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Groundbased and spacecraft telescopic observations, combined with an intensive modeling effort, have greatly enhanced our understanding of hot giant planets and brown dwarfs over the past ten years. Although these objects are all fluid, hydrogen worlds with stratified atmospheres overlying convective interiors, they exhibit an impressive diversity of atmospheric behavior. Hot Jupiters are strongly irradiated, and a wealth of observations constrain the day-night temperature differences, circulation, and cloudiness. The intense stellar irradiation, presumed tidal locking and modest rotation leads to a novel regime of strong day-night radiative forcing. Circulation models predict large day-night temperature differences, global-scale eddies, patchy clouds, and, in most cases, a fast eastward jet at the equator—equatorial superrotation. The warm Jupiters lie farther from their stars and are not generally tidally locked, so they may exhibit a wide range of rotation rates, obliquities, and orbital eccentricities, which, along with the weaker irradiation, leads to circulation patterns and observable signatures predicted to differ substantially from hot Jupiters. Brown dwarfs are typically isolated, rapidly rotating worlds; they radiate enormous energy fluxes into space and convect vigorously in their interiors. Their atmospheres exhibit patchiness in clouds and temperature on regional to global scales—the result of modulation by large-scale atmospheric circulation. Despite the lack of irradiation, such circulations can be driven by interaction of the interior convection with the overlying atmosphere, as well as self-organization of patchiness due to cloud-dynamical-radiative feedbacks. Finally, irradiated brown dwarfs help to bridge the gap between these classes of objects, experiencing intense external irradiation as well as vigorous interior convection. Collectively, these diverse objects span over six orders of magnitude in intrinsic heat flux and incident stellar flux, and two orders of magnitude in rotation rate—thereby placing strong constraints on how the circulation of giant planets (broadly defined) depend on these parameters. A hierarchy of modeling approaches have yielded major new insights into the dynamics governing these phenomena.

A spectroscopic comparison of ten hot-Jupiter exoplanets reveals that the difference between the planetary radius measured at optical and infrared wavelengths allows atmosphere types ranging from clear to cloudy to be distinguished; the difference in radius at a given wavelength correlates with the spectral strength of water at that wavelength, suggesting that haze obscures the signal from water. Diversity in the air on hot Jupiters David Sing et al . present a set of ten broadband exoplanet spectra from Hubble Space Telescope and Spitzer observations that resolve both the optical scattering and infrared molecular absorption spectroscopically. They find that the difference between the planetary radius measured at optical and infrared wavelengths provides a metric that can distinguish between different atmospheric types. Significantly, strong water absorption lines are seen in clear-atmosphere planets, while the weakest features are associated with clouds and hazes, strongly arguing against primordial water depletion during formation, and indicating that clouds and hazes are the cause of weaker spectral signatures. These results clarify the diversity seen in hot Jupiters and illustrate the interplay of clouds, hazes and metallicity in exoplanet atmospheres. Thousands of transiting exoplanets have been discovered, but spectral analysis of their atmospheres has so far been dominated by a small number of exoplanets and data spanning relatively narrow wavelength ranges (such as 1.1–1.7 micrometres). Recent studies show that some hot-Jupiter exoplanets have much weaker water absorption features in their near-infrared spectra than predicted 1 , 2 , 3 , 4 , 5 . The low amplitude of water signatures could be explained by very low water abundances 6 , 7 , 8 , which may be a sign that water was depleted in the protoplanetary disk at the planet’s formation location 9 , but it is unclear whether this level of depletion can actually occur. Alternatively, these weak signals could be the result of obscuration by clouds or hazes 1 , 2 , 3 , 4 , as found in some optical spectra 3 , 4 , 10 , 11 . Here we report results from a comparative study of ten hot Jupiters covering the wavelength range 0.3–5 micrometres, which allows us to resolve both the optical scattering and infrared molecular absorption spectroscopically. Our results reveal a diverse group of hot Jupiters that exhibit a continuum from clear to cloudy atmospheres. We find that the difference between the planetary radius measured at optical and infrared wavelengths is an effective metric for distinguishing different atmosphere types. The difference correlates with the spectral strength of water, so that strong water absorption lines are seen in clear-atmosphere planets and the weakest features are associated with clouds and hazes. This result strongly suggests that primordial water depletion during formation is unlikely and that clouds and hazes are the cause of weaker spectral signatures.

Exoplanets that orbit close to their host stars are much more highly irradiated than their solar system counterparts. Understanding the thermal structures and appearances of these planets requires investigating how their atmospheres respond to such extreme stellar forcing. We present spectroscopic thermal emission measurements as a function of orbital phase (\"phase-curve observations\") for the highly irradiated exoplanet WASP-43b spanning three full planet rotations using the Hubble Space Telescope. With these data, we construct a map of the planet's atmospheric thermal structure, from which we find large day-night temperature variations at all measured altitudes and a monotonically decreasing temperature with pressure at all longitudes. We also derive a Bond albedo of ${0.18}_{-0.12}^{+0.07}$ and an altitude dependence in the hot-spot offset relative to the substellar point.

