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Atmospheric superrotation is circulation whose axial angular momentum per unit mass exceeds that of the underlying body's equator. An eastward equatorial jet is a common manifestation, but it requires a continuing source of equatorial momentum convergence rather than merely a local temperature difference.

In models of exoplanets under tidal locking, day-night heating excites large-scale Rossby waves and Kelvin waves. Their phase structure transports eastward momentum toward the equator and can maintain the jet against drag. This mechanism was demonstrated using an analytic model and numerical calculations of increasing complexity.^[ref] Such a jet can shift a bright atmospheric region eastward, affecting an exoplanet thermal phase curve, although a brightness offset alone does not uniquely measure a wind speed.

Day-night heat redistribution transports absorbed stellar energy from an illuminated hemisphere to a dark one through atmospheric motion or an ocean. A simple comparison uses the atmospheric advection time, $L/U$, and the radiative relaxation time in a planetary atmosphere. Fast transport relative to cooling allows gas to retain heat while crossing the planet, warming the nightside and reducing the day-night contrast.

An exoplanet thermal phase curve measures the resulting variation in brightness as different hemispheres rotate into view. HD 189733 b provides an observed example. The inference remains wavelength-dependent because different opacities sample different pressures. A small brightness contrast can therefore constrain redistribution at the sampled levels without proving that every level has the same temperature.

An exoplanet atmosphere is the gaseous outer region of an exoplanet, where gravity, radiative transfer, chemical reactions and fluid motion jointly determine what emerges into space. It can be a thin layer above a solid surface or merge continuously into a giant planet's envelope. Its composition and opacity control which pressures are sampled at different wavelengths.

Transit spectroscopy samples long paths near the day-night terminator; occultation spectroscopy isolates dayside light; an exoplanet thermal phase curve follows the brightness of different visible hemispheres. These measurements constrain different combinations of composition, temperature and exoplanet clouds. Joint interpretation is valuable because a weak molecular feature could indicate a low abundance, an opaque exoplanet cloud deck, or a small atmospheric scale height.

Exoplanet atmospheric dynamics describes motion in an exoplanet atmosphere and its transport of heat, matter and angular momentum. Uneven stellar heating creates horizontal pressure gradients; rotation deflects the flow through the Coriolis force, while friction and radiative transfer modify its response. The resulting circulation can contain jets, waves and overturning motion.

On a hot Jupiter under tidal locking, permanent day-night heating contrasts drive day-night heat redistribution. Comparing the atmospheric advection time with the radiative relaxation time in a planetary atmosphere estimates whether gas changes hemisphere before losing much heat. An exoplanet thermal phase curve tests the resulting brightness distribution, while disequilibrium chemistry in an exoplanet atmosphere records transport of chemical species.

An exoplanet thermal phase curve measures the change in planetary infrared brightness through the orbit, after modelling the host star's contribution. Around exoplanet secondary eclipse the visible hemisphere is mostly dayside; around primary exoplanet transit it is mostly nightside. The amplitude therefore constrains the longitudinal thermal contrast, while an offset maximum indicates an asymmetric brightness distribution.

Spitzer Space Telescope observations of HD 189733 b demonstrated that this technique could constrain day-night heat redistribution and a displaced bright region even without resolving the planetary disc.^[ref] A phase curve averages light over a visible hemisphere. Its map is therefore model-dependent, and brightness temperatures in one band need not describe all atmospheric pressures.

A general circulation model evolves rotating fluid dynamics together with heating/cooling and suitable transport parameterizations. Atmospheric applications can couple radiative transfer, clouds and chemistry to resolve longitudinal and latitudinal structure. For an exoplanet atmosphere, it can predict winds and an exoplanet thermal phase curve. Unresolved turbulence and cloud microphysics still require approximate closures.

HD 189733 b is a transiting hot Jupiter used to study day-night heat redistribution. Observations with the Spitzer Space Telescope at $8\mu \mathrm{m}$ followed the changing combined light over half an orbital period. Subtracting the star's contribution yielded an exoplanet thermal phase curve, from which a broad longitudinal brightness distribution could be inferred.^[ref]

The bright hemisphere reached its maximum visibility before exoplanet secondary eclipse, implying a hot region displaced from the point directly facing the star. A warm nightside supports transport of absorbed energy by the exoplanet atmosphere. These are brightness temperature measurements in one band; they neither resolve small weather features directly nor uniquely specify the full atmospheric pressure-temperature profile.

A hot Jupiter is a giant exoplanet with a short orbital period and strong stellar irradiance, typically orbiting much closer to its star than Mercury orbits the Sun. Large radius and short orbital period make these bodies favourable for exoplanet transit photometry; large mass and small orbital separation also increase the stellar radial-velocity semi-amplitude.

Strong external heating makes an exoplanet atmosphere very different from that of Jupiter. Tidal locking can maintain nearly permanent illuminated and dark hemispheres, creating a large driving force for day-night heat redistribution. HD 209458 b illustrates transit spectroscopy, while HD 189733 b illustrates an exoplanet thermal phase curve. Their proximity to their stars also motivates planetary migration models: present orbital location need not be the location of formation.

Transmission spectroscopy, eclipse/emission spectroscopy and direct-imaging spectroscopy probe atmospheres in complementary geometries.

A exoplanet transmission spectrum uses the wavelength dependence of primary-transit depth to measure slant-path absorption at the limb. Its long path length gives sensitivity to trace absorbers, but only transiting systems qualify; exoplanet clouds, stellar heterogeneity and the reference-radius/abundance degeneracy complicate interpretation, and the result samples the terminator rather than the dayside.

