Seminar Series

Direct Detection of Spatially Unresolved Exoplanets with Polarimetry

Sloane Wiktorowicz, UC Santa Cruz, Sagan Fellow

19 Sep 2013 - 15:30

The detection of scattered light from exoplanets gives direct access to the structure and composition of their atmospheres. Currently, most spatially unresolved, scattered light experiments focus on nearly edge-on, transiting systems. The temporal changes that occur during planetary occultations are used to suppress systematic errors that would otherwise overwhelm the planetary signal. However, linear polarimetry also has the potential to detect scattered light from exoplanets. This is because the polarization state of light scattered by a planetary atmosphere distinguishes it from both the direct light from the host star and the Wien tail of thermal re-radiation from the planet. Scattered flux should be identifiable regardless of orbital inclination, because both degree and position angle of polarization are modulated continuously throughout the orbit. Orbital inclination, mean number of scattering events, and scattering particle size and index of refraction are discernable with polarimetry. The discovery of spherical droplets of sulfuric acid suspended in the Venusian atmosphere was made with multi-wavelength polarimetry 40 years ago, and technology has matured to the point where such discoveries are now possible for spatially unresolved exoplanets. We will report on the search for scattered light from known exoplanets in B band using the POLISH2 polarimeter on the Lick 3-m telescope. This instrument simultaneously measures all four Stokes parameters (I, Q, U, and V), and it achieves precision within 2.0 times the photon shot noise limit over an entire observing run. The POLISH2 polarimeter is therefore ideally suited for direct detection of spatially unresolved exoplanets. This work is supported by a NExScI Sagan Fellowship, the NASA Origins Program, the UC Lab Fees Research Program, and UCO/Lick Observatory.

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