Dust Properties of the Interstellar Object 3I/ATLAS Revealed by Optical and Near-Infrared Polarimetry

TL;DR

The study leverages independent optical and near-infrared polarimetry of the interstellar object 3I/ATLAS to construct its polarization phase curves (PPC) and polarization color curves (PCC) across 0.64–2.25 μm. By fitting the PPC with a reparameterized linear–exponential form and analyzing the PCC, the authors infer that the observed dust is largely refractory and organized into aggregates of submicron monomers, with a polarization maximum near 1.5–2.0 μm. The PPC remains stable across the water snow line and between inbound and outbound orbital phases, indicating intrinsic dust properties rather than transient volatile activity. The results suggest 3I/ATLAS retains polarimetric characteristics of a primitive cometary planetesimal formed in another planetary system, sharing a universal dust-building-block size scale but differing in composition from typical Solar System comets. This informs our understanding of planetesimal formation diversity and the nature of dust in exoplanetary environments.

Abstract

We present independent polarimetric observations of the interstellar object 3I/ATLAS, including the first near-infrared polarimetric measurements. Using imaging polarimeters, we measured the degree of linear polarization from the visible RC band (0.64 μm) to the near-infrared KS band (2.25 μm), and investigated its dependence on solar phase angle (polarization phase curve; PPC) and wavelength (polarization color curve; PCC). We confirm that the PPC of 3I/ATLAS differs significantly from those of typical Solar System comets, showing an unusually large polarization amplitude. This PPC shows no significant change in the RC band across perihelion passage, despite the perihelion lying within the water snow line. This indicates that the unusual polarimetric behavior of 3I/ATLAS is unlikely to be driven by transient volatile activity, but instead reflects intrinsic optical properties of refractory dust particles. The PCC increases with wavelength over 0.6-1.2 μm and peaks at 1.5-2.0 μm, suggesting that the dominant scattering units are dust aggregates composed of submicron-sized monomers, broadly consistent with interstellar dust and solar-system cometary aggregates. Taken together, our results indicate that 3I/ATLAS preserves polarimetric properties characteristic of a primitive cometary planetesimal formed in another planetary system, with a refractory dust composition that differs from that typically observed among Solar System comets, despite sharing a similar size scale of the aggregate building blocks.

Figures (4)

• Figure 1: Polarization degree ($P_\mathrm{r}$) and polarization position angle ($\theta_\mathrm{r}$) of 3I/ATLAS as a function of phase angle, measured in five bands ($R_\mathrm{C}$, $I_\mathrm{C}$, $J$, $H$, and $K_\mathrm{s}$). Colored symbols denote our visible and near-infrared imaging polarimetric data obtained with different instruments, while orange symbols show previously published measurements of 3I/ATLAS 2025ApJ...992L..29G2025RNAAS...9..338Z. Gray symbols represent polarimetric measurements of Solar System comets compiled in the Database of Comet Polarimetry (DBCP; Kiselev2017), and the pink triangles indicate the PPC of the interstellar comet 2I/Borisov 2021NatCo..12.1797B. The dashed curves show the best-fit linear--exponential models (Equation \ref{['eq:pr']}) to our data.
• Figure 2: Spectral dependence of the polarization degree of 3I/ATLAS at $\alpha = 23.4\degr$ and $30.5\degr$, compared with those of comet C/Hale--Bopp, C/Tsuchinshan--ATLAS, and 1P/Halley. For the comparison comets, polarization degrees in each band are taken from the literature at similar phase angles and, when necessary, are estimated by extrapolation (Appendix B; Kikuchi1987Kiselev20172025ApJ...983L..19L). The dashed and solid curves show Chebyshev polynomial fits.
• Figure 3: PPC of C/Hale-Bopp within each filter
• Figure 4: PPC of C/Tsuchinshan-ATLAS within each filter