One year ago, on July 1 2025, astronomers discovered a fascinating new object moving through the Solar System. Detected by the Asteroid Terrestrial-impact Last Alert System (ATLAS), the object was quickly recognised as something special.
Tearing past at record speed, the comet, known as 3I/ATLAS , is only the third interstellar object ever detected. Born around another star, it was a fleeting visitor that is now heading out of the Solar System at more than 60 kilometres per second, never to return.
The discovery made headlines worldwide - with bizarre claims that it might be an alien spaceship distracting from the true wonders of the object.
In a new paper published in Nature today, a team of scientists (led by Martin Cordiner from the NASA Goddard Space Flight Center) have shed fresh light on the comet's origin. Their results reveal it to be a true galactic fossil, far older than the Solar System .
What are interstellar objects?
Planetary systems are messy places. Along with the planets we know and love, they contain vast numbers of smaller objects - debris left behind from the formation of those systems.
Much of that debris moves on chaotic, unstable orbits. As a result, the Solar System is continually shedding small bodies to interstellar space. Comets and asteroids are pulled around by the gravitational influence of the planets, and are often ejected. They go on to roam the galaxy, floating through the space between the stars.
It has long been thought that, if planetary systems are common in the cosmos, those systems would behave like our own. As a result, the space between the stars should be littered with a vast population of rocky and icy objects - the debris from systems both ancient and new.
The overwhelming majority of interstellar objects will never come close to another star. But with so many objects out there, it's inevitable some will pass through the inner Solar System, coming close enough for us to observe and study them.
The three interstellar objects found so far - 1I/'Oumuamua , 2I/Borisov and 3I/ATLAS - are the first examples of that population we have managed to spot. Such objects have been passing by since the Solar System's birth. But it's only in the past decade that our technology has advanced to the point we can observe them.
A fast-moving ancient comet
Astronomers quickly determined 3I/ATLAS was moving incredibly fast. At its closest to the Sun, it was travelling at more than 68km per second. Even before it felt the Sun's gravitational pull, it was moving at more than 58km/s.
That was the first hint that the comet might be something special.
Purely based on its orbit around our galaxy, astronomers soon realised 3I/ATLAS was potentially older than the Solar System . Most likely, it had been ejected from a star moving in the Milky Way's " thick disk ", a population of ancient stars that formed long before the Sun.
But would it be possible to confirm that hypothesis?
The chemistry of an object born around another star
The new paper in Nature is based on observations of 3I/ATLAS using the James Webb Space Telescope and the Atacama Compact Array , observing the comet at infrared and microwave wavelengths.
Those observations allowed scientists to study the gases emitted by the comet as it swung passed the Sun. They revealed the carbon molecules emitted by the comet contained far less carbon-13 (a heavy isotope of carbon) than is observed for similar objects in the Solar System. Isotopes are different forms of the same element , with identical numbers of electrons and protons, but different numbers of neutrons.
This suggests the comet must be extremely old. Why? The answer boils down to the way material in our galaxy is processed in the interiors of stars.
Almost all the carbon in the Milky Way is produced by stars more massive than the Sun. When those stars die, the carbon they produce through nuclear fusion is returned to the cosmos, to be incorporated in the next generation of stars and planets.
But there's a twist. As stars create carbon in their interiors, they also act to convert carbon-12 (the most common isotope) into carbon-13, through a process called " hot bottom burning ". As time goes by, the fraction of all carbon in the galaxy that is carbon-13 is increasing.
To contain such a low abundance of carbon-13, 3I/ATLAS must have formed at a time when there was less carbon-13 compared to carbon-12 - roughly 12 billion years ago, during the earliest stages of our galaxy's history.
The best is yet to come
Each of the three interstellar visitors we've seen to date has been unique. But what of the future?
The Vera Rubin Observatory , located on a mountain top in northern Chile, has the largest camera ever built strapped to a telescope that is effectively an incredibly fast wide-angle lens.
This makes it an ideal facility for finding interstellar objects as they tear through the Solar System. Astronomers have predicted the observatory could find dozens of interstellar objects in the decade to come.
It will find objects we would otherwise miss, and will find them earlier than otherwise possible.
By finding more of these objects, we will start to be able to view them as a population, rather than individuals, which will shed light on the history of star and planet formation in our galaxy.
Jonti Horner does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
