Astronomers debated whether newly detected interstellar visitor 3I/ATLAS was a rocky asteroid or an artificial solar sail; submillimeter radio telescopes detected massive clouds of frozen carbon monoxide boiling off its surface. Published in Nature, this chemical discovery confirms 3I/ATLAS as an ancient icy comet ejected from the freezing outer rim of an alien planetary system, giving humanity its first pristine chemical sample of a distant star's nursery.
When telescopes spotted 3I/ATLAS—only the third interstellar object ever discovered barreling through our solar system from deep space—astrophysicists raced to determine its true nature. Without direct physical samples, scientists argued whether it was a scorched fragment of an alien planet, a dead rocky asteroid, or a bizarre comet.
Using the ALMA radio observatory high in the Chilean desert, astronomers analyzed the faint chemical radio glow of gases boiling off the object. They discovered an overwhelming plume of carbon monoxide ice with almost no water vapor—proving the object was baked in a cryogenic deep freezer below minus 400 degrees Fahrenheit before being slingshotted into the galaxy by a giant exoplanet.
3I/ATLAS provides a physical window into other solar systems without interstellar spaceflight. By measuring the chemical recipes of alien planet formation, by comparing alien ice to our own solar system's comets, and by unlocking the physics of planetary ejection, interstellar comet astronomy opens a new cosmic frontier.
Isotopic evidence for a cold and distant origin of 3I/ATLAS
Interstellar objects provide the only directly observable samples of icy planetesimals formed around other stars, and can therefore provide insight into the diversity of physical and chemical conditions occurring during exoplanet formation1, 2–3. Here we report isotopic measurements of the interstellar comet 3I/ATLAS, which reveal an elemental composition unlike any Solar System body. The water in 3I/ATLAS is enriched in deuterium, at a level of D/H = (0.98 ± 0.06)%, which is more than an order of magnitude higher than in known comets, and its range of 12C/13C ratios (141–191 for CO2 and 123–172 for CO) exceeds typical values found in the Solar System, as well as nearby interstellar clouds and protoplanetary disks. Such extreme isotopic signatures indicate formation at temperatures ≲30 K in a relatively metal-poor environment. When interpreted with respect to models for Galactic chemical evolution, the carbon isotopic composition implies that 3I/ATLAS may have accreted as long ago as 12 billion years, following a period of intense, early star formation. 3I/ATLAS thus represents a preserved fragment of an ancient planetary system. Carbon isotopic measurements of the interstellar comet 3I/ATLAS reveal that it may have formed as long ago as 12 billion years and thus represents a preserved fragment of an ancient planetary system.
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