Polarization reveals cloudy atmosphere of Exo-planet

The peak amplitude of linear polarization detected recently from an extrasolar hot giant planet, HD 189733b, is a few times 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 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.

 

Fig.: Best-fit models of the observed linear polarization at B band of
HD 189733b. The polarization of HD 189733b expressed as the normalized
and disk-integrated Stokes Q (a) and U (b) for circular orbit (blue)
and for elliptical orbit (red) with eccentricity 0.06 and the longitude
of pericenter at 89°. The observed data, rebinned for equal phase
intervals, are presented by error bars (green). Q and U are on the
scale of 10⁴ .

This article has recently appeared in ApJ Letters  683, Aug 20, 2008, L195 (S. Sengupta)

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