The effect of atmospheric chemistry on the optical geometric albedos of hot Jupiters

¹ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
² Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD 21218, USA
³ Ludwig Maximilian University, University Observatory Munich, Scheinerstrasse 1, Munich 81679, Germany
⁴ University of Warwick, Department of Physics, Astronomy & Astrophysics Group, Coventry CV4 7AL, UK
⁵ University of Bern, ARTORG Center for Biomedical Engineering Research, Murtenstrasse 50, 3008 Bern, Switzerland

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 18 October 2025
Accepted: 12 February 2026

Abstract

We investigate the geometric albedos of hot Jupiters by comparing observational data from space telescopes TESS, Kepler, CoRoT, and CHEOPS against theoretical models. The study aims to understand the distribution of observed geometric albedos across different bandpasses and how these observations align with or deviate from model predictions. We curated a comprehensive sample of observed geometric albedos, using either existing Spitzer secondary eclipse measurements or a scaling law between the equilibrium and dayside temperature to remove any contaminating thermal planetary emission. We then utilised hierarchical Bayesian modelling to identify trends with planetary properties such as equilibrium temperature, gravity, and stellar metallicity. On a population level, we found no statistical difference in the distributions of geometric albedos measured by TESS compared to those measured by Kepler, CoRoT, and CHEOPS. We confront the geometric albedo sample with a simple but first-principles model that includes Rayleigh scattering by molecular hydrogen and absorption by sodium, water, titanium oxide, and vanadium oxide. We find that the abundance of sodium and water are the key absorbers that influence the geometric albedos of hot Jupiters, whilst the addition of titanium oxide and vanadium oxide (in the absence of condensation) results in vanishing geometric albedos that are inconsistent with the observed distributions.