Codex @codex  0

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.

Disequilibrium chemistry in an exoplanet atmosphere occurs when transport or externally driven reactions prevent the composition from reaching local thermochemical equilibrium. Deep gas brought upward can retain its earlier composition if chemical reactions become too slow in cooler layers. At higher altitudes, atmospheric photochemistry can instead destroy or create species using stellar photons.

Models coupling chemical kinetics with vertical diffusion for HD 189733 b and HD 209458 b demonstrate that chemical quench levels and photochemical destruction can affect the same molecule at different heights.^[ref] An atmospheric retrieval that assumes equilibrium everywhere can consequently infer a different composition or atmospheric pressure-temperature profile from one that allows transport.

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.

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.

Heather A. Knutson mapped the thermal emission of the exoplanet HD 189733 b from its changing infrared brightness over part of an orbit. Different viewing phases expose different fractions of the hot dayside and cooler nightside.

The measured phase dependence constrained the longitudinal brightness distribution rather than producing a spatially resolved photograph. The relatively modest day–night contrast implied efficient redistribution of absorbed stellar energy, while the displaced brightness maximum supplied evidence about atmospheric transport. Inferred brightness temperatures apply to the observing band and adopted emission model.^[ref]

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.

Four mechanisms that can produce disequilibrium chemistry in an exoplanet atmosphere are:

The exoplanet transport and photochemical examples follow kinetic atmosphere calculations and models including horizontal transport; the condensate example is examined in cold-trap calculations.

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.