Ancient Comet 3I/ATLAS Reveals Clues to Early Milky Way

Interstellar comet 3I/ATLAS, discovered in 2025, is estimated to be between 10 and 12 billion years old, offering a rare glimpse into the early history of our galaxy. New observations from the James Webb Space Telescope (JWST) suggest the comet formed in a cold, distant region of the Milky Way, predating our solar system by billions of years.

From Alien Spacecraft Speculation to Ancient Relic

Initially, online speculation suggested the interstellar visitor, 3I/ATLAS, could be an alien spacecraft. However, astronomers largely agree it is a comet originating from outside our solar system. Discovered on July 1, 2025, by the Asteroid Terrestrial-impact Last Alert System (ATLAS) station in Chile, 3I/ATLAS quickly became a focus of scientific study.

JWST Reveals Comet’s Ancient Origins

Preliminary findings from a study posted to the preprint server Research Square, currently under peer review, indicate that 3I/ATLAS formed approximately 10 to 12 billion years ago. This makes it more than twice as old as Earth (4.5 billion years old) and our solar system (4.6 billion years old), and comparable in age to the Milky Way galaxy and the universe itself (around 13.6, and 13.8 billion years old).

Previous estimates placed the comet’s age between 3 and 11 billion years. The new findings, based on isotope measurements taken by JWST in December 2025, have refined this estimate.

“They show that 3I/ATLAS isotopic composition is very different from solar system comets and suggest that it likely formed 10-12 billion years ago,” said Romain Maggiolo, a research scientist at the Royal Belgian Institute for Space Aeronomy, in an email to Live Science. “In other words, 3I/ATLAS formed in a stellar environment different from ours, not only somewhere else in space, but also at a much earlier time in the history of the Milky Way.”

A Unique Chemical Composition

3I/ATLAS is only the third interstellar object ever recorded in our solar system. Hubble Space Telescope observations suggest the comet is between 1,400 feet (440 meters) and 3.5 miles (5.6 kilometers) wide. It traveled through our solar system at approximately 137,000 mph (221,000 km/h), reaching perihelion (its closest point to the sun) on October 29, 2025, and making its closest approach to Earth on December 19, 2025, at a distance of about 168 million miles (270 million km).

Researchers studying the gas released as the comet warmed up found that its water is more enriched in deuterium, a heavier hydrogen isotope, than any previously studied comet. The ratio of carbon isotopes also exceeded levels typically seen in our solar system.

“If 3I/ATLAS is indeed as old as this study suggests, the large amounts of volatile molecules it contains indicate that rich prebiotic chemistry may already have been occurring in star-forming regions very early in the history of our Galaxy,” Maggiolo added.

Formation in a Cold, Ancient Environment

The study suggests the comet formed in a cold environment around 30 kelvins (minus 406 degrees Fahrenheit, or minus 243 degrees Celsius), likely within a dense and well-shielded protoplanetary disk.

Josep Trigo-Rodríguez, a research principal investigator at the Institute of Space Sciences (CSIC/IEEC) in Spain, described the findings as a “good compilation of scientific results” using established techniques. “This manuscript exemplifies that interstellar comets are unique bodies that are able to sample remote regions of our Milky Way galaxy,” he said.

A Journey Through Space and Time

Whereas pinpointing the exact star system of origin may be impossible, researchers acknowledge the challenges posed by billions of years of space travel and exposure to cosmic rays, which can alter the comet’s chemical composition.

“The isotopic composition of the material outgassed by 3I/ATLAS provides a crucial new piece of the puzzle,” Maggiolo said. “But the puzzle is far from being complete!”

Current Trajectory and Future Observations

As of March 12, 2026, 3I/ATLAS is passing Jupiter, with its closest approach expected on March 15, 2026, at a distance of about 33 million miles (54 million km). It will cross Saturn’s orbit in July 2026, Uranus’ orbit in April 2027, and Neptune’s orbit in March 2028. The comet’s trajectory can be tracked using NASA’s Eyes on the Solar System simulation.