  We report the detection of the secondary eclipse of the hot Jupiter HD
209458b in optical/visible light using the CHEOPS space telescope. Our
measurement of 20.4 +/- 3.3 ppm translates into a geometric albedo of A_g =
0.096 +/- 0.016. The previously estimated dayside temperature of about 1500 K
implies that our geometric albedo measurement consists predominantly of
reflected starlight and is largely uncontaminated by thermal emission. This
makes the present result one of the most robust measurements of A_g for any
exoplanet. Our calculations of the bandpass-integrated geometric albedo
demonstrate that the measured value of A_g is consistent with a cloud-free
atmosphere, where starlight is reflected via Rayleigh scattering by hydrogen
molecules, and the water and sodium abundances are consistent with stellar
metallicity. We predict that the bandpass-integrated TESS geometric albedo is
too faint to detect and that a phase curve of HD 209458b observed by CHEOPS
would have a distinct shape associated with Rayleigh scattering if the
atmosphere is indeed cloud free.

Earth and Planetary Astrophysics