3I/ATLAS — the second interstellar comet ever spotted in our cosmic neighborhood — may predate our Solar System by more than 3 billion years, according to a research team led by University of Oxford astronomer Matthew Hopkins.
Top view of the Milky Way showing the estimated orbits of both our Sun and 3I/ATLAS. The comet is shown in red dashed lines, and the Sun is shown in yellow dotted lines. The large extent of the comet’s orbit into the outer thick disk is clear, while the Sun stays nearer the core of the Galaxy. Image credit: M. Hopkins / Ōtautahi-Oxford Team / ESA / Gaia / DPAC / Stefan Payne-Wardenaar / CC-BY-SA 4.0.
“All non-interstellar comets such as Halley’s comet formed with our Solar System, so are up to 4.5 billion years old,” Dr. Hopkins said.
“But interstellar visitors have the potential to be far older, and of those known about so far our statistical method suggests that 3I/ATLAS is very likely to be the oldest comet we have ever seen.”
Unlike 1I/ʻOumuamua and 2I/Borisov, the previous two objects to enter the Solar System from elsewhere in the cosmos, 3I/ATLAS appears to be traveling on a steep path through the Milky Way, with a trajectory that suggests it originated from the ‘thick disk’ of our Galaxy — a population of ancient stars orbiting above and below the thin plane where the Sun and most stars reside.
The new research predicts that, because 3I/ATLAS likely formed around an old, thick-disk star, it should be rich in water ice.
“This is an object from a part of the Galaxy we’ve never seen up close before,” said University of Oxford’s Professor Chris Lintott, presenter of the BBC’s The Sky at Night.
“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.”
As it approaches the Sun, sunlight will heat 3I/ATLAS’s surface and trigger cometary activity, or the outgassing of vapor and dust that creates a glowing coma and tail.
Early observations already suggest the comet is active, and possibly larger than either of its interstellar predecessors.
If confirmed, this could have implications for how many similar objects future telescopes, such as the new Vera C. Rubin Observatory, are likely to detect.
It may also provide clues about the role that ancient interstellar comets play in seeding star and planet formation across the Galaxy.
“We’re in an exciting time: 3I/ATLAS is already showing signs of activity,” said Dr. Michele Bannister, an astronomer at the University of Canterbury.
“The gases that may be seen in the future as 3I/ATLAS is heated by the Sun will test our model.”
“Some of the biggest telescopes in the world are already observing this new interstellar object — one of them may be able to find out!”
The astronomers presented their results today at the Royal Astronomical Society’s National Astronomy Meeting 2025 in Durham, the United Kingdom.
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Matthew Hopkins et al. The Galactic Interstellar Object Population in the LSST. NAM 2025
