Interstellar

Extreme Negative Polarisation of New Interstellar Comet 3I/ATLAS

By Keith Cowing

Press Release

September 9, 2025

Filed under 3I/ATLAS, astrochemistry, astrogeology, Centaur Pholus, Comet, icyworld, Interstellar, Spectroscopy, trans-Neptunian object

Extreme Negative Polarisation of New Interstellar Comet 3I/ATLAS — Deep imaging (top) and polarimetric (bottom) maps of 3I from a subset of VLT observations. The colour scale in the imaging maps does not reflect the absolute brightness of the comet. We display isophotes in the last three epochs only due to the large number of background stars in the first images. The colour of each pixel in the polarimetric maps represents the value of polarisation, as shown in the scale to the right. Pixels with values outside this colour scale are given in white and are considered noise. The arrows indicate the directions towards the celestial north and east, as well as the Sun-comet direction and velocity direction of the comet (which mostly overlap) projected onto the sky. X marks the comet photocentre. — astro-ph.EP

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,

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

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