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A triatomic form of oxygen, produced photochemically from molecular oxygen. The ozone layer absorbs ultraviolet radiation; ozone can trace atmospheric oxygen without being directly emitted by metabolism.

An atmospheric thermal inversion is an altitude interval with $dT/dz>0$. Absorption of incoming stellar radiation above the usual thermal-emitting layers can heat the upper gas faster than it cools, producing an inversion. In a semi-grey irradiated atmosphere, a large shortwave-to-infrared opacity ratio favors such high-altitude energy deposition; local infrared emitters and the intrinsic flux also matter.

In the Solar system, Earth and all four giant planets have well-known stratospheric inversions. The ozone layer absorbs ultraviolet sunlight on Earth; methane and photochemical hydrocarbons absorb solar radiation in the giant planets, with aerosols contributing. The giant planets are Jupiter, Saturn, Uranus and Neptune. This refers to their stratospheric temperature rise, not to the gradient at every atmospheric level.

For hot Jupiters, influential factors include the stellar flux and spectrum; the abundances of high-altitude absorbers such as titanium monoxide and vanadium monoxide; atmospheric metallicity of a giant planet and atmospheric carbon-to-oxygen ratio; thermal dissociation, atmospheric photochemistry and condensation; a atmospheric cold trap or atmospheric condensate rainout that removes absorbers; replenishment by vertical mixing; exoplanet clouds and atmospheric hazes; and heat redistribution by circulation. The ratio of visible heating to infrared cooling, rather than a single chemical species in isolation, determines whether an inversion persists.

The pressure axis increases downward in the sketches. Solid red segments indicate efficient convection; blue segments indicate mainly radiative transfer. The boundaries are nominal examples, since radiative-convective boundary pressure depends on opacity, gravity and intrinsic flux.

For Earth, the convective troposphere lies beneath a radiative stratosphere, with the transition near $0.1$--$0.2$ bar. The larger solar-system planets likewise have deep convective regions beneath largely radiative upper atmospheres; their tropopause/upper radiative-convective boundary is commonly of order $0.1$--$1$ bar. Their temperatures differ greatly from Earth's, and detached radiative layers can occur deeper down.

For an irradiated hot Jupiter, absorbed stellar flux maintains a hot, extended, relatively shallow-gradient radiative atmosphere. In models with a weak old-planet intrinsic flux, the deep radiative-convective boundary can lie around $10^2$--$10^3$ bar, with a useful wider model-dependent range of tens to thousands of bars. A thermal inversion can appear at low pressures if visible/UV absorption heats the upper atmosphere. Below the deep boundary the profile joins a convective adiabat.

A young, directly imaged exoplanet on a distant orbit is primarily heated from within. Its photosphere commonly joins the convective interior at order $0.1$--$10$ bar, illustrated here at one bar. The temperature generally rises monotonically with pressure over the infrared-forming layers. Strong stellar heating is absent, so a broad stellar-heated isothermal layer is unnecessary.

Two differences in thermal inversions are their absorbers and their formation conditions. Earth's ozone layer and solar-system hydrocarbon absorption can heat upper layers; the proposed hot-Jupiter absorbers include refractory titanium monoxide/vanadium monoxide or other strong visible absorbers, since ozone and methane-rich cold-planet chemistry are unsuitable at very high temperatures. Hot-Jupiter inversions depend strongly on irradiation, atmospheric cold traps and atmospheric transport, and can occur at mbar-to-sub-bar pressures; Earth and the solar-system giants have cooler, established stratospheres above their shallow tropospheres.

Two differences between the hot Jupiter and distant young-giant profiles are external versus internal heating, and the depth/shape of the radiative zone. The former can have a broad warm radiative layer, a much deeper convective boundary and sometimes an inversion; the latter typically has a steeper outward decrease toward a photosphere, a shallower boundary and no irradiation-driven inversion. These are class trends, not a unique temperature profile for every planet.

Three routes away from local thermochemical equilibrium are:

Vertical mixing, horizontal advection, and photochemistry supply three mechanisms and examples in both exoplanets and Solar-System atmospheres. A photochemical steady state balances production and loss; it is different from a Gibbs free energy minimum. These examples describe mechanisms and model expectations rather than asserting unique observational attribution for every planet.

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.

Let brackets denote number densities and impose a local photochemical steady state. The atomic-oxygen and ozone balances areSubtracting gives $J_1[{\mathrm{O}}_2]=K_4[\mathrm{O}][{\mathrm{O}}_3]$. If photodissociation is the dominant direct ozone loss, $J_3[{\mathrm{O}}_3]\gg K_4[\mathrm{O}][{\mathrm{O}}_3]$, the second balance becomes $K_2[\mathrm{O}][{\mathrm{O}}_2][\mathrm{M}]\simeq J_3[{\mathrm{O}}_3]$. Eliminating atomic oxygen yields the Chapman ozone equilibriumHigh in the atmosphere ultraviolet photons make $J_1$ large but the third-body density is small; low down, $[\mathrm{M}]$ is large but O2-dissociating ultraviolet radiation has been absorbed. Their product peaks at intermediate altitude, producing an ozone layer.

The terrestrial stratosphere lies above the troposphere and has an upward-warming region associated with ultraviolet heating by the ozone layer.