Scattered Light Polarimetry of Close-In Exoplanets

<p>Exoplanet science is at a crossroads. Our dominant characterization technique, transmission spectroscopy, is not ideal for the known planets. Due to calibration constraints, transiting exoplanets around bright stars are required, rendering the bulk of radial velocity-discovered planets unsuitable for study. Furthermore, clouds and hazes, common around close-in exoplanets, reduce the amplitude of spectral features in transmission spectra. This currently hampers study with Hubble, and it will require careful target selection for Webb. Thus, our choice of techniques employed to study atmospheric diversity is, in fact, frustrated by the existence of atmospheric diversity itself.</p> <p>Polarimetry, however, excels at the study of clouds and hazes, because it is sensitive to the size, shape, and index of refraction of scattering particles. Polarimetric observations obtained a century ago, coupled with Mie scattering codes 50 years later, enabled the discovery of concentrated, spherical, 1.1 µm-sized sulfuric acid cloud particles in Venus’ atmosphere. Pioneer 11 observations of Jupiter and Saturn revealed polar hazes, while Pioneer 11 and Cassini Huygens observations of Titan discovered 400 nm haze aggregates composed of 40 nm spherical monomers.</p> <p>The polarimetric signatures of exoplanets will expose the properties of atmospheric scattering particles. While host stars contaminate system polarization and reduce the aperture-integrated polarization amplitude of an orbiting exoplanet to the 10<sup>-5</sup> to 10<sup>-6</sup> level, polarimetry’s differential nature enables ground-based observations to be calibrated to this level. Using our POLISH2 polarimeter, we demonstrate control of systematics to this level, which places the most favorable hot Jupiters in the realm of detectability. We report on the status of a survey of cloudy and clear exoplanets with the POLISH2 polarimeter at the Gemini North 8-m and Lick Observatory 3-m telescopes.</p>