3I/ATLAS mystery deepens: Discovered on July 1 last year, study suggest the cosmic visitor may be older than the Sun, Earth or any other object in our solar system

A space rock that is possibly older than the Sun itself zipped past Earth in December 2025, and an Oxford PhD student who had just submitted his thesis a week earlier was the one who worked out how old it really is. The object, called 3I/ATLAS, is only the third interstellar visitor ever confirmed entering our solar system, and going by the numbers, it may be carrying material from a star system that formed roughly 7 billion years ago, well before our own Sun switched on.

The Discovery That Ruined Someone's Holiday

It started on 1 July 2025, when the ATLAS survey telescope in Chile picked up a strange dot of light moving against the stars. Within hours, telescopes around the world had confirmed something odd about its path: it was not looping around the Sun like a normal comet. It was on a hyperbolic trajectory, meaning it came from outside the solar system entirely and is just passing through, never to return.

That made it the third confirmed interstellar object in history, after 'Oumuamua in 2017 and Borisov in 2019. Scientists named it 3I/ATLAS, after the telescope that first spotted it.


Here is where the story gets a little unusual. According to the Royal Astronomical Society's account of the find, the person who ran the very first detailed analysis of 3I/ATLAS was Matthew Hopkins, an Oxford graduate student who had defended his PhD thesis just seven days earlier. Hopkins had spent years building a statistical tool with co-author Chris Lintott and researchers at the University of Canterbury in New Zealand, a model designed to guess how old and where an interstellar object came from, just by studying its flight path. He called it the Ōtautahi-Oxford Model. He had never actually tested it on a live, freshly discovered object before. Then 3I/ATLAS showed up, and instead of going on the break he had planned after finishing his PhD, Hopkins spent the following days running the numbers.

Why This Comet Might Be Older Than The Sun

The result surprised everyone. Hopkins's model puts the odds at roughly two in three that 3I/ATLAS is older than our own solar system, with a best estimate putting its age at around 7 billion years. For comparison, the Sun is about 4.6 billion years old. If the estimate holds, this would make 3I/ATLAS the oldest comet ever observed by humans.

The clue was in the angle at which it approached. The Milky Way is not one flat blob of stars, it has layers. Most stars, including our Sun, sit in a thin, flat disc that spins around the galaxy's centre. But wrapped around that thin disc is a puffier, older layer called the thick disc, made up of stars that are typically 10 to 12 billion years old and that move on tilted, steep paths rather than staying flat like the younger stars.

The thick disc itself is not the absolute oldest part of the galaxy, that title goes to the stellar halo and the globular clusters, some of which hold stars nearly as old as the universe itself. But those regions are too sparse to realistically send comets our way. The thick disc is the oldest neighbourhood dense enough to actually produce a visitor like this one. And unlike 3I/ATLAS, both 'Oumuamua and Borisov came from the thin disc, meaning they were roughly the Sun's age or younger. This is the first time anything from the thick disc has ever landed in a telescope's crosshairs.

What's Inside a 7-Billion-Year-Old Snowball

So what is this ancient wanderer actually made of? Per EarthSky's coverage of its chemistry, early readings show 3I/ATLAS is rich in water ice, with higher-than-usual amounts of certain volatile compounds compared to comets born inside our own solar system. That chemical fingerprint fits with a comet that formed around an older star with a different mix of raw ingredients than the one that built the Sun and its planets.

Comets are essentially cosmic time capsules — frozen leftovers from the cloud of gas and dust that a star formed out of. A comet built nearly 7 billion years ago is therefore carrying a chemical snapshot of the galaxy from an era far earlier than anything scientists have ever managed to sample directly.

3I/ATLAS made its closest pass by the Sun on 29 October 2025 and swept past Earth on 19 December 2025, staying a safe 168 million miles away the whole time. In that narrow window, Hubble and several other telescopes gathered as much data as they could before the comet began its long journey back out of the solar system, a journey with no return ticket.

The Next One Might Already Be On Its Way

Astronomers reckon 3I/ATLAS will not be the last of its kind, not by a long shot. The Vera C. Rubin Observatory in Chile, which is just starting operations, is expected to spot interstellar objects far more often than older surveys ever could, potentially several a year instead of one every few years. Statistically, roughly one such visitor is thought to pass through the solar system annually; most have simply been too small or too faint to catch. With Rubin's wide, sharp, fast-scanning eye on the sky, that is expected to change fast.

As for 3I/ATLAS itself, it is gone for good. Its hyperbolic path is carrying it back out into interstellar space, never to swing by the Sun again. Whatever happens to it next, whether it drifts for billions more years, gets pulled in by another star system somewhere else in the galaxy, or scatters into the galactic halo, humanity will never get to watch. All that is left is the data already collected, which scientists expect to be studying for years to come.