Optical Albedo Theory of Strongly-Irradiated Giant Planets: The Case of HD 209458b

TL;DR

This paper develops new optical albedo models for close-in giant exoplanets like HD 209458b, comparing them with observational limits and exploring how various atmospheric factors influence their reflective properties.

Contribution

It introduces a suite of albedo models considering different atmospheric conditions and assesses their consistency with observational data, highlighting the diagnostic potential of optical measurements.

Findings

Models without scattering clouds match optical limits.

Albedo behavior varies with wavelength, metallicity, and heat redistribution.

Optical measurements can reveal atmospheric composition and thermal inversions.

Abstract

We calculate a new suite of albedo models for close-in extrasolar giant planets and compare with the recent stringent upper limit for HD 209458b of Rowe et al. using MOST. We find that all models without scattering clouds are consistent with this optical limit. We explore the dependence on wavelength and waveband, metallicity, the degree of heat redistribution, and the possible presence of thermal inversions and find a rich diversity of behaviors. Measurements of transiting extrasolar giant planets (EGPs) at short wavelengths by MOST, Kepler, and CoRoT, as well as by proposed dedicated multi-band missions, can complement measurements in the near- and mid-IR using {\it Spitzer} and JWST. Collectively, such measurements can help determine metallicity, compositions, atmospheric temperatures, and the cause of thermal inversions (when they arise) for EGPs with a broad range of radii, masses, degrees of stellar insolation, and ages. With this paper, we reappraise and highlight the diagnostic potential of albedo measurements of hot EGPs shortward of $\sim$1.3 $\mu$m.