3I/ATLAS Discovery: Interstellar Comets Reveal Unique Water Chemistry Unlike Our Solar System

Interstellar comet 3I/ATLAS showcases unprecedented levels of water and carbon compounds, hinting that it originated around a star vastly different from our Sun—likely billions of years older.

Astronomers have meticulously monitored 3I/ATLAS since its entry into our solar system last year, revealing an astonishing composition rich in carbon dioxide and water. Initial analyses estimate its age at about 8 billion years—almost double that of our Sun.

According to Martin Cordiner and his research team at NASA’s Goddard Space Flight Center, the comet’s deuterium levels (a hydrogen isotope with additional neutrons) are at least ten times higher than any previously recorded comet.

Typically, deuterium exists in minor concentrations in Earth’s oceans, but 3I/ATLAS displays levels more than 40 times greater. “3I/ATLAS continues to amaze us with its revelations about the similarities and differences between its host system and our own,” Cordiner stated. His team utilized the James Webb Space Telescope for these groundbreaking observations.

“It’s truly extraordinary,” commented Paul Hartog from the Max Planck Institute for Solar System Research in Germany. “The ratio of deuterium to hydrogen in its water composition is highly unusual and unexpected.”

Such elevated deuterium levels are generally found in the coldest regions of the Milky Way, suggests Ewain van Dishus of the Leiden Observatory in the Netherlands. “This indicates it likely resides in the outermost disk of the star it orbits, facilitating its ejection,” Dishus explained.

Cordiner and colleagues also identified relatively low levels of carbon-13, an isotope usually created during supernova explosions. The low carbon-13 concentrations found in 3I/ATLAS point to its formation during a time when supernovae contaminants were rare, implying the comet formed around a star system aged between 10 to 12 billion years—again, more than twice the age of our solar system, according to Cordiner.

However, Dishus cautions that the imprecise carbon data means we cannot conclusively determine its exact age.