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8h ago

VLT Confirms Ancient, Non‑Solar Chemistry in Interstellar Comet 3I/ATLAS

New isotope measurements point to formation in the cold outer disk of an ancient low‑metallicity star, reshaping ideas about planetesimal diversity across the galaxy.

This image of interstellar comet 3I/ATLAS was taken on 18 January 2026 with the FORS2 instrument on ESO’s Very Large Telescope (VLT). It is a stack of several images spanning 14 minutes. As the comet moves on the sky, the stars appear as trails in the background.
This image shows part of the spectrum of the interstellar comet 3I/ATLAS, captured in December 2025 with the UVES instrument on ESO’s Very Large Telescope. Image credit: ESO / C. Opitom / Manfroid et al. / O. Hainaut.

Overview

  • A team using ESO’s Very Large Telescope published results in Nature Astronomy on July 6 that measured unusually high carbon and nitrogen isotope ratios in 3I/ATLAS’s coma, confirming the object’s non‑Solar composition.
  • The VLT/UVES data give a carbon‑12/carbon‑13 ratio near 151 and a nitrogen‑14/nitrogen‑15 ratio near 363, values far above those typical of Solar System comets and the local interstellar medium.
  • Independent measurements from JWST and ALMA reported earlier corroborate the VLT findings by showing elevated carbon fractions, extreme deuterium enrichment, and unusual volatile abundances such as high CO, CO2 and methanol.
  • Researchers interpret the combined isotopic and chemical evidence as indicating formation in the cold, outer regions of a low‑metallicity star system and cite model‑dependent age estimates of roughly 10–12 billion years for the comet’s source material.
  • 3I/ATLAS is now receding and fading so new observations are ending, and teams say future facilities like ESO’s ELT and the Rubin Observatory will be needed to study fainter interstellar visitors and test ejection scenarios such as planet scattering or stellar encounters.