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Radiative transfer in planetary atmospheres is usually treated in the static limit, i.e., neglecting atmospheric motions. We argue that hot Jupiter atmospheres, with possibly fast (sonic) wind speeds, may require a more strongly coupled treatment, formally in the regime of radiation-hydrodynamics. To lowest order in v/c, relativistic Doppler shifts distort line profiles along optical paths with finite wind velocity gradients. This leads to flow-dependent deviations in the effective emission and absorption properties of the atmospheric medium. Evaluating the overall impact of these distortions on the radiative structure of a dynamic atmosphere is non-trivial. We present transmissivity and systematic equivalent width excess calculations which suggest possibly important consequences for radiation transport in hot Jupiter atmospheres. If winds are fast and bulk Doppler shifts are indeed important for the global radiative balance, accurate modeling and reliable data interpretation for hot Jupiter atmospheres may prove challenging: it would involve anisotropic and dynamic radiative transfer in a coupled radiation-hydrodynamical flow. On the bright side, it would also imply that the emergent properties of hot Jupiter atmospheres are more direct tracers of their atmospheric flows than is the case for Solar System planets. Radiation-hydrodynamics may also influence radiative transfer in other classes of hot exoplanetary atmospheres with fast winds.
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We present results from a set of simulations using a fully coupled three-dimensional (3D) chemistry-radiation-hydrodynamics model and investigate the effect of transport of chemical species by the large-scale atmospheric flow in hot Jupiter atmospheres. We couple a flexible chemical kinetics scheme to the Met Office Unified Model which enables the study of the interaction of chemistry, radiative transfer and fluid dynamics. We use a newly-released reduced chemical network comprising 30 chemical species that has been specifically developed for application in 3D atmosphere models. We simulate the atmospheres of the well-studied hot Jupiters HD~209458b and HD~189733b which both have dayside--nightside temperature contrasts of several hundred Kelvin and superrotating equatorial jets. We find qualitatively quite different chemical structures between the two planets, particularly for methane (CH$_4$), when advection of chemical species is included. Our results show that consideration of 3D chemical transport is vital in understanding the chemical composition of hot Jupiter atmospheres. 3D mixing leads to significant changes in the abundances of absorbing gas-phase species compared with what would be expected by assuming local chemical equilibrium, or from models including 1D - and even 2D - chemical mixing. We find that CH$_4$, carbon dioxide (CO$_2$) and ammonia (NH$_3$) are particularly interesting as 3D mixing of these species leads to prominent signatures of out-of-equilibrium chemistry in the transmission and emission spectra, detectable with near-future instruments.
We compute models of the transmission spectra of planets HD 209458b, HD 189733b, and generic hot Jupiters. We examine the effects of temperature, surface gravity, and metallicity for the generic planets as a guide to understanding transmission spectra in general. We find that carbon dioxide absorption at 4.4 and 15 microns is prominent at high metallicity, and is a clear metallicity indicator. For HD 209458b and HD 189733b, we compute spectra for both one-dimensional and three-dimensional model atmospheres and examine the differences between them. The differences are usually small, but can be large if atmospheric temperatures are near important chemical abundance boundaries. The calculations for the 3D atmospheres, and their comparison with data, serve as constraints on these dynamical models that complement the secondary eclipse and light curve data sets. For HD 209458b, even if TiO and VO gases are abundant on the day side, their abundances can be considerably reduced on the cooler planetary limb. However, given the predicted limb temperatures and TiO abundances, the models optical opacity is too high. For HD 189733b we find a good match with some infrared data sets and constrain the altitude of a postulated haze layer. For this planet, substantial differences can exist between the transmission spectra of the leading and trailing hemispheres, which is an excellent probe of carbon chemistry. In thermochemical equilibrium, the cooler leading hemisphere is methane-dominated, and the hotter trailing hemisphere is CO-dominated, but these differences may be eliminated by non-equilibrium chemistry due to vertical mixing. It may be possible to constrain the carbon chemistry of this planet, and its spatial variation, with JWST.
