First detection of polarized scattered light from an exoplanetary atmosphere

TL;DR

This paper reports the first direct detection of polarized scattered light from an exoplanet's atmosphere, revealing atmospheric size, orbit orientation, and demonstrating polarimetry's potential in exoplanet studies.

Contribution

It presents the first direct polarization detection of an exoplanet, providing new insights into atmospheric properties and orbital parameters using polarimetric measurements.

Findings

Detected polarization maxima near planetary elongations

Estimated atmospheric size larger than transiting radius

Determined orbital inclination and orientation consistent with previous data

Abstract

We report the first direct detection of an exoplanet in the visible polarized 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 a 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 a north-south direction with a longitude of 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 an 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.