A Comet From Another Star Arrived With an Accent

The ice in comet 3I/ATLAS has an accent, and it is definitely not from around here. When the sun warmed it, its carbon and nitrogen came out in proportions far from the recipe our own comets follow.

3I/ATLAS is only the third object ever caught drifting through our solar system from another star. The ATLAS survey spotted it in July 2025, already shedding gas about 465 million miles (750 million kilometers) from the sun. It was brighter than the two visitors before it, bright enough to read its isotopes.

Isotopes are versions of an element with slightly different weights. Their ratios can shift with the cold and radiation where a comet formed, so they hint at its birthplace. Astronomers led by C. Opitom aimed the European Southern Observatory's Very Large Telescope at the comet in December 2025 and teased faint isotope signals out of the glow of CN, a simple carbon-nitrogen molecule.

Comets born here hold one heavy nitrogen atom for every 150 or so light ones. 3I/ATLAS holds far fewer heavy ones, close to levels seen in the gas between stars. Its carbon leans light too, beyond both our comets and nearby interstellar gas. James Webb Space Telescope readings of its carbon dioxide and carbon monoxide showed carbon just as light.

The team says the pattern could fit a comet that formed far out in the disk of an older star poor in heavy elements. The Milky Way makes more heavy carbon as its chemistry matures, so carbon this light fits an older star.

The nitrogen can't name a home address. And the measurements can't settle whether the gas rose from deep, untouched ice or a surface cooked by billions of years of cosmic rays.

It spent ages crossing the dark between stars. It never lost the accent.

Read the full story at Nature Astronomy, July 6, 2026

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