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CHEOPS geometric albedo of the hot Jupiter HD 209458 b

A. Brandeker, K. Heng, M. Lendl, J. A. Patel, B. M. Morris, C. Broeg, P. Guterman, M. Beck, P. F. L. Maxted, O. Demangeon, L. Delrez, B.-O. Demory, D. Kitzmann, N. C. Santos, V. Singh, Y. Alibert, R. Alonso, G. Anglada-Escudé, T. Bárczy, D. Barrado-Navascues, S. C. C. Barros, W. Baumjohann, T. Beck, W. Benz, N. Billot, X. Bonfils, G. Bruno, J. Cabrera, S. Charnoz, A. Collier Cameron, C. Corral Van Damme, Sz. Csizmadia, M. B. Davies, M. Deleuil, A. Deline, D. Ehrenreich, A. Erikson, J. Farinato, A. Fortier, L. Fossati, M. Fridlund, D. Gandolfi, M. Gillon, M. Güdel, S. Hoyer, K. G Isaak, L. L. Kiss, J. Laskar, A. Lecavelier Des Etangs, C. Lovis, A. Luntzer, D. Magrin, V. Nascimbeni, G. Olofsson, R. Ottensamer, I. Pagano, E. Pallé, G. Peter, G. Piotto, D. Pollacco, D. Queloz, R. Ragazzoni, N. Rando, H. Rauer, I. Ribas, G. Scandariato, D. Ségransan, A. E. Simon, A. M. S. Smith, S. G. Sousa, M. Steller, Gy. M. Szabó, N. Thomas, S. Udry, V. Van Grootel, N. A. Walton, D. Wolter

We report the detection of the secondary eclipse of the hot Jupiter HD 209458 b in optical/visible light using the CHEOPS space telescope. Our measurement of 20.4−3.3+3.2 parts per million translates into a geometric albedo of Ag = 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 Ag for any exoplanet. Our calculations of the bandpass-integrated geometric albedo demonstrate that the measured value of Ag 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 209458 b observed by CHEOPS would have a distinct shape associated with Rayleigh scattering if the atmosphere is indeed cloud free.

techniques: photometric / planetary systems / planets and satellites: atmospheres / planets and satellites: individual: HD 209458 b

The raw and detrended photometric time-series data are only available at the CDS via anonymous ftp to ( or via

Astronomy & Astrophysics
Volume 659, Article Number L4, Number of pages 8
2022 March

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Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia