Interstellar Comet 3I/ATLAS Reveals Unexpected Water Activity Near Sun

The interstellar object 3I/ATLAS, a rare visitor from beyond our solar system, rapidly brightened and appeared bluer than the Sun as it approached perihelion on October 29, 2025, according to observations from various space-based instruments. This celestial interloper, only the third confirmed interstellar object ever observed, offers a unique window into the formation of planetary systems far from our own.

esa.int reported, Scientists have announced a "major breakthrough" with the detection of hydroxyl (OH) gas, a definitive chemical signature of water, escaping from the comet. This finding is particularly unexpected given the comet's distance from the Sun, challenging previous assumptions about interstellar cometary activity.

The detection was made possible by instruments aboard NASA's Neil Gehrels Swift Observatory, which observed a faint ultraviolet glow indicative of water vapor. Auburn University astronomer Dennis Bodewits and his colleagues stated that this discovery is crucial for understanding how interstellar comets evolve.

economictimes.com noted, The comet was losing approximately 40 kilograms of water per second, an unusually high rate for its distance from the Sun, as reported by Live Science on October 9, 2025. This suggests that sunlight is heating small icy grains ejected from the nucleus, causing them to vaporize.

This unexpected water activity allows astronomers to apply the same analytical methods used for solar system comets to 3I/ATLAS, facilitating direct comparisons of planetary system chemistry across the Milky Way Galaxy. Lead author Zexi Xing noted that each interstellar comet has presented surprises, rewriting understanding of planet and comet formation.

sci.news reported, 3I/ATLAS reached its closest point to the Sun, or perihelion, on October 29, 2025, at a distance of 1.36 astronomical units (AU), which is between the orbits of Earth and Mars. Although hidden from Earth-based telescopes during this period due to solar conjunction, space-based observatories continued their crucial monitoring.

The comet's journey through our solar system, first detected on July 1, 2025, by the ATLAS survey telescope in Chile, provides an unprecedented opportunity to study primordial material from another star system. Its hyperbolic orbit confirms its extrasolar origin, making it a valuable "note from another planetary system," as described by Professor Dennis Bodewits.

• esa.int noted, Rarity and Significance of Interstellar Objects: Interstellar objects are incredibly rare visitors, with 3I/ATLAS being only the third confirmed object of its kind to pass through our solar system, following 1I/'Oumuamua in 2017 and 2I/Borisov in 2019. These celestial nomads offer scientists a unique chance to directly sample material from distant planetary systems, providing insights into their chemical composition and physical properties that cannot be obtained through remote observations alone, as noted by researchers on August 18, 2025.

• Detection Methodology and Instruments: The detection of hydroxyl (OH) gas was primarily achieved using the Ultraviolet/Optical Telescope aboard NASA's Neil Gehrels Swift Observatory. This space-based telescope is uniquely capable of observing ultraviolet wavelengths that are largely absorbed by Earth's atmosphere, allowing for the identification of water vapor's faint UV echo even when the comet was far from the Sun. Other instruments, including NASA's Hubble and James Webb Space Telescopes, and ESA's SOHO, have also contributed to observations.

• economictimes.com reported, Unexpected Water Activity: What makes the water detection particularly significant is that it occurred when 3I/ATLAS was nearly three times farther from the Sun than Earth, a distance at which most solar system comets remain relatively inactive. This strong ultraviolet signal suggests that sunlight is heating small icy grains released from the comet's nucleus, causing them to vaporize and form the surrounding gas cloud, as explained by scientists on October 29, 2025.

• Implications for Planet Formation: The study of 3I/ATLAS's composition, particularly its water content, offers crucial insights into the formation of planets and comets in other star systems. Professor Dennis Bodewits of Auburn University highlighted that detecting water from an interstellar comet is like "reading a note from another planetary system," indicating that the chemical ingredients for life are not unique to our own solar system.

• sci.news noted, Comparison with Previous Interstellar Objects: Each interstellar object observed so far has presented unique characteristics. While 1I/'Oumuamua was dry and asteroid-like, and 2I/Borisov was rich in carbon monoxide, 3I/ATLAS is unexpectedly active with water at a significant distance from the Sun. This diversity among interstellar visitors is continually reshaping astronomers' understanding of how planets and comets form around other stars, as stated by lead author Zexi Xing.

• Compositional Details and Age: Beyond water, observations by the James Webb Space Telescope have indicated that 3I/ATLAS is unusually rich in carbon dioxide and also contains carbon monoxide and carbonyl sulfide. The Very Large Telescope also confirmed the presence of cyanide gas and a surprising amount of nickel. Scientists estimate 3I/ATLAS to be approximately 0.8 billion years old, originating from an older population in the thick Galactic disk.

• esa.int reported, Future Observations and Trajectory: After its perihelion on October 29, 2025, 3I/ATLAS will become observable from Earth again in mid-November and will make its closest approach to Earth on December 19, 2025, at a safe distance of 1.8 AU. Astronomers plan to utilize ground-based observations, as well as data from the Hubble and Webb space telescopes, to further characterize the comet in detail during this period.