Transfer of Polarized Light in an (Exo)planetary Atmosphere-Ocean System

Abstract

Fresnel reflection of starlight in the oceans of exoplanets results in two main phenomena: (1) a glint in the exoplanetary ocean, whose size increases with wind speed, and (2) a maximum degree of polarization at the Brewster angle. The corresponding features in the planetary phase curve, i.e. the flux and degree of polarization of the reflected light by the exoplanet orbiting its parent star, have been studied before. However, these studies did not use (1) horizontally inhomogeneous models, allowing the analysis of the glint appearing and hiding behind patchy clouds and (2) a set of consecutive wavelengths together with (3) a gas layer on top of the clouds. We present the phase curves of the (polarized) flux and degree of polarization of oceanic exoplanets with patchy cloud covers and a substellar cloud for tidally-locked planets. We discuss the spectropolarimetric signatures that could potentially be observed in the near future or that may be considered in the design of future telescopes. We show that (1) a previously suggested method to detect an ocean on an exoplanet, based on the shift of the peak value of the degree of polarization towards the Brewster angle, is sensitive to atmospheric surface pressure, (2) the clouds are not necessarily a limiting factor for detecting an ocean when a gas layer on top of the cloud is considered and a set of wavelengths is used, (3) the phase curves of the different wavelengths intersect in one point whose location could help estimating the cloud fraction and that (4) such an intersection in the polarized flux could be observed in the presence of an ocean for surface pressures up to 10 bar and cloud fractions up to 95%, while it never occurs in the absence of an ocean. We conclude that this color reversal in the polarized flux could potentially be used for detecting oceans on exoplanets.