Wednesday, January 6, 2016

Relfected Light Curves of Jupiter and Saturn like Exoplanets

Reflected Light Curves, Spherical and Bond Albedos of Jupiter- and Saturn-like Exoplanets


Dyudina et al


We estimate how the light curve and total stellar heating of a planet depend on forward and backward scattering clouds. To do that, we construct light curves for Jupiter- and Saturn-like planet based on observations. We fit analytical functions to the reflected brightness of Jupiter's and Saturn's surface versus planet's phase. We use Pioneer and Cassini spacecraft images to estimate these functions. These observations cover broad bands at 0.59-0.72 microns and 0.39-0.5 microns, and narrow bands at 0.938 microns (atmospheric window), 0.889 microns (CH4 absorption band), and 0.24-0.28 microns. We simulate the images of the planets at different phases with ray-tracing model of a planet by Dyudina et al. (2005). The full-disk luminosity of these simulated images changes with planet's phase producing the full-orbit light curves. We also derive total planet's reflection integrated in all directions (spherical albedos) for Jupiter, Saturn, and for planets with Lambertian and Rayleigh-scattering atmosphere. For Jupiter, we tune the model to fit the observed full-disk brightness at several phase angles. Jupiter-like atmosphere can produce light curves that are a factor of two fainter at half-phase than the Lambertian planet, given the same geometric albedo at transit. The spherical albedo (and likely the wavelengh-integrated Bond albedo) is lower than for a Lambertian planet. Corresponding absorption of the stellar light and the planet's heating rate would be higher than for a grey Lambertian planet. Lambertian assumption can overestimate spherical albedo by up to a factor of ~ 1.5.

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