulae, catching gravitational nudges from vast distant masses and continuing.
And then, at some point in the last several thousand years, its trajectory brought it toward a small yellow star that had, since its formation four and a half billion years ago, sprouted a family of planets. One of those planets, the third out, had developed something the universe rarely bothers with: an atmosphere, oceans, and eventually, on the surface of one of those oceans, a species that would learn to build telescopes.
On July 1, 2025, one of those telescopes noticed it coming.
The discovery
The telescope was one of the ATLAS array — a set of small survey instruments spread across the world for the specific purpose of scanning the sky for moving objects that shouldn’t be there. This one was at Río Hurtado in Chile, and at some point on the morning of July 1, it recorded an unusual point of light shifting against the background stars.
Within hours, other telescopes had confirmed the trajectory. The object was moving too fast to be a normal comet. Its path was too steep, its velocity too high. This was not something that had originated within the gravitational reach of our Sun. It was a visitor, moving through the inner solar system on a hyperbolic path, on its way in from interstellar space and its way back out.
Only three such objects have ever been identified. 1I/ʻOumuamua, in 2017. 2I/Borisov, in 2019. And now this one, named 3I/ATLAS in honour of the survey that spotted it.
What the Oxford team found
Even before the community had fully absorbed the discovery, a graduate student named Matthew Hopkins at the University of Oxford was already running the numbers.
Hopkins had, for the previous several years, been developing a statistical model with his advisor Chris Lintott and collaborators at the University of Canterbury in New Zealand. The model, which they called the Ōtautahi-Oxford Model, was designed to do something that had never really been possible before — take the trajectory of an interstellar object and estimate, from that trajectory alone, where in the Milky Way it likely came from and how old it likely was.
Interstellar objects preserve, in the geometry of their motion, a partial record of their origin. Different parts of the galaxy have different stellar populations, moving at different velocities and in different directions. An object’s current heading and speed, extrapolated backwards, points toward the region of the galaxy where it was most probably released — usually by being ejected from a young planetary system before its parent star could fully hold onto it.
When Hopkins ran 3I/ATLAS through the model, the output was startling. The comet’s trajectory pointed at the Milky Way’s thick disk — the ancient population of stars that sits above and below the flatter, younger thin disk where our Sun lives.
The thick disk stars are old. Not old like our Sun (which is 4.6 billion years old and considered middle-aged). Old in the deep sense — many of them formed within the first few billion years of the Milky Way’s own existence, some 10 to 12 billion years ago.
If 3I/ATLAS came from that region, then it formed alongside one of those ancient stars, and its own age would be measured on that same scale.
The Oxford team’s statistical best estimate placed it at roughly 7 billion years old, with a two-thirds probability of being older than the solar system.
“This is an object from a part of the galaxy we’ve never seen up close before,” Chris Lintott said in the announcement. “We think there’s a two-thirds chance this comet is older than the solar system, and that it’s been drifting through interstellar space ever since.”
What that actually means
The consequences of this age estimate are worth pausing on.
3I/ATLAS is, by the current best analysis, older than the Earth. Older than the Sun. Older than every planet, every ocean, every mountain range, every ecosystem, every dinosaur, every human, every recorded history. Older than the gravel on your driveway by a factor of about one and a half. Older than the atoms in the marble tombstones humans have carved for their most ancient civilisations — because those atoms, or many of them, were manufactured inside stars that had not yet been born when this comet was already ice.
The rocks on Earth’s oldest continents are, at the extreme, about 4.4 billion years old. The Jack Hills zircon crystals of Western Australia are the oldest confirmed physical fragments of our planet. 3I/ATLAS, if the Oxford estimate holds, predates them by nearly 3 billion years. It was already ancient when the first bacteria appeared on Earth. It was still ancient when the first dinosaurs walked. It has been ancient the entire time human beings have existed as a species, and the entire time our species has been looking up at the sky and wondering.
And it will remain ancient long after we, and every trace of us, are gone. Its current visit to the inner solar system is a brief detour on a journey that has already lasted seven billion years and will continue for at least seven billion more.
What we do with the rest of the visit
3I/ATLAS is currently on its way through the inner solar system. It made its closest approach to the Sun in late October 2025 and is now heading back out. It will not return.
In the months of its visit, telescopes around the world — and in space — have been racing to observe it. Molecular emissions from its coma have been detected. Its composition is being analysed. Every measurement is a small chance to see, up close, material that formed in a completely different part of the galaxy, around a completely different star, seven billion years ago.
We will not see anything like it again. Not this object. And, in all likelihood, not another chance at a piece of the thick disk that has arrived at exactly the right moment in exactly the right trajectory to be observed at close range.
For a species that has only had telescopes for about 400 years, it is a startling piece of luck. A traveller from before our sun existed, arriving briefly, in our neighbourhood, in a decade when we were finally equipped to notice.
The comet does not know we are here. It is just moving.
But somebody, on the third planet out from a small yellow star, is watching it as it passes — for the small remainder of our species’ overlap with the tail end of its seven-billion-year journey.
