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 five broad effective temperature ranges, it is natural to establish five 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 two high-temperature classes: Class IV (900 K $\lesssim$ T$_{\rm{eff}}$ $\lesssim$ 1500 K) for which alkali metal absorption predominates, and Class V (T$_{\rm{eff}}$ $\gtrsim$ 1500 K and/or low surface gravity ($\lesssim$ 10$^3$ cm s$^{-2}$)) for which a high silicate layer shields a significant fraction of the incident radiation from alkali metal and molecular absorption. The resonance lines of sodium and potassium are expected to be salient features in the reflection spectra of Class III, IV, and V 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.Comment: revised LaTeX manuscript accepted to Ap.J.; also available at http://jupiter.as.arizona.edu/~burrows/paper

Spectroscopy of planetary atmospheres in our Galaxy

“…A typical value for solar-system planets is 0.3; other solar-system objects range from about 0.04 (comets) to 0.1-0.2 (asteroids and trans-neptunian objects), with some brighter objects like Venus (0.8) or Saturn's satellite Enceladus (close to 1). In the case of giant exoplanets, Sudarsky et al (2000) predicted albedos of about 0.3 for cold Jupiters (<150 K, NH 3 cloud, class I), about 0.3-0.8 for temperate Jupiters (150 K < T e < 350 K, H 2 O cloud, class II), about 0.1 for warm Jupiters (350 K < T e < 800 K, clear objects with metallic absorption, class III) and 0.02-0.03 for hot Jupiters (800-1500 K, clear objects, class IV). The lowest albedo inferred from observations is 0.025 for the hot-Jupiter TrES-2b (Kipping and Spiegel 2011) in agreement with these predictions, but also upper limits for HD209458b and HD189733b seem to be consistent (Rowe et al 2006).…”

“…CH 4 -After water, methane is one of the most important opacity sources in the layers of exoplanets (Sudarsky et al 2000;Swain et al 2008Swain et al , 2009aSwain et al , 2009b as well as in those of Brown Dwarfs (e.g. Allard et al 1997;Burgasser et al 2006;Nakajima et al 2004).…”

Hot Jupiter secondary eclipses measured byKepler

“…Hot Jupiters are dark, due to the alkali metal (Na and K) line absorption and possibly due to TiO and VO strong molecular absorption bands in the visible (e.g. Seager & Sasselov 2000, Marley et al 1999, Sudarsky et al 2000, 2003.…”

Science case for VLT-Planet Finder

“…in the visible, it depends on the planet radius rather than its mass, and it may be strongly polarized (see Schmid et al this volume, and Stam et al, 2004). Physical processes taking place in the atmosphere of irradiated planets are described in Barman et al (2001) and Sudarsky et al (2000Sudarsky et al ( , 2003. The spectral features of giant planets in the near-infrared (5 MJaged 500 My).…”