The interstellar comet was probably way older than scientists thought

An interstellar comet that recently passed through the solar system may have formed as long as 12 billion years ago, making it one of the oldest planet-building materials ever observed.

Comet 3I/ATLAS is only the third confirmed interstellar object detected in our neck of space. As it passed near the sun in late 2025, it released unusually large amounts of gas, giving scientists a rare chance to study its composition in detail.

Observations from NASA's James Webb Space Telescope focused on the comet's isotopes — different versions of hydrogen and carbon that act like long-lasting chemical fingerprints. Those measurements showed values that do not match any known comet from our own solar system or nearby star-forming regions.

Scientists say the results point to something unusual: Comet 3I/ATLAS likely formed in an extremely cold, chemically primitive region of the early Milky Way and may preserve material from a planetary system that formed more than 7 billion years before the sun and Earth. That would make it about 4 billion years older than some initial predictions, and a rare surviving fragment of the galaxy's earliest days.

"This was a unique opportunity to study an ancient object from the distant galaxy," said NASA astro-chemist Martin Cordiner, lead author of the study, in a statement. "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."

A comet is a small, icy object left over from the beginning of a star system. It is made mostly of frozen gases, water ice, dust, and rock. When it gets close to a star, like the sun, some of that ice warms up and turns directly into gas, skipping over the liquid phase. The gas forms a glowing cloud and sometimes a tail pointing away from the star. 

This foreign comet came from another part of the galaxy and was later ejected — likely by a gravitational scrape from a planet or passing star — before drifting across interstellar space for hundreds of millions of years. Scientists only know about two other interstellar visitors having passed through our neighborhood: 'Oumuamua in 2017, which turned out not to be a comet, and Comet 2I/Borisov in 2019.

Ratios of heavy carbon and heavy hydrogen never seen among comets in our solar system appeared in the interstellar comet 3I/ATLAS that passed through in 2025, according to NIRSpec observations from the James Webb Space Telescope. Credit: NASA / ESA / CSA / STScI / M.Cordiner / Alyssa Pagan

One of the strongest clues about Comet 3I/ATLAS's age and origins comes from its water, which contains an unusually high amount of deuterium, a heavier form of hydrogen. The level is more than 30 times higher than in typical solar system comets. Its carbon isotope ratios also fall outside the range seen in nearby gas clouds and young planetary systems.

Taken together, the chemistry suggests the comet formed in temperatures below about -400 degrees Fahrenheit and has changed very little since then. The findings from the Webb study appear in the journal Nature. 

Researchers also say the carbon data points to a birthplace in a region of the Milky Way that was poor in heavy elements but enriched by earlier generations of massive stars. That matches conditions expected during an early burst of star formation in the galaxy's history.

While the Webb data is enlightening, scientists cannot trace the comet back to a specific star system. Over billions of years, gravitational encounters with other bodies throughout the Milky Way would have scrambled its trajectory beyond reconstruction, so researchers have relied on its chemistry instead of its orbit.

If the age estimate holds, 3I/ATLAS offers a direct glimpse into how some of the first Milky Way planets formed. 

"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," said Stefanie Milam, a co-author from NASA, in a statement. "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 towards learning how common, or uncommon, the conditions for the evolution of life are in the universe."