How space works
How we study planetary atmospheres
Light can carry evidence of gases, clouds, temperature, and motion. Getting that evidence from a distant planet requires separating a faint signal from much brighter surroundings.
Updated
A spectrum spreads light into wavelengths
Atoms and molecules absorb or emit light in characteristic patterns. A spectrum measures how brightness changes with wavelength. Astronomers compare these patterns with laboratory measurements and models to test which gases could explain an observation. Broad coverage and repeated observations can help separate competing explanations.
A single feature is seldom a complete atmospheric inventory. Several gases may overlap in the same wavelength range, and clouds can mute spectral structure. Temperature, pressure, and the instrument's response affect the result. A useful claim identifies the data, the fitted alternatives, and the strength of the evidence rather than presenting every model ingredient as a confirmed detection.
Transmission looks through an atmospheric edge
During a transit, some starlight can pass through the planet's atmospheric limb. An absorbing gas can make the planet appear slightly larger at certain wavelengths. The change is tiny compared with the total light from the star, and the measurement samples the thin region around the apparent edge.
Transmission spectroscopy does not give a complete view of the surface or the whole atmosphere. High clouds or haze may block the deeper layers. Starspots and bright stellar regions can also affect the observed color changes. A flat spectrum could indicate clouds, a compact atmosphere, limited measurement precision, or other possibilities; it is not automatically evidence that no air exists.
Eclipses and phase curves measure heat and light
When a planet moves behind its star, its contribution temporarily disappears from the combined signal. That secondary eclipse can help isolate the planet's emitted or reflected light. A phase curve tracks changes as different portions of the planet face the observer during an orbit.
These methods can test temperature contrasts and atmospheric circulation. A hot dayside and a cool nightside may suggest inefficient heat redistribution, but interpretation still depends on atmospheric properties, surface behavior, and viewing geometry. At some wavelengths a telescope sees high clouds; at others it probes deeper. Every reported temperature belongs to the emitting region and the model used to interpret it.
Close-up missions and distant planets offer different evidence
Spacecraft at Mars, Venus, Titan, and the giant planets can measure atmospheres through instruments in orbit, during entry, or on the ground. Those observations give context for interpreting exoplanet spectra, although no Solar System world represents every distant atmosphere.
In the Codex, a radius or equilibrium-temperature entry does not establish that an atmosphere has been detected. A profile's missing composition remains missing. Follow the individual research reference for atmospheric claims, and check whether a result concerns the whole planet, a limb region, a dayside hemisphere, or one particular gas.