In 2025, astronomers detected a rare interstellar comet passing through the solar system, designated 3I/Atlas. Recent analysis indicates that this icy visitor may be as old as 12 billion years, placing it among the most ancient objects directly studied by scientists. This age estimate suggests the comet originated nearly three times earlier than Earth, offering a unique glimpse into the early universe.

3I/Atlas was identified by its distinctive high velocity and trajectory, which showed it was not gravitationally bound to the sun but had traveled from a different planetary system. Such interstellar objects provide valuable insights into planetary formation processes far beyond the reach of current spacecraft.

A team led by Dr Cyrielle Opitom of the University of Edinburgh used the European Southern Observatory’s Very Large Telescope in Chile to examine the comet’s gaseous envelope, or coma. As sunlight warmed the comet’s surface, ices sublimated into gas, allowing researchers to analyze chemical signatures that could reveal its origin. They focused on cyanogen, a molecule composed of carbon and nitrogen, paying particular attention to isotopic ratios of these elements.

The nitrogen isotopes indicated that the comet likely formed in the cold, outer regions of its parent star system. Meanwhile, measurements of carbon isotopes, specifically the abundance of carbon-13, offer clues about the broader cosmic history of the object. Carbon-13 is produced over successive generations of stars, so a low proportion of this isotope suggests that 3I/Atlas formed around a star from an era when the galaxy was younger and contained fewer heavy elements—conditions described as “low metallicity.” In astronomical terms, metals refer to all elements heavier than hydrogen and helium, which accumulate in the interstellar medium as stars evolve and die.

The combined isotopic evidence points to an origin around a star that existed roughly 10 to 12 billion years ago, far predating the 4.6-billion-year-old sun and its planetary system. This timing places the comet’s formation near the early phases of the Milky Way’s development, making it a rare “fossil” from a primordial planetary system.

Rosemary Dorsey, a researcher at the University of Helsinki, noted that 3I/Atlas offers an exceptional opportunity to study the composition of a system that formed long before the sun existed. As only the third confirmed interstellar visitor observed in detail, the comet’s analysis may help expand understanding of how planetary systems formed throughout cosmic history.