Superrotation is a dynamical regime where the atmosphere circulates around the planet in the direction of planetary rotation with excess angular momentum in the equatorial region. Superrotation is known to exist in the atmospheres of Venus, Titan, Jupiter, and Saturn in the solar system. Some of the exoplanets also exhibit superrotation. Our understanding of superrotation in a framework of circulation regimes of the atmospheres of terrestrial planets is in progress thanks to the development of numerical models; a global instability involving planetary-scale waves seems to play a key role, and the dynamical state depends on the Rossby number, a measure of the relative importance of the inertial and Coriolis forces, and the thermal inertia of the atmosphere. Recent general circulation models of Venus’s and Titan’s atmospheres demonstrated the importance of horizontal waves in the angular momentum transport in these atmospheres and also an additional contribution of thermal tides in Venus’s atmosphere. The atmospheres of Jupiter and Saturn also exhibit strong superrotation. Recent gravity data suggests that these superrotational flows extend deep into the planet, yet currently no single mechanism has been identified as driving this superrotation. Moreover, atmospheric circulation models of tidally locked, strongly irradiated exoplanets have long predicted the existence of equatorial superrotation in their atmospheres, which has been attributed to the result of the strong day-night thermal forcing. As predicted, recent Doppler observations and infrared phase curves of hot Jupiters appear to confirm the presence of superrotation on these objects.

Equatorial superrotation can be generated in global general circulation models (GCMs) when forced with longitudinally varying heating, similar to that postulated in the Matsuno‐Gill model. However, the implications of the classical Matsuno‐Gill theory for equatorial superrotation have not, to date, been addressed. Here, we show that the classic, shallow‐water Matsuno‐Gill solutions do not exhibit equatorial superrotation: although the flow converges westerly momentum from high latitudes to the equator—promoting superrotation—they also contain an artificial source of easterly momentum at the equator that cancels the latitudinal momentum convergence and prevents superrotation from emerging. This artificial momentum source results from a physically inconsistent representation of vertical momentum transport in the model. We show that if the Matsuno‐Gill model is modified to properly account for momentum exchange with an underlying quiescent layer, the solutions naturally exhibit equatorial superrotation, at any forcing amplitude.

'Hot Jupiters' just got hotter 'Hot Jupiter' extrasolar planets are close to their parent stars, so are likely to be tidally locked (like the Earth and Moon), with permanent day and night sides. That raises the question of whether the atmosphere is able to transport energy from the day side to the night side. Infrared data from the Spitzer Space Telescope have now answered that question for the extrasolar planet HD 189733b. Night and day temperatures are similar, around 950–1,200 K, indicating that energy from the irradiated side is efficiently redistributed throughout the atmosphere. A paper going live online this week reports observations of the atmosphere of the extra-solar plant HD 149026b. It’s a very hot Jupiter, the hottest planet known, at about 2,300 K. This matches predictions for a planet where each patch of surface area instantaneously re-emits all absorbed light as a blackbody. ‘Hot Jupiter’ extrasolar planets are expected to be tidally locked because they are close (<0.05 astronomical units, where 1  au is the average Sun–Earth distance) to their parent stars, resulting in permanent daysides and nightsides. By observing systems where the planet and star periodically eclipse each other, several groups have been able to estimate the temperatures of the daysides of these planets 1 , 2 , 3 . A key question is whether the atmosphere is able to transport the energy incident upon the dayside to the nightside, which will determine the temperature at different points on the planet’s surface. Here we report observations of HD 189733, the closest of these eclipsing planetary systems 4 , 5 , 6 , over half an orbital period, from which we can construct a ‘map’ of the distribution of temperatures. We detected the increase in brightness as the dayside of the planet rotated into view. We estimate a minimum brightness temperature of 973 ± 33 K and a maximum brightness temperature of 1,212 ± 11 K at a wavelength of 8 μm, indicating that energy from the irradiated dayside is efficiently redistributed throughout the atmosphere, in contrast to a recent claim for another hot Jupiter 7 . Our data indicate that the peak hemisphere-integrated brightness occurs 16 ± 6° before opposition, corresponding to a hotspot shifted east of the substellar point. The secondary eclipse (when the planet moves behind the star) occurs 120 ± 24 s later than predicted, which may indicate a slightly eccentric orbit.

