Polarized reflected light from the exoplanet HD189733b: First multicolor observations and confirmation of detection

TL;DR

This study presents multicolor polarimetric measurements of exoplanet HD189733b, confirming polarization detection in the blue bands and revealing Rayleigh scattering dominance, thus demonstrating polarimetry's effectiveness in probing exoplanet atmospheres.

Contribution

First multicolor polarimetric observations of HD189733b confirming polarization detection and analyzing atmospheric scattering properties.

Findings

Polarization peaks in blue UB bands (~10^-4)

Rayleigh scattering dominates the polarization signal

Wavelength-dependent albedo similar to Neptune's atmosphere

Abstract

We report first multicolor polarimetric measurements (UBV bands) for the hot Jupiters HD189733b and confirm our previously reported detection of polarization in the B band (Berdyugina et al. 2008). The wavelength dependence of polarization indicates the dominance of Rayleigh scattering with a peak in the blue B and U bands of ~10^-4+/-10^-5 and at least a factor of two lower signal in the V band. The Rayleigh-like wavelength dependence, detected also in the transmitted light during transits, implies a rapid decrease of the polarization signal toward longer wavelengths. Therefore, the nondetection by Wiktorowicz (2009), based on a measurement integrated within a broad passband covering the V band and partly B and R bands, is inconclusive and consistent with our detection in B. We discuss possible sources of the polarization and demonstrate that effects of incomplete cancellation of stellar limb polarization due to starspots or tidal perturbations are negligible as compared to scattering polarization in the planetary atmosphere. We compare the observations with a Rayleigh-Lambert model and determine effective radii and geometrical albedos for different wavelengths. We find a close similarity of the wavelength dependent geometrical albedo with that of the Neptune atmosphere, which is known to be strongly influenced by Rayleigh and Raman scattering. Our result establishes polarimetry as a reliable means for directly studying exoplanetary atmospheres.