Ancient Comet May Predate Solar System

An interstellar object, discovered recently, might be the oldest comet ever observed, possibly existing before our solar system came into being more than three billion years ago, according to researchers.

Interstellar Visitor

Designated 3I/ATLAS, this icy visitor marks only the third object originating from outside our solar system to be detected in our galactic neighborhood. Uniquely, it appears to come from a markedly different part of the Milky Way.

Astronomer **Matthew Hopkins** from the University of Oxford, who is presenting his findings at the Royal Astronomical Society’s National Astronomy Meeting 2025 in Durham, suggests it could be over seven billion years old. He added that this comet may be the most remarkable interstellar visitor to date.

In contrast to the two previous interstellar objects, 3I/ATLAS seems to follow a steep trajectory through the galaxy. Its path indicates an origin within the Milky Way’s ‘thick disk’—an area populated by ancient stars orbiting above and below the plane where the Sun and the majority of stars are located.

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

**Hopkins** added, “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.”

Discovery and Composition

The ATLAS survey telescope in Chile first detected the object on July 1, 2025, at a distance of approximately 670 million km from the Sun.

**Hopkins**’ research indicates that 3I/ATLAS likely originated around an old, thick-disk star and should therefore 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.

**Lintott** stated, “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.”

A side-on view of the Milky Way, showing the estimated orbits of both our Sun and the 3I/ATLAS comet. 3I/ATLAS is shown in red dashed lines, and the Sun is shown in yellow dotted lines. The large extent of 3I’s orbit vertically into the outer thick disk is clear, while the Sun stays nearer the plane of the galaxy. **M. Hopkins**/Ōtautahi-Oxford team. Base map: ESA/Gaia/DPAC, **Stefan Payne-Wardenaar**, CC-BY-SA 4.0

Future Observations

As 3I/ATLAS gets closer to the Sun, the sunlight will warm its surface, leading to cometary activity—the release of gas and dust that forms a glowing coma and tail. Initial observations suggest that the comet is already active and potentially larger than 1I/’Oumuamua (2017) and 2I/Borisov (2019).

Confirmation of its size could influence expectations for future telescope detections, such as those by the Vera C. Rubin Observatory. It might also shed light on the function of ancient interstellar comets in seeding star and planet formation across the galaxy.

**Dr. Michele Bannister**, co-author from the University of Canterbury in New Zealand, noted, “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.”

**Bannister** added, “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 team discovered 3I unexpectedly, as they prepared for survey operations with the Vera C. Rubin Observatory. Their model forecasts that this observatory will detect between 5 and 50 interstellar objects.

According to co-researcher **Dr. Rosemary Dorsey** from the University of Helsinki, “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.”

“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,” **Dorsey** said.

The team’s analysis is based on a model developed during **Hopkins**’ doctoral research, which simulates interstellar object properties using their orbits and likely stellar origins.

One week before the comet was discovered, **Hopkins** defended his thesis. When 3I/ATLAS was announced, he was about to begin a vacation. Instead, he began comparing real-time data with his predictions.

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

**Hopkins** and his colleagues have released their analysis as a preprint on arXiv. Their model, named the Ōtautahi–Oxford Model, represents the first real-time application of predictive modeling to an interstellar comet.

For those hoping to view 3I/ATLAS, it should be visible through a reasonably sized amateur telescope in late 2025 and early 2026. Currently, there are 4,600 known comets in our solar system (Space.com).