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

ABSTRACT We simulate the geometric albedo spectra of hot Jupiter exoplanets HD 209458 b and WASP-43 b, based on global climate model 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 subsolar point, largely due to the inclusion of strong optical absorbers TiO and VO in the general circulation model for the latter causes a temperature inversion. In both cases our models find low-geometric albedos – 0. 026 for WASP-43 b and 0. 028 for HD 209458 b when averaged over the CHaracterizing ExOPlanet Satellite bandpass of 0. 35–1. 1 μm – 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.