Probing the physical properties of directly imaged gas giant exoplanets through polarization

It has been becoming clear that the atmospheres of the young, self-luminous extrasolar giant planets imaged to date are dusty. Planets with dusty atmospheres may exhibit detectable amounts of linear polarization in the near-infrared, as has been observed from some field L dwarfs. The asymmetry required in the thermal radiation field to produce polarization may arise either from the rotation-induced oblateness or from surface inhomogeneities, such as partial cloudiness. While it is not possible at present to predict the extent to which atmospheric dynamics on a given planet may produce surface inhomogeneities substantial enough to produce net non-zero disc-integrated polarization, the contribution of rotation-induced oblateness can be estimated. Using a self-consistent, spatially homogeneous atmospheric model and a multiple scattering polarization formalism for this class of exoplanets, we show that polarization of the order of 1 per cent may arise due to the rotation-induced oblateness of the planets. The degree of polarization for cloudy planets should peak at the same wavelengths at which the planets are brightest in the near-infrared. The observed polarization may be even higher if surface inhomogeneities exist and play a significant role. Polarized radiation from self-luminous gas giant exoplanets, if detected, provides an additional tool to characterize these young planets and a new method to constrain their surface gravity and masses.