Uniformly hot nightside temperatures on short-period gas giants

TL;DR

This study finds that hot Jupiters have uniformly warm nightsides around 1100 K, likely due to thick clouds, challenging existing cloud-free atmospheric models and highlighting the importance of multi-wavelength phase curve observations.

Contribution

The paper provides evidence that hot Jupiter nightsides are uniformly warm due to thick clouds, offering new insights into their atmospheric dynamics and cloud properties.

Findings

Nightside temperatures cluster around 1100 K.

Cloud-free models do not predict this clustering.

Thick clouds likely cause the uniform temperatures.

Abstract

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 space. Thermal flux from the nightside of several hot Jupiters has been measured, 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 1100 K, with a slight upward trend as a function of stellar irradiation. The clustering is not predicted by cloud-free atmospheric circulation models. 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 nightsides.