Short-period gas giants (hot Jupiters) on circular orbits are expected to be tidally locked into synchronous rotation, with permanent daysides that face their host stars and permanent nightsides that face the darkness of space1. Thermal flux from the nightside of several hot Jupiters has been detected, meaning energy is transported from day to night in some fashion. However, it is not clear exactly what the physical information from these detections reveals about the atmospheric dynamics of hot Jupiters. Here we show that the nightside effective temperatures of a sample of 12 hot Jupiters are clustered around 1,100 K, with a slight upward trend as a function of stellar irradiation. The clustering is not predicted by cloud-free atmospheric circulation models2,3,4. This result can be explained if most hot Jupiters have nightside clouds that are optically thick to outgoing longwave radiation and hence radiate at the cloud-top temperature, and progressively disperse for planets receiving greater incident flux. Phase-curve observations at a greater range of wavelengths are crucial to determining the extent of cloud coverage, as well as the cloud composition on hot Jupiter nightsides5,6.
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The authors thank T. Bell for providing the updated energy balance model code, T. Komacek for providing his analytic day–night temperature difference code and L. Kreidberg for an early look at her WASP-103b phase-curve paper. Thanks to J. Mendonça for providing the phase-curve parameters from his WASP-43b paper. Thanks to E. Pass for an early look at her Gaussian process temperature estimate results. Thanks to J. Bean and V. Parmentier for helpful discussion and comments about the manuscript.
The authors declare no competing interests.
Peer review information: Nature Astronomy thanks Thaddeus Komacek and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Keating, D., Cowan, N.B. & Dang, L. Uniformly hot nightside temperatures on short-period gas giants. Nat Astron 3, 1092–1098 (2019). https://doi.org/10.1038/s41550-019-0859-z
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