Measure transit depth vs wavelength to probe exoplanet atmospheres—see how atmospheric scale height affects absorption
Transmission spectroscopy is a powerful technique for characterizing exoplanet atmospheres. When a planet transits its host star, starlight passes through the planet's atmosphere at the terminator (the day-night boundary). Different molecules absorb light at specific wavelengths, creating a wavelength-dependent transit depth that reveals the atmospheric composition.
The key parameter is the atmospheric scale height H = kT/(μg), where k is Boltzmann's constant, T is temperature, μ is mean molecular weight, and g is surface gravity. The scale height determines how much the atmosphere extends above the planet's solid surface. A hotter, lower-gravity planet with lighter molecules has a larger scale height and thus stronger spectral features.
The effective radius of the planet varies with wavelength: Reff(λ) = Rp + H × τ(λ), where τ(λ) is the wavelength-dependent optical depth. At wavelengths where molecules absorb strongly, the atmosphere becomes opaque higher up, increasing the effective radius and transit depth. The transit depth is (Reff/R*)², typically measured in parts per million (ppm).
High-altitude clouds and hazes can obscure molecular features by providing wavelength-independent (gray) opacity that blocks light before it reaches deeper absorption layers. This is a major challenge in transmission spectroscopy—many hot Jupiters show muted or flat spectra due to thick cloud decks. The interplay between aerosols and molecular absorption determines the observable spectrum.
Transmission spectroscopy requires precise measurements of transit depth at many wavelengths, typically achieved with space telescopes like HST, JWST, or future missions. The spectral resolution and wavelength coverage determine which molecules can be detected. JWST's infrared sensitivity has revolutionized the field, revealing detailed chemical inventories of exoplanet atmospheres including biosignature gases and exotic chemistry.
The first atmospheric detection via transmission spectroscopy came in 2002 when Hubble detected sodium in HD 209458b's atmosphere. Since then, the technique has detected water vapor, methane, carbon dioxide, and even clouds in dozens of exoplanets. JWST's first exoplanet spectrum, released in 2022, showed unprecedented detail in WASP-96 b's atmosphere, ushering in a new era of atmospheric characterization.