More than two hundred extrasolar planets were discovered by indirect methods of Doppler spectroscopy, photometric transits, microlensing, and pulsar timing. However, the direct detection of exoplanets, enabling a study of their physical properties, remains a challenge. Thus far atmospheres of transiting planets and only of one non-transiting planet have been detected. Polarimetry is a powerful technique for detecting directly starlight that is scattered in a planetary atmosphere and, thus, possesses the information on its geometry, chemistry, and thermodynamics.

The light scattered in the planetary atmosphere is linearly polarized perpendicular to the scattering plane. It is best characterized by the Stokes parameters q and u, normalized to the total flux. In general, when the planet revolves around the parent star, the scattering angle changes and the Stokes parameters vary. If the orbit is close to circular, two peaks per orbital period can be observed. The observed polarization variability is thus expected to exhibit the orbital period of the planet and reveal the inclination, eccentricity, and orientation of the orbit, and, if detected with high enough polarimetric accuracy, also the nature of scattering particles in the planetary atmosphere.

In December 2007 we reported the first direct detection of the light scattered in the atmosphere of an extrasolar planet (Berdyugina et al. 2007). We presented results of our polarimetric monitoring of the known exoplanet HD189733b in the B band. We interpret the data employing a model based on the Lambert sphere approximation and Rayleigh scattering and deduced the radius of the sphere and the orbit orientation on the sky plane (see Fig. 1). Our findings open new vast opportunities to explore the exoplanetary atmospheres as well as to determine radii and true masses, and hence densities, of non-transiting planets.

Figure 1.  The orbit of HD189733b as projected on the sky. Solid and dashed lines indicate parts of the orbit in front of and behind the sky plane, respectively. The orange circle in the center depicts the star and the blue circle on the orbit the planet. The reconstruction is made for i = 94° and W=196°.  Positive directions of Stokes q and u and orientations on the sky are also shown.

The inlet in the low left corner shows the data (dots) and the models (red line) running with the planetary orbital phase. The polarization maxima in Stokes q are observed near planetary elongations.