Astronomers are continuing to gather data on 3I/ATLAS, the third confirmed interstellar object (ISO) to pass through our solar system — and possibly the most intriguing yet. Since its discovery on July 1, 2025, new analyses have revealed clues about its origins, composition, age, and potential for future observation. Here’s what the latest research tells us.
A hyperbolic orbit revealed by ZTF
On July 1, the NASA-funded ATLAS survey telescope in Chile spotted the object in the constellation Sagittarius and reported it to the Minor Planet Center.
But the first sign that 3I/ATLAS was something unusual came from the Zwicky Transient Facility (ZTF), a robotic survey camera at Caltech’s Palomar Observatory.
Just hours after ATLAS discovered the object, astronomer Quanzhi Ye of the University of Maryland identified pre-discovery images of the comet taken by ZTF on June 28 and 29. These extended the object’s observed orbit from a time span of just three hours to three days — a change that revealed a strongly hyperbolic trajectory.
In a July 11 Caltech press release, Ye explained: “The new three-day arc suggested a strongly hyperbolic orbit, which means the comet is just passing through our solar system and will not come back.” He added, “This prompted speculation on community mailing lists that the object might be interstellar.”
He recalled the excitement of the moment: “I was trembling because I didn’t want to make a mistake that prompted speculation of the body being interstellar. There were a lot of intense email exchanges. We were all very excited.”
A thick-disk origin and a size surprise
With its interstellar nature confirmed, scientists quickly began analyzing 3I’s motion, composition, and structure. A recent paper from a team led by Matthew Hopkins of the University of Oxford, U.K., suggests 3I/ATLAS is the first known ISO to originate from the Milky Way’s thick disk — a population of stars that orbit above and below the thin galactic plane. The authors also write that the object “predicts a high water mass fraction, which may become observable shortly.”
They also find that 3I may be unusually large — perhaps as large as around 6 miles (10 kilometers). That would make it up to ten times larger than 2I/Borisov, the next-largest known ISO, and even more so compared to 1I/ʻOumuamua. This unexpected size suggests that the population of interstellar objects may include more large bodies than previously thought.
These insights come from the Ōtautahi–Oxford Model, developed by Hopkins and colleagues, which uses predictive modelling of an interstellar comet to determine its properties.
Possibly the oldest comet ever seen
Now, this new statistical modeling may place 3I/ATLAS in even more exclusive territory. Presenting at the Royal Astronomical Society’s National Astronomy Meeting in Durham, U.K., Hopkins stated: “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.”
Because 3I likely formed around an old, thick-disk star, it may be more than 7 billion years old. “This is an object from a part of the galaxy we’ve never seen up close before,” said co-author and Oxford astronomer Chris Lintott. “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.”
Their findings also hint at what’s to come from the Vera C. Rubin Observatory. “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,” said Rosemary Dorsey of the University of Canterbury in Christchurch, New Zealand.
And for Hopkins, the timing couldn’t have been more unexpected. Having just defended his doctoral thesis on interstellar objects a week before, he found himself thrust into the spotlight. “Rather than the quiet Wednesday I had planned, I woke up to messages like ‘3I!!!!!!!!!!’ It’s a fantastic opportunity to test our model on something brand new and possibly ancient.”
What comes next
3I/ATLAS is expected to reach its closest approach to the Sun around Oct. 30, passing just inside the orbit of Mars. As it gets closer, astronomers hope to learn more about its composition, structure, and history — and perhaps catch a rare glimpse of a primordial fragment from a distant corner of the galaxy.
