2025ApJ...992L..29G

We present the first polarimetric observations of the third discovered interstellar object (ISO), 3I/ATLAS (C/2025 N1, or 3I), obtained preperihelion with FORS2 at the Very Large Telescope, ALFOSC at the Nordic Optical Telescope, and FoReRo2 at the 2 m Ritchey-Chrétien-Coudé telescope, over a phase angle range of 7$\mathop{.}\limits^{\unicode{x000b0}}$7-22$\mathop{.}\limits^{\unicode{x000b0}}$4. This marks the second-ever polarimetric study of an ISO, the first distinguishing 2I/Borisov from most solar system comets by its higher positive polarization. Our polarimetric measurements as a function of phase angle reveal that 3I is characterized by a deep and narrow negative polarization 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 polarization 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 spatially resolved. These findings may demonstrate that 3I represents a distinct type of comet, expanding the diversity of known interstellar bodies.