An exoplanet secondary eclipse isolates the dayside thermal/reflected spectrum by removing the planet behind the star. It constrains brightness temperatures, molecules and thermal inversions, but the planet/star contrast is small, a suitable orbital geometry is needed, and temperature-abundance degeneracies remain. A exoplanet thermal phase curve extends this method to longitudinal information.

Exoplanet direct imaging spatially separates a planet from its star and permits repeated spectroscopy without requiring a transit. It is effective for young, luminous, widely separated giant planets, but demands high contrast and angular separation; old small planets close to their stars are difficult. Inferred gravities, radii and masses can depend strongly on atmosphere and evolutionary models.

Three signatures are a displaced exoplanet thermal phase curve maximum, the day-night temperature contrast, and wind-induced Doppler shifts.

The maximum of an exoplanet thermal phase curve need not coincide with secondary eclipse. An eastward hot spot and a maximum before eclipse indicate eastward advection/atmospheric superrotation, provided reflected light and exoplanet clouds are accounted for.

The amplitude and shape of an exoplanet thermal phase curve measure day-night heat redistribution: a warm nightside and reduced day-night contrast imply effective transport relative to radiative relaxation time in a planetary atmosphere. Wavelength-dependent contrasts can reveal different circulation efficiencies at different contribution pressures.

High-resolution transmission or emission spectra can show line-centroid shifts/broadening relative to the orbital velocity. A limb blueshift is consistent with a dayside-to-nightside flow toward the observer; rotation and limb asymmetry affect the pattern. Such Doppler spectroscopy probes velocity directly, while phase curves probe the transported temperature pattern.

One approach uses exoplanet transmission spectra. Three aerosol signatures are:

A second approach uses the dayside spectrum and brightness obtained with exoplanet secondary eclipses, optionally supported by orbital phase variations. Its three useful aerosol signatures are:

Transmission measures aerosol extinction along the limb; eclipse/reflection measurements test dayside scattering and emission. No single flattened feature proves clouds: small atmospheric scale height, low molecular abundances, stellar heterogeneity and thermal-gradient degeneracies must be considered. Joint wavelength coverage and geometries make the inference stronger.

Three research directions natural in the 2017 setting are:

Small-planet atmospheres, formation through composition, and atmospheric dynamics/variability provide three concrete emerging directions. The examples describe research aims as of the paper's date, rather than treating later discoveries or later operating missions as established in 2017.

Three observational signatures constrain exoplanet atmospheric dynamics:

Opacity, exoplanet cloud distributions, and pressure-dependent brightness temperature affect the phase-curve interpretation. These measurements constrain circulation through atmospheric models; a single offset or line shift does not uniquely determine a three-dimensional wind field.

Three major research directions, appropriate to the scientific questions posed in 2018, are:

These are research goals rather than claims that the relevant mechanisms or habitable atmospheres have already been established.

A close-in giant is often in synchronous rotation after tidal locking, giving persistent dayside heating and nightside cooling. Its contrast is controlled by the competition between radiative relaxation time in a planetary atmosphere, wind transport characterized by the atmospheric advection time, wave adjustment, and drag. When heat transport is fast compared with radiation, day-night heat redistribution lowers the contrast; when radiation is fast, each hemisphere stays closer to its local radiative balance.

For a rough atmospheric column estimate,At comparable pressure, higher planetary equilibrium temperature sharply shortens radiative relaxation, tending to increase the day-night contrast. Wind speeds, rotation, and magnetic drag can modify this trend; dissociation and recombination can carry additional heat in very hot atmospheres.

At higher altitude, lower pressure generally means shorter radiative relaxation and a larger contrast. Infrared bands with larger opacity probe these higher layers, while lower-opacity windows sample deeper layers with longer cooling times and more effective redistribution. A wavelength-dependent exoplanet thermal phase curve can therefore reveal how the contrast and hot-region displacement change with pressure.

First refine the ephemeris, planetary mass, stellar radius, and stellar variability using exoplanet transit photometry and the radial-velocity method. Then combine observations that probe different regions rather than relying on one spectrum. A present-day programme could use the following complementary measurements; in the 2019 setting of the paper, James Webb Space Telescope observations would have been a future capability.

Together these address aerosols, molecular composition, elemental enrichment, vertical thermal structure, horizontal heat transport, winds, and escape. Repeat key events and monitor stellar activity, since stellar contamination and instrumental trends can imitate atmospheric signals. Use actual brightness, saturation limits, and predicted feature amplitudes to choose observing modes and exposure times.

The wavelength ranges and time-series capabilities are documented in the NIRISS SOSS guide, NIRSpec overview, MIRI spectroscopy guide, and ESO's CRIRES description.

The orbital variation of a planet's reflected stellar light as the visible illuminated area changes. In a simple spherical model, $F_p/F_{\ast }=A_g(R_p/a{)}^2\Phi (\alpha )$, where $\Phi (0)=1$ and $\alpha$ is the star-planet-observer angle. Exoplanet clouds can alter the geometric albedo and the angular scattering law; this observable differs from an exoplanet thermal phase curve.

The Spitzer Space Telescope provided infrared observations of exoplanet secondary eclipses and exoplanet thermal phase curves. Its original mission included infrared spectroscopy; its warm mission retained the 3.6 and 4.5 micrometre imaging bands.