Ancient Interstellar Comet 3I/ATLAS Offers Clues to Early Milky Way
Recent observations from the James Webb Space Telescope (JWST) reveal that interstellar comet 3I/ATLAS may be between 10 and 12 billion years old, making it one of the oldest objects ever detected passing through our solar system. The comet, discovered speeding through the cosmic region around the Sun, initially sparked speculation about being an alien spacecraft, but astronomers have confirmed its origin lies outside our solar system.
Discovery and Trajectory of 3I/ATLAS
Comet 3I/ATLAS was first detected by the Asteroid Terrestrial-Impact Last Alert System (ATLAS) survey in Chile on July 1, 2025. NASA is tracking and studying this rare interstellar comet as it passes through our solar system. Unlike comets native to our solar system, 3I/ATLAS is traveling on a hyperbolic trajectory, meaning it’s a one-time visitor and won’t orbit the Sun. It reached its closest approach to the Sun on October 29, 2025, and came within approximately 270 million kilometers of Earth on December 19, 2025.
JWST Reveals Comet’s Ancient Origins
Analysis of the comet’s isotopic composition, conducted using JWST observations in December 2025, suggests it likely formed in a cold and distant region of the Milky Way between 10 and 12 billion years ago. James Webb Space Telescope observations revealed the presence of carbon dioxide (CO₂) and water (H₂O). This age estimate refines previous estimates that ranged from 3 to 11 billion years. According to Romain Maggiolo, a research scientist at the Royal Belgian Institute for Space Aeronomy, the isotopic composition of 3I/ATLAS is significantly different from that of comets within our solar system, indicating its formation in a different stellar environment and at a much earlier time in the galaxy’s history.
Unique Composition and Implications
The comet’s size is estimated to be between 440 meters and 5.6 kilometers in diameter, based on observations from the Hubble Space Telescope. Analysis of the gases released by 3I/ATLAS revealed a higher concentration of deuterium – a heavier isotope of hydrogen – and a greater carbon isotope ratio than typically found in our solar system. Maggiolo suggests that if the age estimate is accurate, the comet’s abundance of volatile molecules could indicate that rich prebiotic chemistry occurred in star-forming regions very early in the Milky Way’s history. The comet likely formed in a very cold environment, around -243 degrees Celsius, possibly within a dense protoplanetary disk.
Further Observations and Future Trajectory
Numerous space missions have observed 3I/ATLAS, including the SPHEREx Observatory, the Psyche spacecraft, the Lucy spacecraft, and the PUNCH mission. The MAVEN mission also detected hydrogen surrounding the comet, and the Mars Reconnaissance Orbiter (MRO) and the Perseverance rover captured images of the comet from Mars. Currently, 3I/ATLAS is moving away from the Sun and is expected to pass near Jupiter on March 15, 2026, before continuing its journey through the orbits of Saturn, Uranus, and Neptune in the coming years.
Josep Trigo-Rodríguez from the Institute of Space Sciences (CSIC-IEEC) notes that this research demonstrates interstellar comets are unique objects capable of providing samples from remote regions of the Milky Way galaxy.