The dark days are overcast: Iron-bearing clouds on HD 209458 b and WASP-43 b can explain low dayside albedos

TL;DR

This study uses climate and cloud models to explain the low dayside albedos of hot Jupiters HD 209458 b and WASP-43 b, highlighting the role of Fe-bearing clouds in darkening their atmospheres.

Contribution

It demonstrates that Fe-bearing clouds can account for low albedos, offering an alternative to cloud-free models for hot Jupiter atmospheres.

Findings

WASP-43 b is predicted to be cloudy throughout its dayside.

HD 209458 b has a clear upper atmosphere at the hot sub-solar point.

Low geometric albedos (~0.026-0.028) match observations.

Abstract

We simulate the geometric albedo spectra of hot Jupiter exoplanets HD 209458 b and WASP-43 b, based on global climate model (GCMs) post-processed with kinetic cloud models. We predict WASP-43 b to be cloudy throughout its dayside, while HD 209458 b has a clear upper atmosphere around the hot sub-solar point, largely due to the inclusion of strong optical absorbers TiO and VO in the GCM for the latter causes a temperature inversion. In both cases our models find low geometric albedos - 0.026 for WASP-43b and 0.028 for HD 209458 b when averaged over the CHEOPS bandpass of 0.35 - 1.1 microns - indicating dark daysides, similar to the low albedos measured by observations. We demonstrate the strong impact of clouds that contain Fe-bearing species on the modelled geometric albedos; without Fe-bearing species forming in the clouds, the albedos of both planets would be much higher (0.518 for WASP-43 b, 1.37 for HD 209458 b). We conclude that a cloudy upper or mid-to-lower atmosphere that contains strongly absorbing Fe-bearing aerosol species, is an alternative to a cloud-free atmosphere in explaining the low dayside albedos of hot Jupiter atmospheres such as HD 209458 b and WASP-43 b.