NASA’s Webb says interstellar comet 3I/ATLAS formed 10 to 12 billion years ago
NASA astronomers used the James Webb Space Telescope to detect unusual chemistry in the interstellar comet 3I/ATLAS, a result that points to a birth between 10 and 12 billion years ago, before the Sun existed. The findings were published on June 22, 2026, in the journal Nature. The comet carries about 30 times more deuterium, a heavy form of hydrogen, than comets that formed in the Earth’s cosmic neighborhood.
The measurements were taken on December 22 and 23, 2025, after the comet had passed closest to the Sun and was moving back out into interstellar space. Heat from the Sun turned old ice in the comet’s nucleus straight into gas, forming a bright cloud called a coma. The NIRSpec near-infrared spectrograph on Webb analyzed that gas in detail.
3I/ATLAS is the third interstellar object confirmed by astronomers, after 1I/ʻOumuamua and 2I/Borisov. NASA’s ATLAS asteroid alert system, which the agency funds, discovered it and gave it its name. To make the observations, a team led by astrochemist Martin Cordiner, at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, won approval to interrupt the observatory’s regular schedule.
Cordiner, the lead author of the study, said the comet offered a rare chance to examine an ancient object from a distant galaxy, probably older than the Sun and the Solar System. He added that the data give direct information about that distant time and place, and also show how unusual our own Solar System may be.
Read also: 3I/ATLAS interstellar comet: 57 km/s hyperbolic orbit confirms it came from another star
Deuterium levels point to a frigid birthplace for the comet
The high deuterium ratio indicates the comet formed in an extremely cold environment, at 30 kelvins or lower, roughly minus 243 degrees Celsius. Under those conditions, the material that built the comet absorbed heavy radiation but never warmed enough to turn its heavy water ice into the ordinary water found on Earth. Researchers say the ice stayed frozen from the moment the comet formed.
The NIRSpec spectra also detected only trace amounts of carbon-13 compared with carbon-12, the lighter isotope of that element. Carbon-13 builds up slowly in interstellar gas as generations of stars are born and die, so its scarcity suggests the comet came from an early era. That era, known as cosmic noon, was a period of intense star formation billions of years before the Sun, which is 4.5 billion years old, came into being.
- Estimated age: 10 billion to 12 billion years, more than twice the age of the Solar System.
- Deuterium: about 30 times the level found in comets from the Solar System, with a D/H ratio of 0.98 percent in the water analyzed.
- Carbon isotope ratios: 141 to 191 for CO2 and 123 to 172 for CO, both beyond typical Solar System values.
- Birthplace conditions: a cold, dense interstellar cloud at temperatures below minus 243 degrees Celsius, where the comet stayed frozen from formation onward.
A second team used a Chilean telescope to test the comet’s origin
Astronomer Cyrielle Opitom, of the University of Edinburgh in the United Kingdom, led a separate investigation with the Very Large Telescope (VLT), operated by the European Southern Observatory (ESO) in Chile. That analysis examined carbon and nitrogen isotope variations in cyanide released by the comet. It confirmed the object was born on the cold outer edge of a disk orbiting an ancient star.
For researchers, mapping comets from other star systems can show whether the chemical ingredients that led to life on Earth are common or rare in the universe. Webb is run by NASA in partnership with the European Space Agency (ESA) and the Canadian Space Agency (CSA).
Keep reading: NASA mobilizes planetary defense telescopes for comet 3I/ATLAS
Stefanie Milam, a NASA Goddard researcher and co-author of the study, said that finding these rare isotopes is fascinating, but the main goal is to understand the possibilities for prebiotic chemistry elsewhere in the galaxy. She noted that the Solar System and Earth are the only known place where chemistry led to life, so studying interstellar objects helps show how common those conditions are.
Despite progress in reconstructing where 3I/ATLAS formed, astronomers have not identified the star or planetary system from which it was ejected. A separate study in Nature Astronomy, based on ALMA observations and led by researchers at the University of Michigan, reported a water deuterium ratio above 6.6 x 10^-3, more than 40 times Earth’s ocean value. Random gravitational interactions over billions of years in the Milky Way keep scientists from tracing the comet’s orbit back more than 10 million years, while the object follows a hyperbolic path that will carry it permanently out of the Solar System.
