Scattered polarized radiation of extrasolar circumplanetary rings

Abstract

Aims. We have investigated the impact of circumplanetary rings consisting of spherical micrometer-sized particles on the net scattered light polarization of extrasolar gas giants. Methods. Using the three-dimensional Monte Carlo radiative transfer code POLARIS, we studied the impact of the macroscopic parameters that define the ring, such as its radius and inclination, and the chemical composition of the ring particles on the net scattered polarization. For the spherical ring particles, we applied the Mie scattering theory. We studied the flux and polarization of the scattered stellar radiation as a function of planetary phase angle and wavelength from the optical to the near-infrared. Results. For the chosen grain size distribution, the dust particles in the ring show strong forward scattering at the considered wavelengths. Thus, the reflected flux of the planet dominates the total reflected and polarized flux at small phase angles. However, the scattered and polarized flux of the ring increase at large phase angles and exceeds the total reflected planetary flux. For large rings that contain silicate particles, the total reflected flux is dominated by the radiation scattered by the dust in the ring at all phase angles. As a result, the orientation of polarization is parallel to the scattering plane at small phase angles. In contrast, for a ring that contains water ice particles, the orientation of polarization is parallel to the scattering plane at large phase angles. Depending on the ring inclination and orientation, the total reflected and polarized flux show a specific distribution as well. Large particles show a strong polarization at large phase angles compared to smaller particles. For a Jupiter-like atmosphere that contains methane and aerosols, methane absorption features are missing in the spectrum of a ringed planet. Conclusions. Scattering of the stellar radiation by dust in circumplanetary rings of extrasolar planets results in unique features in the phase-angle- and wavelength-dependent reflected and polarized net flux. Thus, exoplanet polarimetry provides the means to study not only the planetary atmosphere and surface, but also to identify the existence and constrain the properties of exoplanetary rings.