A comet that drifted between the stars for millions or even billions of years before entering our Solar System carries water with roughly 30 times more heavy hydrogen than a typical comet here, suggesting its ice formed in an extraordinarily cold alien planetary system.

In 2025, astronomers discovered 3I/ATLAS, only the third confirmed interstellar object. 1 year later, its heavy water revealed clues to a cold alien planetary system

In 2025, astronomers discovered 3I/ATLAS, only the third confirmed interstellar object. 1 year later, its heavy water revealed clues to a cold alien planetary system. ALMA and the James Webb Space Telescope found high deuterium levels in its water. The result suggests that its ice formed below about 30 kelvin and preserved chemical evidence from another planetary system for billions of years before entering the Solar System.

A comet that drifted between the stars for millions or even billions of years before entering our Solar System carries water with roughly 30 times more heavy hydrogen than a typical comet here, suggesting its ice formed in an extraordinarily cold alien planetary system.

For most of its existence, 3I/ATLAS was an invisible piece of ice moving between the stars. Then its hyperbolic path carried it through the Solar System, sunlight warmed its surface, and gas escaping from the nucleus gave astronomers a chemical sample of material made around another star. The strongest clue was not simply water, but the balance between two kinds of hydrogen inside it. A 2026 Nature Astronomy analysis found that the water in 3I/A…

Astronomers discovered a comet passing through our solar system in July 2025 carrying organic molecules formed in a distant star system—the third interstellar object ever confirmed—and evidence now suggests this ancient traveller may have spent billions of years wandering the galaxy before we spotted it.

Comet 3I/ATLAS gave astronomers something genuinely rare: a chance to identify organic molecules in material released by a confirmed visitor from another planetary system. The ATLAS survey telescope at Río Hurtado, Chile, reported the object on July 1, 2025. Its unusually fast, open orbit showed that it was passing through rather than circling the Sun. Observations made as sunlight warmed the comet later detected methanol, hydrogen cyanide and m…