2008ApJ...683L.195S

The peak amplitude of linear polarization detected recently from an extrasolar hot giant planet, HD 189733b, is a fewx10^–4, more than an order of magnitude higher than all theoretical predictions. Although Rayleigh scattering off H₂ and He may 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 submicron size dust grains in the upper visible atmosphere which supports the observational finding of an almost featureless 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° ±6° which interestingly coincides with the observationally inferred location of the peak hemisphere-integrated brightness.