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The atmospheric carbon-to-oxygen ratio compares the elemental abundances of carbon and oxygen in the atmosphere. It influences molecular composition under chemical equilibrium, but condensation, disequilibrium chemistry in an exoplanet atmosphere, and incomplete spectral coverage complicate its inference from molecular bands.

The atmospheric metallicity of a giant planet describes its atmospheric inventory of elements heavier than hydrogen and helium. It must specify a convention: an elemental abundance ratio relative to hydrogen, an enhancement relative to a reference composition, or a total heavy-element mass fraction are related but different quantities. A quoted enrichment also needs to identify its reference, such as the host star.

Atmospheric retrieval estimates molecular abundances, which must then be connected to elemental inventories through a model of chemical equilibrium or disequilibrium chemistry in an exoplanet atmosphere. Atmospheric condensate rainout can hide elements in particles below the observed levels. Thus an atmospheric enrichment need not directly equal the heavy-element fraction of the whole exoplanet interior.

A chemical system is out of local chemical equilibrium when reaction adjustment cannot keep pace with transport, imposed changes or external driving. Comparing the chemical relaxation time with transport and irradiation times identifies plausible departures. Disequilibrium chemistry in an exoplanet atmosphere combines this principle with mixing, horizontal advection and atmospheric photochemistry.

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.

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.

These are three distinct ways to maintain disequilibrium chemistry in an exoplanet atmosphere; each compares chemistry with a different transport or irradiation process.

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.

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.

Four possible molecular signatures of disequilibrium chemistry in an exoplanet atmosphere, measured through an exoplanet transmission spectrum or exoplanet emission spectrum, are:

The first two signatures can be consequences of the same process and are not independent evidence for two mechanisms. Changes in atmospheric carbon-to-oxygen ratio, atmospheric metallicity of a giant planet, exoplanet cloud coverage, and the atmospheric pressure-temperature profile can mimic abundance changes. A convincing inference compares several molecular bands with a chemically consistent equilibrium model rather than identifying one unusual band alone.

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.

Assume all three planets retain approximately their original hydrogen-helium bulk composition. Stellar encounters change orbital energy and irradiation, not automatically the planet's elemental abundances. Their atmospheric structures can become approximately stationary long before their interiors finish cooling.

For the unperturbed Jupiter-like planet at $a\simeq 5.2\mathrm{au}$, the zero-albedo globally averaged planetary equilibrium temperature is about $122\mathrm{K}$. Stellar light heats the outer atmosphere, while internal cooling supports a deeper temperature gradient and convection. At cooler pressures, chemical equilibrium favors methane and ammonia; condensate clouds can include ammonia at high levels and water deeper down. Disequilibrium chemistry in an exoplanet atmosphere can preserve carbon monoxide or other species from deeper layers.

For the inward-migrated planet at $a\simeq 0.052\mathrm{au}$, the irradiation-only planetary equilibrium temperature is ten times higher, about $1220\mathrm{K}$, because $T_{\mathrm{eq}}\propto a^{-1/2}$. Its irradiated planetary atmosphere has a heated radiative exterior, potentially strong day-night differences, and a deep radiative-convective boundary. At suitable pressures, chemical equilibrium increasingly favors carbon monoxide over methane; water remains important, while alkali absorption and high-temperature condensates can matter. An atmospheric thermal inversion depends on absorbers, clouds, and irradiation and is not guaranteed simply by migration.

The ejected object is a rogue planet. Its irradiation-based planetary equilibrium temperature becomes very small, but its actual emitting temperature is set primarily by internal cooling and Kelvin-Helmholtz contraction. After only several million years it can remain warm and self-luminous, with deep convection and an outward-cooling radiative atmosphere. Its photospheric chemistry and clouds depend on that cooling temperature; they cannot be inferred from the absence of a host star alone. None of these cases fixes an exact pressure-temperature profile without a cooling model, opacities, and atmospheric composition.

At fixed temperature and pressure, a closed reacting system is in thermochemical equilibrium when its Gibbs free energy is minimal subject to elemental-abundance constraints. For every independent reaction,with a positive second variation in every allowed direction.

An atmosphere can be driven into disequilibrium chemistry in an exoplanet atmosphere by vertical mixing faster than chemical conversion, which causes chemical quenching; ultraviolet atmospheric photochemistry; and atmospheric escape. Lightning, energetic particles, horizontal transport, condensation, and rainout provide further mechanisms.

Three observable signatures of disequilibrium chemistry in an exoplanet atmosphere are vertically quenched carbon monoxide in methane-favored cool layers or methane in carbon-monoxide-favored hot layers; quenched ammonia-nitrogen chemistry; and photochemical products such as hydrogen cyanide, sulfur dioxide, hydrocarbons, or hazes. Their abundances constrain mixing, ultraviolet irradiation, reaction kinetics, and atmospheric escape.

Solar-System methane and gravity-field measurements imply that Jupiter is enriched in heavy elements by a few times solar, Saturn by roughly an order of magnitude, and Uranus and Neptune by tens of times solar. This motivates an inverse planetary mass-metallicity trend: lower-mass planets acquire a larger heavy-element fraction from solids relative to hydrogen-helium gas. Carbon-to-oxygen and nitrogen abundances also constrain ice lines, planetesimal accretion, core erosion, and migration.

Uniform exoplanet transmission and emission surveys can test whether this trend is universal across mass, equilibrium temperature, age, and host metallicity. Measurements of water, methane, carbon monoxide, carbon dioxide, ammonia, and sulfur species can separate bulk enrichment from carbon-to-oxygen ratio, clouds, and disequilibrium, thereby constraining where and how planets formed.