As interstellar Comet 3I/ATLAS began moving away from the Sun in December 2025, astronomers took the opportunity to turn the powerful NASA/ESA/CSA 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.
Webb captured detailed data, including chemical ratios of carbon and deuterium, also known as heavy hydrogen, that are not found in Solar System comets. The results surprised researchers. Working backward, astronomers used the components that make up Comet 3I/ATLAS to understand the environment in which it formed.
A paper detailing the findings was published on 22 June 2026 in the journal Nature.
The comet’s name comes from its status as the third confirmed interstellar comet, meaning it originated outside the Solar System, and the telescope that first spotted it, the NASA-funded ATLAS (Asteroid Terrestrial-impact Last Alert System).
“This was a unique opportunity to study an ancient object from the distant Galaxy, probably pre-dating our Sun and Solar System,” said astro-chemist Martin Cordiner of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the study. “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 NIRSpec (Near-Infrared Spectrograph) instrument to study the comet.
NIRSpec revealed exceptionally high levels of deuterium, about 30 times more than seen in Solar System comets. 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 any long-term warmth that would have reprocessed its 'heavy water' ice, with deuterium, into the type of H2O ice we are familiar with on Earth.
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, as 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.