Comet 3I/ATLAS is a visitor to our Solar System from deep space, having formed elsewhere in the Galaxy and only temporarily passing through our cosmic neighbourhood.
While astronomers regularly discover and observe many comets in our night sky, the vast majority of those spotted originated within our Solar System.
More on 3I/ATLAS
Interstellar comets like 3I/ATLAS are much more rare.
In fact, it's one of only three such interstellar visitors ever discovered, the other two being 1I/ʻOumuamua (2017) and 2I/Borisov (2019).
Why are these objects so rare, why have we only ever seen three, and how can we discover more?
Dr Matthew Hopkins is a researcher at the University of Oxford in the UK, who primarily looks at interstellar objects and galactic evolution.
His study of 3/ATLAS revealed it could be the oldest comet we've ever seen.
We spoke to Matthew to find out more.
What is 3I/ATLAS and what makes it so interesting?
3I/ATLAS is an interstellar object that was discovered passing through our Solar System by the Asteroid Terrestrial-impact Last Alert System (ATLAS) survey in July 2025.
During the survey, we built a model: the Ōtautahi-Oxford interstellar object population model.
This model can, among other things, tell us about the age of a transiting body like 3I/ATLAS.
I noticed that 3I/ATLAS was bobbing up and down on its orbit around the Milky Way.
That was a light-bulb moment for me, because I knew that older stars also follow a similar pattern in their orbits.
When we did all the statistics and looked at the correlations, we found that 3I/ATLAS was probably over seven billion years old.
That's about twice the age of our Solar System.
Why have we only ever found three interstellar visitors?
Firstly, they need to be close enough that they are reflecting enough sunlight for us to be able to see them.
These objects don’t tend to reflect a lot of light, and tend to be very dark in colour.
Additionally, we need to determine that the object is a moving point of light and not just something that appears in one image and is never seen again.
All this means asteroids and comets are generally difficult to find. Ones like 3I/ATLAS, which come from outside the Solar System, are even harder.
There are a lot of them, though. Around 50 are predicted to be within the orbit of Jupiter at any time.
But there are also around five million asteroids in that volume of space, so when it comes to finding interstellar objects, the odds are stacked against us.
How do you distinguish an interstellar object from a Solar System asteroid?
When we see the object moving, we can then work out what path it’s taking as it makes its way through the Solar System.
If something is bound to the Sun, its path will look like a circle or an ellipse.
Interstellar objects have orbits that take a different form, which we call hyperbolic.
This means they come in on a straight line, bend around the Sun, and then leave again on a straight line.
Are we likely to detect more interstellar objects?
The upcoming Vera C Rubin Observatory is going to make a big difference.
Rubin is a very sensitive telescope looking at huge areas of the sky.
It’s going to conduct what’s called the Legacy Survey of Space and Time (LSST), which is a 10-year survey of the southern sky.
How long will 3I/ATLAS be around for?
The timescale is months. We discovered it in July 2025, when it was in the orbit of Jupiter, and it was only just visible then.
Right now, it’s going behind the Sun and is near its closest approach. In roughly a year, it will be fading from the visibility of most of our telescopes.
What else can we learn from 3I/ATLAS?
It’s a really interesting object.
Firstly, its existence implies that there were planetesimals (or tiny planets) in the Galaxy a very long time ago.
And what we know about the composition of 3I/ATLAS also tells us these planetesimals contained heavier elements than hydrogen and helium.
Seven billion years ago, the Galaxy would have looked very different to how it does today, and 3I/ATLAS is like seeing a piece of that earlier Universe in our lifetimes.
What’s next for researchers?
We have to wait for 3I/ATLAS to emerge from behind the Sun. After that, we will start to try to understand how it evolves.
Our team will be studying its chemical composition, because that will tell us about its parent bodies.
On a bigger scale, we will be anticipating the LSST survey by the Vera C Rubin Observatory, as this will help us to spot many more of these interstellar objects.
Can we see 3I/ATLAS from our back garden?
3I/ATLAS is going to remain pretty faint throughout its passage through the Solar System, so will probably be out of reach of all but the biggest amateur telescopes.
However, comets can be unpredictable and can undergo sudden increases in brightness due to bursts of activity, so we might get lucky.
