A Groundbreaking Discovery in Polarimetric Observations

Astronomers have achieved a remarkable milestone with the first polarimetric observations of the newly discovered interstellar comet, 3I/ATLAS (C/2025 N1). This stellar phenomenon was scrutinized using advanced tools like FORS2/VLT, ALFOSC/NOT, and FoReRo2/RCC, revealing intriguing insights into its properties.

A Unique Polarisation Profile

The observations indicate that 3I/ATLAS features a deep and narrow negative polarisation branch, reaching a striking minimum of -2.7% at just 7 degrees phase angle. This is a groundbreaking finding—previous studies of interstellar objects, such as 2I/Borisov, showed higher positive polarisation values. The key to 3I lies in its unprecedented polarimetric signature, marking it distinctly among both asteroids and comets.

Hints of a Water-Ice Composition

At extremely small phase angles, the polarisation measurements align with certain small trans-Neptunian objects, hinting at the possibility that 3I/ATLAS may harbor water ice. This intriguing connection is further supported by independent spectroscopic evidence that suggests a reddish hue, characteristic of such icy bodies.

Imaging Insights and New Classifications

The imaging data has confirmed the presence of a diffuse coma surrounding the comet from the earliest observations. However, strong polarimetric features were not spatially resolved. These findings might indicate that 3I represents a new class of comet, expanding the catalog of known interstellar entities.

A Collaborative Achievement

This pioneering study is the result of collaborative efforts by an extensive team of astronomers, including Zuri Gray, Stefano Bagnulo, and Galin Borisov, among others. Their work opens exciting avenues for further research into the composition and characteristics of interstellar comets.