Cloudy atmosphere of the extra-solar planet HD189733b: A possible explanation of the detected B-band polarization

Abstract

The peak amplitude of linear polarization detected recently from an extraso- lar hot giant planet HD 189733b, is a few times of 10−4 , more than an order of magnitude higher than all theoretical predictions. Rayleigh scattering off H2 and He may although give rise to a planet-star flux ratio of the order of 10−4 in the blue band, it cannot account for the high polarization unless the planet has an unusually extended atmosphere. Therefore, it is suggested that the high polariza- tion should be attributed to the presence of a thin cloud of sub-micron size dust grains in the upper visible atmosphere which supports the observational finding of an almost feature-less transmission spectrum in the optical with no indication of the expected alkaline absorption features. It is found that the polarimetry observation allows for a small eccentricity of the orbit that is predicted from the time delay of the secondary eclipse of the planet. The estimated longitude of the ascending node is 16o ± 6 which interestingly coincides with the observationally inferred location of the peak hemisphere-integrated brightness.