Is it a bird? Is it a plane? No, it’s a mysterious interstellar interloper spotted passing through our Solar System.
The comet, named 3I/ATLAS, was discovered by NASA’s Asteroid Terrestrial-impact Last Alert System (ATLAS) survey telescope.
Currently 675 million km (420 million miles) away, 3I/ATLAS could be the oldest comet ever seen.
“Rather than the quiet Wednesday I had planned, I woke up to messages like ‘3I!!!!!!!!!!’,” says Matthew Hopkins, one of the astronomers who discovered the comet.
Hopkins and the team developed the Ōtautahi–Oxford model to predict the properties of interstellar objects based on their orbits and possible origins.
How old is 3I/ATLAS?
Non-interstellar comets, like Halley’s Comet, formed within our Solar System and are no older
than the Solar System’s 4.5-billion-year lifespan.
Interstellar visitors, however, are not formed around the Sun and can be much older.
The team’s modelling suggests 3I/ATLAS is more than seven billion years old – the oldest comet identified to date.
“It’s a fantastic opportunity to test our model on something brand new and possibly ancient,” Hopkins continues.
A visitor from deep space
3I/ATLAS is only the third-known interstellar object to have been observed, following 2I/Borisov in 2019 and ‘Oumuamua in 2017.
Unlike its predecessors, it is travelling on a much steeper path through the Galaxy, implying it originated in a completely different region of the Milky Way.
It’s thought the comet formed in the Milky Way’s ‘thick disc’, a region of ancient stars that contains about 10 per cent of our Galaxy’s stellar mass.
If it does originate from around an old star in the thick disc, that implies that the comet should be rich in water-ice.
“This is an object from a part of the Galaxy we’ve never seen up close before,” says Professor Chris Lintott, who co-authored the study.
As 3I/ATLAS nears the Sun, its surface will heat up, allowing scientists to observe whether the expulsion of vapour and dust will form a classic glowing tail.
As well as having implications for how we detect interstellar comets, astronomers also hope 3I/ATLAS will provide clues about the role of interstellar objects in star and planet formation within our Galaxy.
Was Earth an interstellar wanderer?
Words: Chris Lintott
Five billion years ago or so, another interstellar object passed through our part of the Galaxy.
This wanderer, perhaps similar to 3I/ATLAS, became trapped in the nebula from which the Sun was forming.
Within the disc of material around our newly born star, the object began to accrete material, growing quickly into a more substantial body – one that would become the planet Earth.
Is this true?
Astronomers Suzanne Pfalzner and Michele Bannister proposed this idea following the arrival of ‘Oumuamua, and the romance of it grabbed me immediately.
Maybe the arrival of 3I will help us understand whether this idea about our cosmic origins holds up.
This article appeared in the September 2025 issue of BBC Sky at Night Magazine