CHEOPS geometric albedo of the hot Jupiter HD209458 b

Vol. 659
1. Letters to the Editor

CHEOPS geometric albedo of the hot Jupiter HD209458 b

The albedo of a planet determines how much starlight enters into its atmosphere and hence its global energy budget, and it also provides a key diagnostic of the physical and chemical properties of this atmosphere. Albedos have been measured for a few exoplanets through the depth of their occultation of their star, but up to now that has been limited to hot planets which orbit so close to their star that their thermal emission extends into the visible range. Their occultation depth, therefore, measures the sum of their reflected light and thermal emission, and evaluating their albedo requires a large and model-dependent correction for the latter. Using the exquisite photometric precision provided by the CHEOPS mission, Brandeker et al. measured the depth of the occultation of HD 209458 by its planet, which is cool enough so that thermal emission negligibly contributes to the occultation. This provides the cleanest measurement to date of an exoplanet albedo and shows that HD 209458b is very dark, with Ag = 0.096±0.016.