To test the hypothesis that the zonal jets on Jupiter and Saturn result from energy injected by thunderstorms into the cloud layer, forced-dissipative numerical simulations of the shallow-water equations in spherical geometry are presented. The forcing consists of sporadic, isolated circular mass pulses intended to represent thunderstorms; the damping, representing radiation, removes mass evenly from the layer. These results show that the deformation radius provides strong control over the behavior. At deformation radii <2000 km (0.03 Jupiter radii), the simulations produce broad jets near the equator, but regions poleward of 15°–30° latitude instead become dominated by vortices. However, simulations at deformation radii >4000 km (0.06 Jupiter radii) become dominated by barotropically stable zonal jets with only weak vortices. The lack of midlatitude jets at a small deformation radii results from the suppression of the beta effect by column stretching; this effect has been previously documented in the quasigeostrophic system but never before in the full shallow-water system. In agreement with decaying shallow-water turbulence simulations, but in disagreement with Jupiter and Saturn, the equatorial flows in these forced simulations are always westward. In analogy with purely two-dimensional turbulence, the size of the coherent structures (jets and vortices) depends on the relative strengths of forcing and damping; stronger damping removes energy faster as it cascades upscale, leading to smaller vortices and more closely spaced jets in the equilibrated state. Forcing and damping parameters relevant to Jupiter produce flows with speeds up to 50–200 m s−1 and a predominance of anticyclones over cyclones, both in agreement with observations. However, the dominance of vortices over jets at deformation radii thought to be relevant to Jupiter (1000–3000 km) suggests that either the actual deformation radius is larger than previously believed or that three-dimensional effects, not included in the shallow-water equations, alter the dynamics in a fundamental manner.

Telescopic observations and space missions to Jupiter have provided vast information about Jupiter's cloud level winds, but the depth to which these winds penetrate has remained an ongoing mystery. Scheduled to be launched in 2011, the Jupiter orbiter Juno will make high‐resolution observations of Jupiter's gravity field. In this paper we show that these measurements are sensitive to the depth of the internal winds. We use dynamical models ranging from an idealized thermal wind balance analysis, using the observed cloud‐top winds, to a full general circulation model (GCM). We relate the depth of the dynamics to the external gravity spectrum for different internal wind structure scenarios. In particular, we predict that substantial Jovian winds below a depth of 500 km would lead to detectable (milligal‐level) gravity anomalies with respect to the expected gravity for a planet in solid body rotation.

The atmospheres of the four giant planets of our Solar System share a common and well-observed characteristic: they each display patterns of planetary banding, with regions of different temperatures, composition, aerosol properties and dynamics separated by strong meridional and vertical gradients in the zonal (i.e., east-west) winds. Remote sensing observations, from both visiting spacecraft and Earth-based astronomical facilities, have revealed the significant variation in environmental conditions from one band to the next. On Jupiter, the reflective white bands of low temperatures, elevated aerosol opacities, and enhancements of quasi-conserved chemical tracers are referred to as ‘zones.’ Conversely, the darker bands of warmer temperatures, depleted aerosols, and reductions of chemical tracers are known as ‘belts.’ On Saturn, we define cyclonic belts and anticyclonic zones via their temperature and wind characteristics, although their relation to Saturn’s albedo is not as clear as on Jupiter. On distant Uranus and Neptune, the exact relationships between the banded albedo contrasts and the environmental properties is a topic of active study. This review is an attempt to reconcile the observed properties of belts and zones with (i) the meridional overturning inferred from the convergence of eddy angular momentum into the eastward zonal jets at the cloud level on Jupiter and Saturn and the prevalence of moist convective activity in belts; and (ii) the opposing meridional motions inferred from the upper tropospheric temperature structure, which implies decay and dissipation of the zonal jets with altitude above the clouds. These two scenarios suggest meridional circulations in opposing directions, the former suggesting upwelling in belts, the latter suggesting upwelling in zones. Numerical simulations successfully reproduce the former, whereas there is a wealth of observational evidence in support of the latter. This presents an unresolved paradox for our current understanding of the banded structure of giant planet atmospheres, that could be addressed via a multi-tiered vertical structure of “stacked circulation cells,” with a natural transition from zonal jet pumping to dissipation as we move from the convectively-unstable mid-troposphere into the stably-stratified upper troposphere.

The nature and structure of the observed east-west flows on Jupiter and Saturn have been a long-standing mystery in planetary science. This mystery has been recently unraveled by the accurate gravity measurements provided by the Juno mission to Jupiter and the Grand Finale of the Cassini mission to Saturn. These two experiments, which coincidentally happened around the same time, allowed the determination of the overall vertical and meridional profiles of the zonal flows on both planets. This paper reviews the topic of zonal jets on the gas giants in light of the new data from these two experiments. The gravity measurements not only allow the depth of the jets to be constrained, yielding the inference that the jets extend to roughly 3000 and 9000 km below the observed clouds on Jupiter and Saturn, respectively, but also provide insights into the mechanisms controlling these zonal flows. Specifically, for both planets this depth corresponds to the depth where electrical conductivity is within an order of magnitude of 1 S m −1 , implying that the magnetic field likely plays a key role in damping the zonal flows. An intrinsic characteristic of any gravity inversion, as discussed here, is that the solutions might not be unique. We analyze the robustness of the solutions and present several independent lines of evidence supporting the results presented here.