3I/ATLAS Formed Before the Sun, Possibly 9 Billion Years Ago

Q: What secrets about ancient, alien star systems does the glowing plasma tail of comet 3I/ATLAS hide?

The plasma tail of 3I/ATLAS appears to preserve a chemical record of an extremely cold, distant, and likely very ancient alien planetary system. The clearest clue is the comet’s unusually high ratio of ionised molecular nitrogen to ionised carbon monoxide, measured with the WEAVE spectrograph on the William Herschel Telescope. Nitrogen is easily lost unless ices form in deep cold, so the strong N₂⁺ signal indicates that 3I/ATLAS likely formed below about 33 kelvin, roughly –240°C, far beyond its home star in the outer disk of its original system. The detection of five ions in the plasma tail — including N₂⁺, CO⁺, CO₂⁺, H₂O⁺ and CH⁺ — also gives the richest ion inventory yet for any interstellar object. That tail chemistry matches a wider pattern from other telescopes. VLT measurements found unusually high carbon-12/carbon-13 and nitrogen-14/nitrogen-15 ratios in cyanide gas, while JWST detected exceptionally deuterium-rich water and unusual carbon isotope ratios in CO and CO₂. Together, those signatures point to formation at temperatures near or below 30 kelvin in a low-metallicity environment unlike the young Solar System. The broader implication is that 3I/ATLAS may be a preserved fragment of an old planetary system, possibly 10 to 12 billion years old. Its tail is not just glowing gas; it is a readout of primordial chemistry, showing that volatile-rich icy bodies were forming in the frozen outskirts of ancient star systems early in the Milky Way’s history.

3 articles · Updated · Sky at Night Magazine · Sep 9

Carbon isotope measurements from Webb, ALMA and the Very Large Telescope indicate 3I/ATLAS formed around an old, low-metallicity star and may be up to twice the Sun’s age.
Those data, combined with the comet’s trajectory from an older region of the galaxy, point to formation in a very cold environment and show such stars could build planetary-system material.
The object became the third interstellar visitor ever identified after ATLAS spotted it in July 2025, but its unusual brightness let astronomers track it from July 2025 to March 2026 and gather far richer data than for earlier finds.
Webb and VLT observations also found a carbon-dioxide-rich coma and unusually strong nickel and iron signatures, adding to earlier evidence that 3I/ATLAS differs markedly from typical Solar System comets.
Rubin Observatory’s new survey could find about 1 interstellar object a year, potentially turning rare one-off detections into a population that astronomers can compare across star systems.