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Atmospheric escape removes gas from a planet through thermal motion, hydrodynamic outflow, photochemistry, and interactions with stellar radiation and plasma.

Atmospheric photochemistry begins when a molecule absorbs a photon and dissociates or ionizes. The products can drive reaction networks whose steady abundances differ strongly from thermochemical equilibrium.

This is the atmospheric instance of a scale height.

For an isothermal ideal-gas atmosphere, $H=k_{B}T/(\mu m_{H}g)$. A strong transmission feature spanning $N_H$ scale heights has approximate transit-depth amplitude $2R_p{N}_{H}H/R_{\ast }^2$.

CRIRES is a high-resolution infrared spectrograph. Its upgraded form supports molecular spectroscopy of exoplanet atmospheres and measurements based on the Doppler effect. See ESO's instrument overview.

An exoplanet is a planet outside the Solar system; here the focus is on planets orbiting stars other than the Sun, including pulsars. Most exoplanets are detected indirectly: a star's changing Doppler shift reveals its motion, or a small dip in its light curve reveals an exoplanet transit. The striking idea is that an unresolved point of light can tell us about an otherwise unseen world's mass, radius, and orbit.

The measurements answer different questions. Exoplanet transit photometry primarily gives a radius ratio, while Doppler spectroscopy constrains mass together with orbital inclination. Combining them with the host star's properties gives a mean mass density, ${\bar{\rho }}_p=3M_p/(4\pi R_p^3)$, useful for exoplanet interior models. Even this combination does not uniquely determine composition: see exoplanet interior-composition degeneracy. Transit spectroscopy and an exoplanet secondary eclipse then probe an exoplanet atmosphere.

The discovery history has two distinct milestones. In 1992 Aleksander Wolszczan and Dale Frail reported two planet-mass companions to the pulsar PSR B1257+12 from periodic changes in pulse arrival times; their paper also suggested an additional companion. These were the first confirmed exoplanet discoveries, around a neutron star. ^[ref]. Wolszczan's contemporary conference account, archived by NASA, likewise distinguishes the two reported companions from a possible third. ^[ref].

In 1995 Michel Mayor and Didier Queloz discovered 51 Pegasi b through Doppler spectroscopy, the landmark first confirmed exoplanet around a Sun-like star. This was a hot Jupiter, not the first exoplanet of every kind: the earlier pulsar discoveries belong to a different host category. ^[ref]. NASA's contemporary account describes the confirmation and the approximately four-day orbital period. ^[ref].

An exoplanet atmosphere controls the planet's spectrum, heat redistribution, chemistry, clouds, and atmospheric escape.

Exoplanet atmospheric dynamics includes day-night circulation, zonal jets, waves, vortices, storms, and heat redistribution under irradiation, rotation, drag, and magnetic effects.

An exoplanet cloud is a suspended condensate aerosol that scatters or absorbs radiation. Its particle sizes, composition, vertical extent, and horizontal coverage shape transmission, reflection, and thermal-emission spectra.

Exoplanet direct imaging spatially separates planetary light from host-star light. It is most sensitive to young, hot, self-luminous giant planets on wide orbits around nearby stars.

An exoplanet emission spectrum is the wavelength-dependent thermal radiation emerging from the planet. Vertical temperature gradients turn opacity differences into absorption or emission features.

An exoplanet transmission spectrum measures the wavelength-dependent area blocked as starlight passes tangentially through the atmosphere during transit.

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.

Atmospheric haze consists of small suspended particles, often produced photochemically. A haze can flatten molecular features or produce a wavelength-dependent scattering slope.

Hot-Jupiter radius inflation is the anomalously large radius of many strongly irradiated gas giants relative to isolated cooling models. Delayed cooling and energy deposition by tides, atmospheric circulation, or electrical currents are leading contributions.

Methods of detecting exoplanets infer or resolve an exoplanet through its effects on the host star's light or through its own light. The observable determines what can be learned:

An exoplanet secondary eclipse measures the loss of the exoplanet's light when it passes behind its star, complementing exoplanet transit photometry. A detection method also selects which systems are easiest to find: deeper exoplanet transits favour larger radius ratios, and larger stellar radial-velocity semi-amplitudes favour more massive companions, shorter orbital periods, and larger $\sin i$. These preferences must be accounted for when inferring the underlying exoplanet population.