Free Astronomy Magazine September-October 2026

47 SEPTEMBER-OCTOBER 2026 ASTRO PUBLISHING T his image shows part of the spectrum of interstellar comet 3I/ATLAS, captured between 6 and 26 December 2025 with the UVES instrument on ESO’s Very Large Telescope (VLT). Using UVES, astronomers studied the spectral signatures of cyanide, a molecule comprised of a carbon atom and a nitrogen one. More pre- cisely, they looked at their isotopic ra- tios: the relative amounts of different forms of the same atoms. These ratios are sensitive to the conditions under which 3I/ATLAS formed, and are not expected to change much as the comet travels through space. The spectrum shown here contains spectral features produced by 12 C, an isotope of carbon with 6 protons and 6 neutrons, and 13 C, which has 7 neutrons instead. These features are very faint, but as- tronomers know the exact wavelength regions where to look for them in the spectrum of the comet. Adding to- gether several of these wavelength re- gions averages out the noise, making the real features pop out. The team performed similar measurements with two isotopes of nitrogen, 14 N and 15 N. By comparing the 12 C/ 13 C and 14 N/ 15 N ratios with those measured in Solar System comets and in the discs of material around young stars, the team concluded that 3I/ATLAS likely formed in the outskirts of the disc around a star older than the Sun. [ESO/C. Opitom, J. Manfroid et al. Comet image: O. Hainaut] ! second was too faint — this was not the case for 3I/ATLAS. Thanks to the object’s unprecedented brightness, Opitom, Manfroid, Hutsemékers and their team were able to measure the comet’s isotopic ratios: the relative amounts of different forms of the same element. Using the UVES instrument on ESO’s VLT, the team measured ratios of carbon and nitrogen isotopes in cyanide molecules present in the gas around the comet. These ratios are known to be a good indicator of a comet’s origin, as they are very sen- sitive to the physical conditions in the formation environment and are not expected to change much as the comet travels on through space. “Unlike comets from our Solar Sys- tem, this interstellar visitor carries unusually high carbon and nitrogen isotopic ratios,” explains Aravind Kr- ishnakumar, a researcher at the Uni- versity of Liège and co-author on the new study. A similar study led by Martin Cordiner at the NASA God- dard Space Flight Center, that was published at the end of June in Na- ture , found a similar isotopic ratio of carbon, as well as elevated levels of deuterium, also called heavy hydro- gen. The study used data from the James Webb Space Telescope, a joint project of the US, European and Canadian space agencies. Overall, the findings by Opitom’s team indicate that the comet likely formed in the outer regions around an old, ‘low-metallicity’ star. A low- metallicity star is one with few ele- ments heavier than helium in its composition, that is thought to have formed when the Universe was much younger — and less chemically rich — than it is now. The team sus- pects that 3I/ATLAS therefore origi- nated around a star much older than the Sun. “3I/ATLAS is a really excit- ing opportunity to probe the com- position of another planetary sys- tem, one that formed long before our Sun and Solar System even ex- isted,” says co-author Rosemary Dorsey, a researcher at the Univer- sity of Helsinki, Finland. Evidence from the studies by the different teams points to 3I/ATLAS being more than twice as old as the Sun. As 3I/ATLAS moves away from the Sun and gets progressively fainter, its observations at the VLT are also nearing their end. ESO’s upcoming Extremely Large Telescope (ELT) will allow similar measurements for fu- ture interstellar objects, including those less bright than 3I/ATLAS. “The field of interstellar objects is still very new, and we do not really know what to expect. Every time a new one is discovered, we have new surprises,” Opitom concludes.