Transmission & Emission Spectroscopy: Physical Mechanisms

A transmission spectrum records small changes in apparent transit radius with wavelength. Emission measurements can compare system light before and during secondary eclipse or separate planet light directly. Molecular features, scattering slopes, clouds, and temperature gradients shape the result.

Atmospheric gases absorb and emit at characteristic wavelengths. During transit, wavelengths with stronger opacity are blocked at higher altitudes; in emission, the atmospheric temperature-pressure profile controls whether molecular bands appear in absorption or emission.

Space telescopes have detected water, carbon-bearing molecules, alkali metals, clouds, and thermal structures in exoplanet atmospheres. Repeated observations and retrieval models quantify molecular abundances with uncertainties and degeneracies.

At wavelengths where opacity is stronger, a transit samples higher atmospheric altitude. Emission features depend on the temperature-pressure profile and whether layers are hotter or cooler with height.