Brightness and Polarization of Extrasolar Planet Oceans and Atmospheres

We have developed a model to determine if orbital variation in reflected starlight from extrasolar planets could eventually be used to detect surface oceans on Earth-like planets. We then used this model to simulate the orbital variation in brightness and polarization of light scattered by different types of exoplanets, including ocean planets. The hypothetical exoplanets we modeled are Earth-sized planets orbiting a Sun-like star at 1 AU; we assume orbits that are edge-on to our line of sight, because this geometry maximizes the water signatures we seek. The modeled planets have Earth-like atmospheres (including Rayleigh scattering, absorption, aerosols, and clouds) and surfaces that are either ocean-covered or diffusely scattering (similar to Earth deserts). We confirm earlier findings that ocean planets with thin atmospheres exhibit a brightness peak before and after transit (near orbital longitude = 30 degrees), and show a strong polarization peak near orbital longitude = 74 degrees. However, we also show that, on an exoplanet with Earth-like Rayleigh scattering, these signatures are almost completely hidden by the atmosphere when observing in the waveband 0.5 to 1.0 um. At longer wavelengths out to 2.3 um, Rayleigh scattering can be essentially eliminated, but aerosol scattering tends to obscure both the polarization signature and the brightness peak, unless aerosols are very thin by Earth standards. Clouds and wind-driven waves on an ocean surface can also mask water signatures, and atmospheric scattering on dry planets can create a polarization signature similar to that from an ocean planet. In summary, our model shows that detecting a water ocean on an Earth-like planet using polarized and unpolarized glint from the water surface may be more difficult than expected. However polarization adds another dimension which can be used, in combination with variations in orbital brightness and polarization, to seek oceans, atmospheric water aerosols, and water clouds on extrasolar planets. Additionally, the presence and direction of the polarization vector could be used to determine if an observed object is a planet or a background stair, and constrain the planet's orbital inclination.