Albedo and Reflection Spectra of Extrasolar Giant Planets

Abstract

We generate theoretical albedo and reflection spectra for a full range of extrasolar giant planet (EGP) models, from Jovian to 51-Pegasi class objects. Our albedo modeling utilizes the latest atomic and molecular cross sections, a Mie theory treatment of extinction by condensates, a variety of particle size distributions, and an extension of the Feautrier radiative transfer method which allows for a general treatment of the scattering phase function. We find that due to qualitative similarities in the compositions and spectra of objects within each of four broad effective temperature ranges, it is natural to establish four representative EGP albedo classes: a ``Jovian'' class (T$_{\\rm eff} \\lesssim 150$ K; Class I) with tropospheric ammonia clouds, a ``water cloud'' class (T$_{\\rm eff} \\sim 250$ K; Class II) primarily affected by condensed H$_2$O, a ``clear'' class (T$_{\\rm eff} \\gtrsim 350$ K; Class III) which lacks clouds, and a high-temperature class (T$_{\\rm{eff}}$ $\\gtrsim$ 900 K; Class IV) for which alkali metal absorption predominates. The resonance lines of sodium and potassium are expected to be salient features in the reflection spectra of Class III and Class IV objects. We derive Bond albedos and effective temperatures for the full set of known EGPs and explore the possible effects of non-equilibrium condensed products of photolysis above or within principal cloud decks. As in Jupiter, such species can lower the UV/blue albedo substantially, even if present in relatively small mixing ratios.