Transmission spectroscopy

Transmission spectroscopy analyzes starlight filtered through a planet’s atmosphere as the planet passes in front of its star. Wavelength-dependent absorption reveals atmospheric gases and aerosols.

Transit spectroscopy: Spectroscopic study of a planet as it transits its star, using changes in starlight to infer planetary properties. Transmission spectroscopy is the atmospheric-analysis method within this broader transit-based approach.
Exoplanet atmosphere: The gaseous envelope surrounding a planet orbiting a star beyond the Solar System. Transmission spectra provide direct constraints on these atmospheres.
Emission spectroscopy: Spectroscopy that studies radiation emitted by a source, often across different wavelengths. It samples a planet’s emitted light rather than starlight passing through its atmospheric limb.
Astronomical transit: An event in which one celestial body passes in front of another from an observer’s viewpoint. A planet’s transit creates the changing starlight used for transmission spectroscopy.
Transmission spectrum degeneracy: The ambiguity by which different atmospheric compositions and structures can produce similar transmission spectra. Cloud height, abundances, and temperature can trade off in fitting the same observed features.
Transit depth: The fraction of a star’s observed light blocked by a transiting object. Its wavelength-by-wavelength variation forms the transmission spectrum.
Sodium in exoplanet atmospheres: Sodium absorption detected in the atmospheres of planets outside the Solar System. Prominent sodium lines were among the first atomic features identified in an exoplanet transmission spectrum.
Direct imaging of exoplanets: Observation of a planet’s light separately from the much brighter light of its star. Direct imaging avoids the transit geometry required for transmission spectroscopy.
Exoplanet: A planet orbiting a star outside the Solar System. The technique is used chiefly to study exoplanet atmospheres during their transits.
Starspot contamination: Bias in planetary transit measurements caused by unocculted or occulted regions of differing stellar brightness. Stellar surface heterogeneity can mimic or distort wavelength-dependent atmospheric signals.

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