Circular polarization as a probe of cloud properties and asymmetries in giant exoplanet atmospheres

Polarimetry is a promising method for characterizing clouds in exoplanetary atmospheres. For planets in the Solar System, circular polarization measurements complement linear polarimetry by providing additional information on cloud particle properties. As the disk-integrated circular polarization is zero for symmetric planets, observing intrinsic circular polarization of spatially unresolved exoplanets requires stable spatial asymmetries such as circumplanetary rings. We investigated the potential of circular polarization measurements at optical and near-infrared wavelengths to determine optical properties of cloud particles in the atmospheres of giant exoplanets and characterize asymmetries. For 20 selected cloud condensates that cover a large range of refractive indices, we calculated optical properties assuming Mie scattering theory. The circular polarization of starlight scattered by cloudy exoplanets was calculated with Monte Carlo radiative transfer simulations. To explain the connection between optical properties and planetary circular polarization, we derived an interpretative model of the first two scattering orders. Planetary hemispheres with atmospheres including cloud particles with a large imaginary part, k, of the refractive index show distinct circular polarization phase curves dominated by scattering first by gaseous molecules and then by cloud particles. The intrinsic degree of circular polarization, P_ . When the cloud particles have a low k, they instead induce even smaller but more predictable circular polarization dominated by scattering solely by cloud particles. The predictability allows for a characterization of the underlying asymmetry. For example, this could be used to determine which half of the planet is covered by a circumplanetary ring. is at most 3⋅ 10^ -4 Circular polarization of starlight reflected by giant exoplanets is sensitive to cloud particle composition and large-scale asymmetries but remains a subtle signal. While promising for characterizing clouds under favorable conditions, practical detection requires technological advances in polarimetry and careful disentanglement from stellar background signals.