First detection of polarized scattered light from an exoplanetary atmosphere

We report the first direct detection of an exoplanet in polarized scattered light. The transiting planet HD189733b is one of the very hot Jupiters with shortest periods and, thus, smallest orbits, which makes them ideal candidates for polarimetric detections. We obtained polarimetric measurements of HD189733 in the $B$ band well distributed over the orbital period and detected two polarization maxima near planetary elongations with the peak amplitude of $\sim2\cdot10^{-4}$. Assuming Rayleigh scattering, we estimated the effective size of the scattering atmosphere (Lambert sphere) to be 1.5$\pm$0.2 $R_{\rm J}$, which is 30% larger than the radius of the opaque body previously inferred from transits. If the scattering matter fills the planetary Roche lobe, the lower limit of the geometrical albedo can be estimated as 0.14. The phase dependence of polarization indicates that the planetary orbit is oriented almost in the north-south direction with the longitude of the ascending node $\Omega$=(16\degr or 196\degr)$\pm$8\degr. We obtain independent estimates of the orbit inclination $i$=98\degr$\pm$8\degr and eccentricity $e$=0.0 (with the uncertainty of 0.05) which are in excellent agreement with values determined previously from transits and radial velocities. Our findings clearly demonstrate the power of polarimetry and open a new dimension in exploring exoplanetary atmospheres even for systems without transits.