KLD1
Keynote (SB): Topic: 3I/ATLAS, with a focus on composition and isotopic ratios
Co-organized by SB
Abstract
Comets are remnants of Solar System formation. The passage of a bright comet-like interstellar object through the Solar System had long been anticipated, as it offers a unique opportunity to investigate the chemical and isotopic properties of icy planetesimals formed in other planetary systems and, in turn, to constrain the diversity of planetary formation environments across the Galaxy. The interstellar comet 3I/ATLAS was discovered on 2025 July 1 on a highly hyperbolic orbit, when it was 5 au from the Sun. As it approached the Sun, its gaseous activity increased rapidly, enabling detailed observational studies. It was observed worldwide with numerous ground- based and space-based observatories, including several spacecraft. The comet reached perihelion on 2025 October 29. In this talk, I will present the main observational results, with particular emphasis on the chemical and isotopic composition of 3I/ATLAS. These measurements reveal significant differences from Solar System comets and suggest that 3I/ATLAS is a preserved fragment of an ancient planetary system that formed in a relatively metal-poor Galactic environment.