The third interstellar object passing through our Solar System has a cold and distant origin

Embargoed until: Publicly released: 2026-06-23 01:00

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International astronomers say they have potentially tracked the home of 3I/ATLAS, the third-known visitor to our Solar System from interstellar space. The team used some of our best telescopes to work out what isotopes (versions of atoms with the same number of protons but different numbers of neutrons) were present on the comet, and say 3I/ATLAS must have formed in a cold environment (close to –243 °C), and likely began its life 12 billion years ago in a primordial ancient planetary system.

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From: Springer Nature

A cold and distant origin for the interstellar object 3I/ATLAS

I/ATLAS, the third-known interstellar visitor to pass through our Solar System, may have formed in an ancient planetary system around 12 billion years ago. Isotopic evidence, presented in Nature this week, reveals that the object has a composition unlike anything else in the Solar System.

3I/ATLAS, discovered in July 2025, is the third confirmed interstellar object to pass through our inner Solar System. Despite detailed observations of this object, the precise age, origin, and path of 3I/ATLAS have been uncertain. Estimates of its age have ranged from 3–10 billion years old based on its velocity. Measurements of the ratio of isotopes (different versions of chemical elements) can offer clues about the physical and chemical conditions where 3I/ATLAS formed.

Martin Cordiner and colleagues report isotopic measurements of 3I/ATLAS obtained from observations from JWST and the ALMA observatory. Ratios of hydrogen isotopes provide insights into the temperature and radiation in the environment in which 3I/ATLAS formed, while carbon isotope ratios can help to locate the object’s home interstellar gas cloud. The water in 3I/ATLAS contains 10 times more deuterium (a hydrogen isotope) than other known comets, and the carbon ratios exceed typical values found in the Solar System, as well as nearby interstellar clouds and protoplanetary disks. The authors propose that the observed values indicate that 3I/ATLAS formed in a cold environment, less than 30 K (roughly –243 °C). The carbon composition implies that 3I/ATLAS may have formed 12 billion years ago, following a period of intense, early star formation in its host environment.

These findings suggest that 3I/ATLAS came from a primordial ancient planetary system, the authors conclude.

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conference: Nature

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Organisation/s: NASA Goddard Space Flight Center, Greenbelt, MD, USA

Funder: This work is based on observations made with the NASA/ESA/ CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. We acknowledge Matthew Hopkins for input on the Galactic (dynamical) origin of 3I/ATLAS. We gratefully acknowledge the assistance of optical observers who submitted astrometric observations of 3I/ATLAS in the weeks leading up to our observations, to help refine the ephemeris position. In particular, we thank J. Chatelain, , M. Frissell, E. Gomez, S. Greenstreet, W. Hoogendam, C. Holt, K. Meech, H. W. Lin, T. Lister, L. Salazar Manzano, T. Santana-Ros, D. Seligman, D. Singh, Q. Ye, Q. Zhang. Supporting astrometric observations were obtained by the Comet Chasers school outreach program (https://www.cometchasers.org/), led by Helen Usher, which is funded by the UK Science and Technology Facilities Council (via the DeepSpace2DeepImpact Project), the Open University and Cardiff University. It accesses the LCOGT telescopes through the Schools Observatory/Faulkes Telescope Project (TSO2025A-00 DFET-The Schools’ Observatory), which is partly funded by the Dill Faulkes Educational Trust, and through the LCO Global Sky Partners Programme (LCOEPO2023B-013). Observers included individuals and representatives from the following schools and clubs: E. Maciulis, A. Bankole, J. Bower, O. Roberts, participants on the British Astronomical Associations’ Work Experience project 2025 from The Coopers Company & Coborn School, Upminster, UK; St Marys Catholic Primary School, Bridgend, UK; J. M. Perez Redondo & Students: A. Matea, L. Guillamet, A. Montoy, and A. Martin from Institut d’Alcarr`as, Catalonia, Spain; Louis Cruis Astronomy Club, Brazil; Jelkovec High School, Zagreb, Croatia, and C. Wells at a British Astronomical Association event. ALMA is a partnership of ESO, NSF (USA), NINS (Japan), NRC (Canada), NSC and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The JAO is operated by ESO, AUI/NRAO and NAOJ. The NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.. This research has made use of NASA’s Astrophysics Data System Bibliographic Services. This research has made use of data and/or services provided by the International Astronomical Union’s Minor Planet Center.

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