Context. Atmospheric superrotating flows at the equator are an almost ubiquitous result of simulations of hot Jupiters, and a theory explaining how this zonally coherent flow reaches an equilibrium has been developed in the literature. However, this understanding relies on the existence of either an initial superrotating or a sheared flow, coupled with a slow evolution such that a linear steady state can be reached. Aims. A consistent physical understanding of superrotation is needed for arbitrary drag and radiative timescales, and the relevance of considering linear steady states needs to be assessed. Methods. We obtain an analytical expression for the structure, frequency and decay rate of propagating waves in hot Jupiter atmospheres around a state at rest in the 2D shallow water beta plane limit. We solve this expression numerically and confirm the robustness of our results with a 3D linear wave algorithm. We then compare with 3D simulations of hot Jupiter atmospheres and study the non linear momentum fluxes. Results. We show that under strong day night heating the dynamics does not transit through a linear steady state when starting from an initial atmosphere in solid body rotation. We further show that non linear effects favour the initial spin up of superrotation and that the acceleration due to the vertical component of the eddy momentum flux is critical to the initial development of superrotation. Conclusions. Overall, we describe the initial phases of the acceleration of superrotation, including consideration of differing radiative and drag timescales, and conclude that eddy-momentum driven superrotating equatorial jets are robust, physical phenomena in simulations of hot Jupiter atmospheres.
[Abridged] A key hypothesis in the field of exoplanet atmospheres is the trend of atmospheric thermal structure with planetary equilibrium temperature. We explore this trend and report here the first statistical detection of a transition in the near-infrared (NIR) atmospheric emission between hot and ultra-hot Jupiters. We measure this transition using secondary eclipse observations and interpret this phenomenon as changes in atmospheric properties, and more specifically in terms of transition from non-inverted to inverted thermal profiles. We examine a sample of 78 hot Jupiters with secondary eclipse measurements at 3.6 {mu}m and 4.5 {mu}m measured with Spitzer Infrared Array Camera (IRAC). We measure the deviation of the data from the blackbody, which we define as the difference between the observed 4.5 {mu}m eclipse depth and that expected at this wavelength based on the brightness temperature measured at 3.6 {mu}m. We study how the deviation between 3.6 and 4.5 {mu}m changes with theoretical predictions with equilibrium temperature and incoming stellar irradiation. We reveal a clear transition in the observed emission spectra of the hot Jupiter population at 1660 +/- 100 K in the zero albedo, full redistribution equilibrium temperature. We find the hotter exoplanets have even hotter daysides at 4.5 {mu}m compared to 3.6 {mu}m, which manifests as an exponential increase in the emitted power of the planets with stellar insolation. We propose that the measured transition is a result of seeing carbon monoxide in emission due to the formation of temperature
We report 78 secondary eclipse depths for a sample of 36 transiting hot Jupiters observed at 3.6- and 4.5 microns using the Spitzer Space Telescope. Our eclipse results for 27 of these planets are new, and include highly irradiated worlds such as KELT-7b, WASP-87b, WASP-76b, and WASP-64b, and important targets for JWST such as WASP-62b. We find that WASP-62b has a slightly eccentric orbit e cos(omega) = 0.00614+/- 0.00064, and we confirm the eccentricity of HAT-P-13b and WASP-14b. The remainder are individually consistent with circular orbits, but we find statistical evidence for eccentricity increasing with orbital period in our range from 1 to 5 days. Our day-side brightness temperatures for the planets yield information on albedo and heat redistribution, following Cowan and Agol (2011). Planets having maximum day side temperatures exceeding ~ 2200K are consistent with zero albedo and distribution of stellar irradiance uniformly over the day-side hemisphere. Our most intriguing result is that we detect a systematic difference between the emergent spectra of these hot Jupiters as compared to blackbodies. The ratio of observed brightness temperatures, Tb(4.5)/Tb(3.6), increases with equilibrium temperature by 100 +/- 24 parts-per-million per Kelvin, over the entire temperature range in our sample (800K to 2500K). No existing model predicts this trend over such a large range of temperature. We suggest that this may be due to a structural difference in the atmospheric temperature profile between the real planetary atmospheres as compared to models.
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