Dominique Bockelée-Morvan
ABSTRACT
Giuseppe Piazzi discovered Ceres, opening a new chapter in our exploration of small bodies. In homage to this pioneering achievement, I am pleased to present a new class of objects whose study offers unique insights into the diversity of planetary systems throughout the Galaxy: interstellar comets.
The first interstellar object, 1I/’Oumuamua, was discovered in October 2017. There was no detection of coma gases and dust particles, but astrometric measurements suggested the presense of gravitational forces related to cometary activity. The second interstellar object, 2I/Borisov discovered in 2019, showed cometary activity and spectroscopic properties remarkably similar to comets of the solar system.
This talk will focuss on the interstellar comet 3I/ATLAS, discovered on a highly hyperbolic orbit on July 1, 2025, as it was at 5 au from the Sun. Subsequent studies revealed a rapid increase of its gaseous activity as it approached the Sun, making detailed investigations possible. It passed its perihelion on 29 October 29, 2025 and was observed worldwide from numerous observatories, including both ground-based facilities and space-based instruments and space missions. The nucleus ices of 3I/ATLAS exhibit compositional and isotopic properties distinct from those of Solar System comets, suggesting that its natal planetary system formed in a galactic environment different from our own.