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Add as preferred source This image, provided by NASA, shows the interstellar comet 3I/Atlas captured by the Hubble Space Telescope on Nov. 30, 2025, about 178 million miles (286 million kilometers) from Earth. (NASA, ESA, STScI, D. Jewitt (UCLA), M.-T. Hui (Shanghai Astronomical Observatory), J. DePasquale (STScI) via AP, File) The comet that rambled past us from another star last year likely originated in a cold, isolated corner of the galaxy that had yet to gel into its own solar system, astronomers reported Thursday.

Comet 3I/Atlas is only the third interstellar visitor to be confirmed and quite possibly the oldest. Scientists estimate it could be up to 11 billion years old, more than twice as old as the sun.

A team led by the University of Michigan used the ALMA observatory in Chile's Atacama Desert to examine the comet last fall. The errant but harmless iceball was discovered last summer, giving NASA and the European Space Agency plenty of time to aim multiple space telescopes at it as it zoomed past Mars in October and made its closest approach to Earth in December. It's now well past Jupiter on its way out of our solar system for good, still visible only to the professionals.

In the study, scientists said they detected extremely high amounts of deuterium, or heavy hydrogen, in the comet's water. That suggests that the comet originated in a place considerably colder—before the star of this solar system even formed—than our own cosmic neighborhood, said the University of Michigan's Teresa Paneque-Carreno.

While our sun may have been surrounded by other newborn stars as it was forming, she noted, this comet's home star could have been more of a loner, leading to less heating and colder conditions.

The comet's precise place of origin is still unknown. Observations by the Hubble Space Telescope put the size of its nucleus somewhere between a quarter-mile and 3.5 miles (440 meters and 5.6 kilometers). It's hurtling away at 137,000 mph (220,000 kph).

Linking all these "puzzle pieces together may give an idea to how the planet-forming conditions were at these early times," Paneque-Carreno said in an email.

The first known interstellar object to stray into our celestial backyard—Oumuamua—was discovered by a telescope in Hawaii in 2017. Comet 2I/Borisov followed in 2019, named for the Crimean amateur astronomer who first spotted it.

Luis E. Salazar Manzano et al, Water D/H in 3I/ATLAS as a probe of formation conditions in another planetary system, Nature Astronomy (2026). DOI: 10.1038/s41550-026-02850-5

Journal information: Nature Astronomy

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Comet 3I/Atlas, the third confirmed interstellar comet, likely originated in a cold, isolated region of the Milky Way, predating the formation of its home star and possibly the Sun. High deuterium levels in its water indicate formation under extremely cold conditions. The comet is estimated to be up to 11 billion years old, with its precise origin still undetermined.

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