Astronomers have identified 3I/ATLAS, a mysterious interstellar comet hurtling through our solar system, as potentially the oldest comet ever observed. Detected on July 1, 2025, by the NASA-funded ATLAS survey telescope in Chile, this cosmic visitor is estimated to be over 7 billion years old—predating our own 4.5-billion-year-old solar system by billions of years. Unlike typical comets that originate within our solar system, 3I/ATLAS hails from the Milky Way’s ancient “thick disk,” offering a rare glimpse into the galaxy’s distant past. As scientists race to study its water-ice-rich composition and vivid coma and tail, 3I/ATLAS promises to unlock secrets about the formation of stars and planets across the cosmos.
Discovery and Characteristics 🚀
3I/ATLAS was first spotted on July 1, 2025, by the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey telescope in Chile.
Unlike 1I/'Oumuamua, which was thought to be more asteroid-like, 3I/ATLAS has displayed clear signs of cometary activity, including a faint coma (a fuzzy cloud of gas and dust) and a short tail. This confirms its nature as a comet, actively releasing volatile materials as it approaches the Sun. Early observations also suggest that 3I/ATLAS might be larger than its interstellar predecessors, with an estimated nucleus diameter potentially ranging up to 24 kilometers.
Why is it considered so old? 🕰️
The primary reason astronomers believe 3I/ATLAS is exceptionally old stems from its trajectory and origin within the Milky Way galaxy. Researchers, including Matthew Hopkins from the University of Oxford, presented their findings at the Royal Astronomical Society's National Astronomy Meeting 2025.
Their analysis suggests that 3I/ATLAS originated from the "thick disk" of the Milky Way.
The statistical method used to estimate its age, which correlates the ages and velocities of objects with their vertical motion out of the galactic plane, indicates a high probability (around two-thirds chance) that 3I/ATLAS is indeed older than our Solar System.
Future Observations and Significance 🔭
3I/ATLAS will make its closest approach to the Sun around October 30, 2025, at a distance just inside the orbit of Mars.
Astronomers worldwide are racing to study 3I/ATLAS as it provides an unprecedented opportunity to analyze a celestial body that likely formed in a vastly different environment and era of the galaxy. Its composition, particularly its water ice abundance and elemental makeup, will be key to verifying its thick-disk origin and understanding galactic chemistry and the origins of planetary systems beyond our own. The discovery of 3I/ATLAS also suggests that future telescopes, like the Vera C. Rubin Observatory, may discover even more interstellar objects, potentially revealing between 5 and 50 such visitors in the coming years.