Bizarre Comet From Deep Space Defies Everything We Know About Icy Bodies
Astronomers studying a comet that originated outside our solar system discovered it exhibits light-scattering behavior never before seen in any asteroid or comet. The finding suggests the object's surface composition differs fundamentally from local comets, offering new clues about how planetary systems form in distant star systems.
Originaltitel: Extreme Negative Polarization of New Interstellar Comet 3I/ATLAS
<p>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 7:7–22:4. 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<sup>∘</sup>, and an inversion angle of 17<sup>∘</sup>—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.</p>