Recently discovered interstellar comet 3I/ATLAS may be a very old object indeed.
A new visitor to the inner solar system may in fact be an extremely old object.
All eyes are on the newly discovered interstellar object 3I/ATLAS, currently inbound to the inner solar system. Initial observations have revealed that it’s rich in water ice, and it’s believed that it originated from the Milky Way’s thick disk, which hosts a population of ancient stars that orbit above and below the galactic plane. This could mean that 3I/ATLAS is billions of years older than our solar system, making it the oldest comet ever discovered. It should reveal more as it heats up and outgasses as it gets closer to the Sun.
We wrote about the breaking news and the discovery of 3I/ATLAS on July 2nd, along with prospects for spotting the interstellar comet before and after perihelion later this year.
Comet 3I/ATLAS, remotely captured using the iTelescope 0.51-meter reflector on July 2nd. (note, it was still known by its provisional designation of A11pl3Z at this time). Credit: Filipp Romanov.
Now, a study presented at the recent meeting of the Royal Astronomical Society in Durham hints that Comet 3I/ATLAS may be something special indeed. Specifically, the comet may pre-date the formation of our solar system by over three billion years.
Already, each of the three known interstellar objects—1I/ ‘Oumuamua, 2I/ Borisov, and 3I/ATLAS—have proven to be distinctive examples in their own right.
Comet 3I/ATLAS appears to hail from the outer thick disk of the Milky Way, versus the thin inner disk where stars like our Sun typically reside. Astronomers extrapolate this from the relatively steep path of 3I/ATLAS’s orbit around the galaxy. Ancient stars tend to reside in the Milky Way’s thick disk population. If 3I/ATLAS formed and was subsequently ejected from such a system long ago, it could be on the order of over seven billion years old.
A side-view simulation of our Milky Way Galaxy, showing the orbit of 3I/ATLAS (in red) versus our Sun. Credit: M. Hopkins/Ōtautahi-Oxford team. Base map: 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,” says lead researcher Matthew Hopkins (University of Oxford) in a recent press release. “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.”
A speedy space-dot: Comet 3I/ATLAS shortly after discovery on July 1st. Credit: ATLAS/University of Hawaii/NASA.
3I/ATLAS is currently approaching the inner solar system from the direction of the Scutum-Sagittarius constellation border, while the perihelion passage of 1.36 Astronomical Units (AU) near the Sun will eject it in the direction of the constellation Gemini, in the general direction of the star Zeta Geminorum afterwards. 3I/ATLAS is approaching our solar system at a speedy 68 kilometers per second near perihelion. For context, the fastest spacecraft, NASA’s Parker Solar Probe hit 191 kilometers per second in 2024 at perihelion, just 6.9 million kilometers or just under 10 solar radii from the Sun.
The Sun and our solar system are moving at 13.4 kilometers per second (versus the local standard of rest) in the direction of a point just south of the bright star Vega (known as the Apex of the Sun’s Way) on our quarter of a billion-year journey around the Milky Way Galaxy.
This provides researchers a chance to apply what’s known as the Ōtautahi-Oxford Model for interstellar objects into actual practice, comparing real-time data to predictions. This model uses data from the Gaia survey along with formation chemistry of the galactic disk to model the expected properties for the population of interstellar objects.
You can see just how difficult the discovery of 3I/ATLAS was, against the star-dense field along the plane of the Milky Way in this wide-field view, versus the inset showing the comet. Credit: ATLAS/University of Hawaii/NASA.
3I/ATLAS could be the first known interstellar object from the thick disk population seen, and could prove to be a water ice-rich object as well. We’ll know shortly as it nears perihelion on October 29th and heats up, growing a coma and tail characteristic of solar system comets.
The recently commissioned Vera C. Rubin Observatory is expected to find up to 50 interstellar objects (ISOs) over the next decade as it surveys the sky. This added knowledge will allow astronomers to understand the population of interstellar objects as a whole, and predict just how common—or rare—they really are.
Riding along with 3I/ATLAS would provide a mind-boggling journey in time and space. After several billion years in deep-space, a bright star looms ahead. For a brief few months, that star becomes a Sun, heating the surface of the long-frozen comet. As an added bonus, one of the planets huddled near the star hosts an intelligent civilization, determined to understand its place in the Universe, and what the fleeting passage of 3I/ATLAS promises to tell them about the unfolding story of the galaxy and their place in it. The final story of 3I/ATLAS isn’t over yet.