- The James Webb Space Telescope studied the interstellar comet 3I/ATLAS as it passed through our solar system.
- Its observations have revealed that the comet contains an unusual set of chemicals, making it very different from the comets in our own solar system.
- Researchers estimate that 3I/ATLAS may have formed 10–12 billion years ago.
ESA originally published this article on June 22, 2026. Edits by EarthSky.
Interstellar Comet 3I/ATLAS is over 10 billion years old
As the interstellar comet 3I/ATLAS began to move away from the sun in December 2025, astronomers took the opportunity to turn the James Webb Space Telescope in its direction and capture detailed measurements of its chemical components. The comet was freshly warmed from its closest pass by the sun, and its ancient ice had been converted to a bright coma of gas ideal for observation.
In a surprise to the researchers, Webb detected ratios of chemicals that are not found in comets in our solar system. Working backward, astronomers have now used the components that make up comet 3I/ATLAS to understand the environment in which it formed. And one revelation is that 3I/ATLAS appears to be 10-12 billion years old. That’s nearly three times the age of our solar system.
The researchers published a paper detailing the findings on June 22, 2026, in the peer-reviewed journal Nature.
Studying ancient Comet 3I/ATLAS
The comet’s name comes from its status as the 3rd confirmed interstellar comet, meaning it originated outside the solar system. The comet was first spotted by the Asteroid Terrestrial-impact Last Alert System (ATLAS).
According to the study’s lead author, astrochemist Martin Cordiner of NASA’s Goddard Space Flight Center in Greenbelt, Maryland:
This was a unique opportunity to study an ancient object from the distant galaxy, probably predating our sun and solar system. On the one hand, we get direct insight into that distant time and place, and on the other, we learn something about how unusual our own solar system may be.
Cordiner and the research team joined astronomers from many sub-disciplines in taking the opportunity to get a look at 3I/ATLAS on its journey through the solar system. They received approval to interrupt Webb’s planned schedule of observations to make use of its Near-Infrared Spectrograph (NIRSpec) instrument to study the comet.
NIRSpec revealed exceptionally high levels of deuterium, also known as heavy hydrogen. In fact, it contains about 30 times more deuterium than the comets in our solar system. This implies that 3I/ATLAS may have originated in a very cold system much earlier in the history of our galaxy.
During its formation, the material that became incorporated into 3I/ATLAS was likely exposed to plenty of radiation, but not to any long-term warmth that would have reprocessed its heavy water ice, containing deuterium, into the type of H2O ice we are familiar with on Earth.
Comet 3I/ATLAS is older than our solar system
Additionally, NIRSpec showed only traces of carbon-13 compared to lighter-weight carbon-12. This also points to a very old origin for 3I/ATLAS, since stellar systems become enriched with carbon-13 over time as generations of stars are born and die in the galaxy. That is why there are higher levels of carbon-13 in our system, around our sun, which formed relatively recently: 4.5 billion years ago.
The research team estimates that 3I/ATLAS could have formed as long as 10 to 12 billion years ago, during the universe’s cosmic noon, when star formation was at its height. Its young origin system was likely ensconced in a relatively cold, dense cloud. The abundance of heavy water shows that 3I/ATLAS spent its formative years in a deeply frozen state.
A separate study using the European Southern Observatory’s Very Large Telescope, led by astronomer Cyrielle Opitom of the University of Edinburgh, complements Webb’s findings with an analysis of 3I/ATLAS’s carbon and nitrogen varieties in the form of the chemical cyanide.
Stefanie Milam of NASA Goddard and co-author of the study said:
For us as scientists, finding these rare isotopes is fascinating, but the bigger picture here is looking at the possibilities of prebiotic chemistry elsewhere in the galaxy. So far, we know of only one place in the vast cosmos where chemical ingredients led to life: our solar system, our Earth. Analysis of these interstellar objects is a major step toward learning how common, or uncommon, the conditions for the evolution of life are in the universe.
Bottom line: The Webb space telescope has found chemical clues to the origin of interstellar comet 3I/ATLAS. It appears to be several times older than our solar system.
Read more: Comet 3I/ATLAS has methane, unexpected discovery reveals
Read more: Interstellar comet 3I/ATLAS born in a cold environment
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