CHEOPS geometric albedo of the hot Jupiter HD 209458b

TL;DR

This study reports a precise optical measurement of HD 209458b's geometric albedo using CHEOPS, indicating a cloud-free atmosphere dominated by Rayleigh scattering, with implications for future phase curve observations.

Contribution

First robust optical geometric albedo measurement of HD 209458b confirming a cloud-free, Rayleigh scattering atmosphere with stellar metallicity consistency.

Findings

Geometric albedo A_g = 0.096 +/- 0.016

Consistent with a cloud-free, Rayleigh scattering atmosphere

TESS bandpass albedo likely too faint for detection

Abstract

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.