Exoplanet atmospheric dynamics

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.

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.

This factor converts substellar incident flux into the representative flux used in an atmospheric model. Uniform global averaging gives $f_r=1/4$; dayside averaging gives $1/2$ if albedo is treated separately. The convention must be stated when defining an irradiation temperature.

If a fraction $ϵ$ of total absorbed stellar power reaches the night side, blackbody balance gives

A column near pressure $p$ contains mass per area of order $p/g$. Linearizing its blackbody cooling around temperature $T$ gives the estimatewhere $c_p$ is specific heat capacity at constant pressure. This estimate omits details of wavelength-dependent radiative transfer, but captures faster relaxation in hotter and lower-pressure atmospheric layers.

The atmospheric advection time over length $L$ at wind speed $U$ is ${\tau }_{\mathrm{adv}}\sim L/U$. Day-night heat redistribution tends to be efficient when this time and the relevant wave-adjustment time are shorter than the radiative relaxation time in a planetary atmosphere.

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.

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.