Exoplanet atmosphere

An exoplanet atmosphere is the gaseous envelope surrounding a planet beyond the Solar System. Its composition, temperature, and circulation can reveal how the planet formed and evolved.

Transmission spectroscopy: A method that measures how a planet’s atmosphere filters starlight during a transit. Absorption features in transmitted starlight reveal gases in the atmosphere’s outer layers.
Atmospheric composition: The proportions of gases and particles that make up an atmosphere. Composition is inferred from spectral features and helps distinguish atmospheric formation and evolution.
Hot Jupiter: A gas giant orbiting very close to its star, with an orbital period typically measured in days. These intensely irradiated planets provide strong atmospheric signals and were early targets for characterization.
51 Pegasi b: The first exoplanet discovered orbiting a Sun-like star, announced in 1995. Its discovery established hot Jupiters as a surprising planetary population and opened a major target class.
Airless body: A celestial body with no substantial gaseous envelope. An airless planet lacks the absorption and emission signatures used to study exoplanet atmospheres.
Emission spectroscopy: A method that analyzes radiation emitted by an astronomical object across different wavelengths. Thermal emission from an exoplanet can reveal atmospheric temperatures and molecular signatures.
Water vapor: The gaseous form of water, which absorbs radiation at characteristic wavelengths. Water vapor has been detected in some exoplanet atmospheres and is a major spectral target.
Super-Earth: An exoplanet larger than Earth but smaller than Neptune, with a composition that may be rocky or volatile-rich. Atmospheric measurements can help distinguish rocky super-Earths from planets with thick gas envelopes.
HD 209458 b: A transiting hot Jupiter that became the first exoplanet observed to transit its star. Transit observations of this planet enabled early detections of atmospheric sodium and escaping hydrogen.
Terrestrial planet: A planet composed primarily of silicate rock or metal, with a solid surface. Rocky planets may have thin secondary atmospheres unlike the deep envelopes of gas giants.

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