Reflected light from 3D exoplanetary atmospheres and simulation of HD 209458b

TL;DR

This paper models 3D exoplanetary atmospheres to show they can significantly reduce reflected light levels, explaining the low albedos observed in some exoplanets, and highlights the importance of 3D structure in atmospheric reflectivity.

Contribution

First study to analyze how 3D atmospheric structures affect reflected light and albedo in exoplanets, providing insights into their observed darkness.

Findings

Reflected light can be over 50% lower in 3D atmospheres compared to smooth models.

3D atmospheres allow deeper starlight penetration, reducing scattering and increasing absorption.

Geometric albedos can reach up to 0.45 in some cases, but are generally much lower.

Abstract

We present radiation transfer models that demonstrate that reflected light levels from three dimensional (3D) exoplanetary atmospheres can be more than 50% lower than those predicted by models of homogeneous or smooth atmospheres. Compared to smooth models, 3D atmospheres enable starlight to penetrate to larger depths resulting in a decreased probability for the photons to scatter back out of the atmosphere before being absorbed. The increased depth of penetration of starlight in a 3D medium is a well known result from theoretical studies of molecular clouds and planetary atmospheres. For the first time we study the reflectivity of 3D atmospheres as a possible explanation for the apparent low geometric albedos inferred for extrasolar planetary atmospheres. Our models indicate that 3D atmospheric structure may be an important contributing factor to the non-detections of scattered light from exoplanetary atmospheres. We investigate the self-shadowing radiation transfer effects of patchy cloud cover in 3D scattered light simulations of the atmosphere of HD209458b. We find that, for a generic planet, geometric albedos can be as high as 0.45 in some limited situations, but that in general the geometric albedo is much lower. We conclude with some explanations on why extrasolar planets are likely dark at optical wavelengths.