  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 $16^o\pm6$ which interestingly coincides with the
observationally inferred location of the peak hemisphere-integrated brightness.
