Interstellar comet 3I/ATLAS may be nearly 12 billion years old — so ancient its star system may no longer exist

The interstellar comet that recently dominated headlines, 3I/ATLAS, could be between 10 and 12 billion years old, a new assessment of the comet's isotopic composition has shown. This so-called "invader" in our solar system is only the third object on record to enter our cosmic neighborhood from beyond.

If these new age estimates of the comet are true, they would suggest 3I/ATLAS was born within a few billion years of the birth of the Milky Way.

When it was discovered in 2025, 3I/ATLAS was speeding in at 36 miles (58 kilometers) per second relative to the sun. It is the fastest comet ever seen, vastly exceeding the velocities of its predecessors 1I/'Oumuamua and 2I/Borisov. And according to theory, the faster an interstellar object is travelling, the older it must be because it must have experienced multiple space-object encounters and gravitational slingshots with other stars that would accelerate it to a high velocity.

So, based on the comet's velocity, astronomers Aster Taylor and Darryl Seligman of the University of Michigan and Michigan State University, respectively, determined 3I/ATLAS has a "kinematic" age somewhere between 3 billion and 11 billion years old. That's a large range with substantial uncertainty — but now, a new study led by NASA Goddard's Martin Cordiner has come out in favor of the older end of that range, based on the comet's isotopic composition.

From studies with the James Webb Space Telescope's Near-Infrared Spectrometer (NIRSpec), Cordiner and his team measured both the ratio of carbon-12 to carbon-13 in 3I/ATLAS and also how enriched 3I/ATLAS's water is with the molecule deuterium, which is one of two stable isotopes of hydrogen. Both these properties are important tools for deducing the age and origin of the comet.

Isotopes are atoms of the same element that have the same number of protons but differing numbers of neutrons. Carbon-12 is the regular form of carbon, with 6 protons and 6 neutrons. Carbon-13 is the isotope, with 6 protons and 7 neutrons. Deuterium has one proton and one neutron (as opposed to your regular old hydrogen, which has one proton and no neutrons).

The carbon isotopes are found on 3I/ATLAS in compounds such as carbon monoxide and carbon dioxide, and even organic molecules such as methanol, formaldehyde and methane.

NIRSpec found that 3I/ATLAS contains far more carbon-12 relative to carbon-13 than anything seen in our solar system, or indeed even in nearby planet-forming disks around other stars, or local molecular clouds. This tells us that 3I/ATLAS ain't from around here, at least.

Carbon-13 becomes enriched over time within the interstellar medium and the molecular clouds that form stars. A low abundance of carbon-13 relative to carbon-12 therefore indicates that 3I/ATLAS must have formed a long time ago — before carbon-13 was able to build up to modern-day levels.

We can turn to models of galactic evolution to hazard an estimate as to exactly how long ago that was.

After the Milky Way formed about 13 billion years ago, it underwent a starburst: a huge bout of star formation. Many of these stars quickly evolved into red giants before casting adrift their outer layers and forming a planetary nebula while leaving behind their hot, inert core, which is what we call a white dwarf.

When in a close binary system with another star, a white dwarf can steal so much material that it ignites a thermonuclear explosion on its surface. We call this a nova, and such events are prodigious producers of carbon-13. A rapid burst of nova explosions is expected to have therefore occurred during the first four billion years of the Milky Way's history. For 3I/ATLAS to contain such a low ratio of carbon-13 relative to carbon-12, yet still contain some heavy elements, it must have formed in the middle of all this, before the carbon-13 abundance had a chance to build up in the galaxy.

This would actually place 3I/ATLAS's age as 10–12 billion years old.

The enrichment of deuterium in 3I/ATLAS also tells us about the interstellar comet's origins. Deuterium can replace one or both of the ordinary hydrogen atoms in water, which is what scientists mean by deuterium enrichment. The enrichment of deuterium in 3I/ATLAS's water has a D/H ratio that is an order of magnitude greater than in typical comets that formed in our solar system.

This degree of enrichment takes place in certain environments. Water-ice can become enriched in deuterium at temperatures less than 30 degrees above absolute zero (30 kelvin/–243 degrees Celsius/–405 degrees Fahrenheit), which is typical of interstellar clouds, and in an environment relatively poor in heavy elements, which points back to a formation early in the history of our galaxy.

Comets form alongside planets, so if these findings are correct, then 3I/ATLAS may very well be a relic of one of the earliest planetary systems in the galaxy. Can 3I/ATLAS tell us anything about these early planets that it formed alongside?

Comets, being icy objects, are thought to form in the more distant reaches of planet-forming disks, away from the heat of their young star that would otherwise vaporize ices. The boundary in a planet-forming disk between where water exists as a vapor or liquid, and where it exists as ice, is called the snow line.

"We believe that cometary materials in general are representative of the building blocks of planets outside the water snow-line in the protoplanetary disk," Cordiner told Space.com. "So the same is probably true of interstellar comets, and they provide unique insights into what extrasolar planets could be formed from."

Scientists are still building up a complete picture of 3I/ATLAS' chemical inventory, but there are a few things they can say at this stage.

"2I/Borisov and 3I/ATLAS both show a relatively carbon-rich composition compared to solar system comets," Cordiner said. "At the very least, this implies the abundant presence of carbon in the originating planetary system. 3I/ATLAS is also very water-rich."

The presence of deuterium, and various carbon and oxygen compounds, is indicative of a fairly complex chemistry that took place on icy dust grains from which 3I/ATLAS' planetary system likely formed, telling us that organic molecules and water were important components of planets even that early in the universe's history.

However, 3I/ATLAS' true point of origin remains a mystery, and likely always will. Tracing its path back more than 10 million years becomes difficult if not impossible, because of gravitational interactions between 3I/ATLAS and the stars that it passes close to, which perturb its trajectory.

However, the knowledge of its age narrows things down a little bit.

The Milky Way's disk is split into two parts — a narrow, 1,000-light-year-deep disk where most of our galaxy's star-formation now takes place (and where our sun was born), which is located inside a more diffuse and deeper thick disk (about 3,000 light-years deep). Observations of stars in the thick disk by the European Space Agency's Gaia mission suggest the thick disk began forming 13 billion years ago, whereas the thin disk is thought to be much younger, forming about 9 billion years ago. If these ages are correct, then 3I/ATLAS may have come from a thick-disk star.

"That seems more probable the older [3I/ATLAS] is," said Cordiner.

Indeed, 3I/ATLAS is so ancient that the star system that produced it may not even exist any longer. Is 3I/ATLAS truly a relic from a lost era of planet formation?

The findings are currently awaiting peer review, but are available via a pre-print.