Extreme Negative Polarisation of New Interstellar Comet 3I/ATLAS

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°.

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°, and an inversion angle of 17° — 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.

Zuri Gray, Stefano Bagnulo, Galin Borisov, Yuna G. Kwon, Alberto Cellino, Ludmilla Kolokolova, Rosemary C. Dorsey, Grigori Fedorets, Mikael Granvik, Eric MacLennan, Olga Muñoz, Philippe Bendjoya, Maxime Devogèle, Simone Ieva, Antti Penttilä, Karri Muinonen

Comments: 2 Figures, 2 Tables, submitted to ApJL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2509.05181 [astro-ph.EP] (or arXiv:2509.05181v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2509.05181
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Submission history
From: Zuri Gray
[v1] Fri, 5 Sep 2025 15:28:18 UTC (805 KB)
https://arxiv.org/abs/2509.05181
Astrobiology,