Cloudy Atmosphere of the Extra-solar Planet HD189733b : A Possible Explanation of the Detected B-band Polarization

The peak amplitude of linear polarization detected recently from an extrasolar 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 $H_2$ 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 polarization 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 ${16}^o\pm 6$ which interestingly coincides with the observationally inferred location of the peak hemisphere-integrated brightness.