A recent study suggests that the interstellar comet 3I/ATLAS could be between 10 and 12 billion years old. This new insight came from analyzing its isotopic composition. 3I/ATLAS is special; it’s only the third comet we’ve recorded entering our solar system from outside. If this age estimate holds, it means 3I/ATLAS was formed just a few billion years after the Milky Way itself.
When astronomers discovered 3I/ATLAS in 2025, it was zooming through space at an astonishing 36 miles (58 kilometers) per second. This speed makes it the fastest comet we’ve ever observed, ahead of others like 1I/’Oumuamua and 2I/Borisov. The theory holds that older interstellar objects tend to travel faster because they’ve interacted with many stars and objects over billions of years. By studying its speed, experts Aster Taylor and Darryl Seligman placed the comet’s age between 3 billion and 11 billion years. However, a more recent study led by Martin Cordiner at NASA Goddard points towards the older end of that spectrum.
Using the James Webb Space Telescope, Cordiner’s team measured the isotopes of carbon in 3I/ATLAS. They found a significant amount of carbon-12 compared to carbon-13. In the universe, carbon-13 takes time to accumulate. A low level of carbon-13 suggests that 3I/ATLAS was formed long ago, before this isotope became prevalent.
Interestingly, 3I/ATLAS also showed an unusual level of deuterium in its water. Deuterium is a heavy form of hydrogen, and its presence indicates that 3I/ATLAS formed in very cold conditions, typical of early interstellar clouds. These insights suggest that this comet might be a remnant from one of the universe’s earliest planetary systems.
In examining the context of 3I/ATLAS’s origin, we can look at the Milky Way’s history. Shortly after it formed around 13 billion years ago, there was a significant burst of star formation. During this “starburst,” many stars evolved into red giants, shedding their outer layers and creating heavy elements like carbon-13. For 3I/ATLAS to have such a low carbon-13 ratio while still containing some heavier elements means it likely formed during this starburst phase, about 10-12 billion years ago.
3I/ATLAS not only holds clues to its own origins but may also provide insights into the formation of planets beyond our solar system. Cordiner notes that, like solar system comets, interstellar comets may mirror the building blocks needed for planetary systems. This makes 3I/ATLAS a unique object for understanding how planets may have formed under different cosmic conditions.
As astronomical technology advances, scientists are becoming increasingly adept at analyzing celestial objects. Before long, we may learn even more about ancient comets like 3I/ATLAS, unraveling the mysteries they carry from the depths of time. Meanwhile, studies like these deepen our understanding of the universe’s history and the processes that shape it.
Current findings await peer review but are accessible through a pre-print publication.
