A 7-Billion-Year-Old Ice Ball Just Entered Our Solar System

A mysterious, ice-rich comet named 3I/ATLAS has entered our solar system — and it may be the oldest comet ever observed.

Estimated to be over 7 billion years old, this interstellar visitor likely originated from the Milky Way’s thick disk, far beyond the familiar realm of our Sun and planets.

Discovery of a Possible Ancient Comet

A newly identified interstellar object could be the oldest comet ever detected, with scientists suggesting it might have formed more than three billion years before our solar system.

Named 3I/ATLAS, this icy traveler is rich in water and is only the third known object to visit us from outside the solar system. What makes it even more unusual is that it appears to have arrived from an entirely different part of the Milky Way than any of its predecessors.

This mysterious object could be more than seven billion years old, according to University of Oxford astronomer Matthew Hopkins, who discussed his findings at the Royal Astronomical Society’s National Astronomy Meeting 2025 in Durham. He believes 3I/ATLAS might be the most extraordinary interstellar object scientists have studied so far.

An Origin in the Milky Way’s Thick Disk

Unlike the previous two objects to enter our solar system from elsewhere in the cosmos, 3I/ATLAS appears to be traveling on a steep path through the galaxy, with a trajectory that suggests it originated from the Milky Way’s ‘thick disk’ – a population of ancient stars orbiting above and below the thin plane where the Sun and most stars reside.

“All non-interstellar comets, such as Halley’s comet, formed within our solar system, so they are up to 4.5 billion years old,” 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.”

Detection and Early Predictions

The object was first spotted on July 1, 2025, by the ATLAS survey telescope in Chile, when it was about 670 million km from the Sun.

Hopkins’ 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 Professor Chris Lintott, co-author of the study and 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.”

Comet Activity and Astronomical Significance

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, 1I/’Oumuamua (spotted in 2017) and 2I/Borisov (2019).

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.

Testing Models with Live Observations

“We’re in an exciting time: 3I is already showing signs of activity. The gases that may be seen in the future as 3I is heated by the Sun will test our model,” said co-author Dr. Michele Bannister, of the University of Canterbury in New Zealand.

“Some of the biggest telescopes in the world are already observing this new interstellar object – one of them may be able to find out!”

A Surprise That Boosts Future Discovery Potential

The discovery of 3I caught the team by surprise. It happened as they were gearing up for the beginning of survey operations with the Vera C. Rubin Observatory, which their model predicts will discover between 5 and 50 interstellar objects.

“The solar system science community was already excited about the potential discoveries Rubin will make in the next 10 years, including an unprecedented number of interstellar objects,” said co-researcher Dr. Rosemary Dorsey, of the University of Helsinki.

“The discovery of 3I suggests that prospects for Rubin may now be more optimistic; we may find about 50 objects, of which some would be similar in size to 3I. This week’s news, especially just after the Rubin First Look images, makes the upcoming start of observations all the more exciting.”

Predictive Models and Real-Time Validation

The team’s findings come from applying a model developed during Hopkins’ doctoral research, which simulates the properties of interstellar objects based on their orbits and likely stellar origins.

Just a week before the comet’s discovery, Hopkins had defended his thesis, and when 3I/ATLAS was announced, he was set to go on holiday. Instead, he found himself comparing real-time data to his predictions.

“Rather than the quiet Wednesday I had planned, I woke up to messages like ‘3I!!!!!!!!!!’,” said Hopkins. “It’s a fantastic opportunity to test our model on something brand new and possibly ancient.”

When and How to See 3I/ATLAS

The researchers’ model, dubbed the Ōtautahi–Oxford Model, marks the first real-time application of predictive modelling to an interstellar comet.

For those keen to catch a glimpse of 3I/ATLAS, it should be visible through a reasonably-sized amateur telescope in late 2025 and early 2026.

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