We present the first polarimetric observations of the third discovered interstellar object, 3I/ATLAS (C/2025 N1), obtained pre-perihelion with FORS2/VLT, ALFOSC/NOT, and FoReRo2/RCC, over a phase angle range of 7.7-22.4{\deg}. This marks the second ever polarimetric study of an interstellar object, the first distinguishing 2I/Borisov from most Solar System comets by its higher positive polarisation. Our polarimetric measurements as a function of phase angle reveal that 3I is characterised by an deep and narrow negative polarisation branch, reaching a minimum value of -2.7% at phase angle 7{\deg}, and an inversion angle of 17{\deg} -- a combination unprecedented among asteroids and comets, including 2I/Borisov. At very small phase angles, the extrapolated slope of the polarisation phase curve is consistent with that of certain small trans-Neptunian objects and Centaur Pholus, consistent with independent spectroscopic evidence for a red, possibly water-ice-bearing object. Imaging confirms a diffuse coma present from our earliest observations, though no strong polarimetric features are spatial resolved. These findings may demonstrate that 3I represents a distinct type of comet, expanding the diversity of known interstellar bodies.
@article{arxiv.2509.05181,
title = {Extreme Negative Polarisation of New Interstellar Comet 3I/ATLAS},
author = {Zuri Gray and Stefano Bagnulo and Galin Borisov and Yuna G. Kwon and Alberto Cellino and Ludmilla Kolokolova and Rosemary C. Dorsey and Grigori Fedorets and Mikael Granvik and Eric MacLennan and Olga Muñoz and Philippe Bendjoya and Maxime Devogèle and Simone Ieva and Antti Penttilä and Karri Muinonen},
journal= {arXiv preprint arXiv:2509.05181},
year = {2025}
}