An interstellar object currently passing through our solar system might be one of the oldest comets we have ever seen, originating from a star billions of years older than our own.

Comet 3I/ATLAS was spotted earlier this month near the orbit of Jupiter, moving at about 60 kilometres a second and estimated to be 20 kilometres across. It is the third known interstellar object found in our solar system, and will pass close to Mars in October before heading away from our sun.

Matthew Hopkins at the University of Oxford and his colleagues modelled the comet’s speed and trajectory to work out where it came from, using data from ESA’s Gaia spacecraft that mapped a billion stars in our galaxy. It looks like it originated near a region of our 13-billion-year-old galaxy called the thick disk, containing older stars and sitting above the thin disk in which our sun orbits.

“Thick disk objects are faster,” says Hopkins, whereas the prior two known interstellar objects – ‘Oumuamua in 2017 and Comet Borisov in 2019 – were slower. “Their velocities were what we’d expect for a thin disk object.”

The team’s modelling suggests 3I/ATLAS comes from a star that is at least 8 billion years old, almost twice the age of our sun, and possibly even older. “It could be the oldest comet we’ve ever seen,” says Hopkins. It is thought that interstellar objects are more likely to be ejected early in a star’s life, perhaps flung out by passing stars or interactions with giant planets.

Older stars are likely to have a lower metal content than our sun, which would also result in a higher water content for their comets, says Hopkins. If that is true, we could start to see large amounts of water spewing from the comet as it approaches the sun in the coming months.

This would probably be its first encounter with another star, giving us a glimpse at pristine material billions of years older than Earth. “We think most interstellar objects that we see will be encountering a star for the first time, even if they’re 8 billion years old,” says Hopkins. “They would have been wandering in deep space until they got near us.”