Uniformly Hot Nightside Temperatures on Short-Period Gas Giants
The paper investigates the phenomenon of hot Jupiters, short-period gas giants, and their atmospheric dynamics, specifically focusing on the thermal characteristics of their nightsides. The study reveals that the nightside effective temperatures of a sample of twelve hot Jupiters demonstrate a surprising uniformity, clustering around 1100 K, with a minor upward trend related to increasing stellar irradiation. This finding deviates from predictions made by existing cloud-free atmospheric circulation models.
The study leveraged full orbit infrared phase curves from previously published data to calculate the nightside brightness and effective temperatures. Utilizing Gaussian Process regression, the researchers estimated each planet's bolometric flux and consequently determined its nightside effective temperature. Notably, the study addresses the issue of phase curves indicating negative unphysical flux values by implementing constraints to ensure non-negative flux distributions.
The clustering of nightside temperatures around 1100 K is inconsistent with cloud-free models. However, the presence of optically thick nightside clouds is a plausible explanation, as such clouds could produce an emission spectrum corresponding to the cloud-top temperatures. These clouds are hypothesized to disperse gradually as the stellar irradiation increases, thereby maintaining the uniform temperature across different planets. This phenomenon necessitates further investigations through phase curve observations at a broader range of wavelengths to better ascertain cloud composition and coverage.
Two models were employed to fit the nightside temperature data: a semi-analytic energy balance model and an analytic dynamical model. The former suggests that a common wind velocity across different planets, coupled with hydrogen dissociation and recombination processes, could account for the observed uniformity in temperature, providing a superior fit than models excluding hydrogen effects. The latter incorporated radiation, advection, magnetic drag, Coriolis forces, and gravity waves, predicting wind velocities that match the day-night temperature contrasts obtained from general circulation models. The analysis indicated that observably similar wind velocities in hot Jupiters suggest more universal atmospheric dynamics than previously assumed.
The strong numerical evidence provided in this study supports the presence of clouds as a critical factor in shaping hot Jupiter atmospheres. This insight has significant implications for our understanding of exoplanet atmospheres, indicating a commonality in atmospheric characteristics across diverse exoplanets. The study advocates for the integration of realistic cloud physics into three-dimensional atmospheric models and the inclusion of magnetic effects for more accurately predicting heat distribution, particularly for ultra-hot Jupiters.
Future research directions emphasize the need for enhanced observational capabilities, such as those provided by the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope and the Ariel space mission, to test these hypotheses and further unravel the complex atmospheric dynamics at play on these intriguing exoplanets.
In conclusion, the paper contributes fundamentally to exoplanet atmospheric sciences by challenging existing models and offering a comprehensive explanation for nightside temperature uniformity, significantly enhancing our understanding of hot Jupiter properties and guiding future observational and theoretical studies.
