The stable climate of KELT-9b,★

Jones, K.; Morris, B. M.; Demory, B. -O.; Heng, K.; Hooton, M. J.; Billot, N.; Ehrenreich, D.; Hoyer, S.; Simon, A. E.; Lendl, M.; Demangeon, O. D. S.; Sousa, S. G.; Bonfanti, A.; Wilson, T. G.; Salmon, S.; Csizmadia, Sz.; Parviainen, H.; Bruno, G.; Alibert, Y.; Alonso, R.; Anglada, G.; Bárczy, T.; Barrado, D.; Barros, S. C. C.; Baumjohann, W.; Beck, M.; Beck, T.; Benz, W.; Bonfils, X.; Brandeker, A.; Broeg, C.; Cabrera, J.; Charnoz, S.; Collier Cameron, A.; Davies, M. B.; Deleuil, M.; Deline, A.; Delrez, L.; Erikson, A.; Fortier, A.; Fossati, L.; Fridlund, M.; Gandolfi, D.; Gillon, M.; Güdel, M.; Isaak, K. G.; Kiss, L. L.; Laskar, J.; Lecavelier des Etangs, A.; Lovis, C.; Magrin, D.; Maxted, P. F. L.; Nascimbeni, V.; Olofsson, G.; Ottensamer, R.; Pagano, I.; Pallé, E.; Peter, G.; Piotto, G.; Pollacco, D.; Queloz, D.; Ragazzoni, R.; Rando, N.; Ratti, F.; Rauer, H.; Reimers, C.; Ribas, I.; Santos, N. C.; Scandariato, G.; Ségransan, D.; Smith, A. M. S.; Steller, M.; Szabó, Gy. M.; Thomas, N.; Udry, S.; Van Grootel, V.; Walter, I.; Walton, N. A.; Wang Jungo, W.
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Astronomy and Astrophysics

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Even among the most irradiated gas giants, so-called ultra-hot Jupiters, KELT-9b stands out as the hottest planet thus far discovered with a dayside temperature of over 4500 K. At these extreme irradiation levels, we expect an increase in heat redistribution efficiency and a low Bond albedo owed to an extended atmosphere with molecular hydrogen dissociation occurring on the planetary dayside. We present new photometric observations of the KELT-9 system throughout 4 full orbits and 9 separate occultations obtained by the 30 cm space telescope CHEOPS. The CHEOPS bandpass, located at optical wavelengths, captures the peak of the thermal emission spectrum of KELT-9b. In this work we simultaneously analyse CHEOPS phase curves along with public phase curves from TESS and Spitzer to infer joint constraints on the phase curve variation, gravity-darkened transits, and occultation depth in three bandpasses, as well as derive 2D temperature maps of the atmosphere at three different depths. We find a day-night heat redistribution efficiency of ~0.3 which confirms expectations of enhanced energy transfer to the planetary nightside due to dissociation and recombination of molecular hydrogen. We also calculate a Bond albedo consistent with zero. We find no evidence of variability of the brightness temperature of the planet, excluding variability greater than 1%

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