3I/ATLAS: a comet from deep in a galaxy older than the solar system

The recently discovered interstellar object 3I/ATLAS may turn out to be the oldest comet ever recorded by scientists.

According to astronomer Matthew Hopkins of Oxford University, the celestial body could be more than 7 billion years old, making it potentially older than the solar system itself. He made the announcement at the UK's National Astronomy Conference (NAM 2025) in Durham.

Read more: The Galactic Interstellar Object Population in the LSST.

Matthew J. Hopkins et al, From a Different Star: 3I/ATLAS in the context of the Ōtautahi-Oxford interstellar object population model, arXiv (2025). DOI: 10.48550/arxiv.2507.05318, doi.org/10.48550/arXiv.2507.05318

3I/ATLAS became only the third known object from outside the solar system to come close to Earth, and the first to come from another region of the Milky Way galaxy. Its trajectory, researchers said, is different from previous interstellar objects - the comet follows a steep path out of the Milky Way's so-called "thick disc" - an area populated by ancient stars above and below the thin plane where the Sun is located.

"Ordinary comets, such as Halley's Comet, formed at the same time as the solar system and are up to 4.5 billion years old," Hopkins explains. - "But interstellar objects have much more potential, and according to our model, it is 3I/ATLAS that is probably the oldest of all."

The comet was detected on 1 July 2025 by the ATLAS telescope in Chile when it was about 670 million kilometres from the Sun. Scientists suggest the object is made up mostly of water ice and has already started to show signs of activity - vaporising volatile matter as it approaches the Sun.

"This is an object from a part of the galaxy that we've never observed this close before," said Professor Chris Lintott, co-author of the study and presenter of BBC science programme The Sky at Night. - "There is about a two-thirds chance that this comet is older than the solar system itself."

Already the comet is showing activity, as confirmed by early observations. It may even be larger than two previous interstellar visitors, 'Oumuamua (1I, 2017) and 2I/Borisova (2019). If these data are confirmed, the discovery of 3I/ATLAS could have a significant impact on future observing campaigns, including surveys with the new Vera C. Rubin.

"We are in for an exciting time: 3I is already showing signs of life. If we can capture the outgassing as it is heated by the Sun, it will be a great test of our model," said Dr Michelle Bannister from the University of Canterbury, New Zealand.

The team of scientists used the new Ōtautahi-Oxford model, developed as part of Hopkins' PhD thesis. This is the first-ever application of a real-time predictive model to an interstellar comet. Scientists say the Rubin telescope will be able to detect five to 50 similar objects in the future.

"The solar system science community was already expecting major discoveries with the launch of the Rubin telescope. And the finding of 3I makes these prospects even more encouraging," says Dr Rosemary Dorsey from the University of Helsinki.

Those who want to see 3I/ATLAS with their own eyes should wait until late 2025 or early 2026, when the object is predicted to be available for observation with an amateur telescope.