Transmission spectroscopy is a technique used to gather details about the chemical composition and the extent of the atmosphere of a transiting exoplanet. As the light from the host star passes through the atmosphere of the planet, some of the light is absorbed by the atoms, molecules or grains present in the atmosphere, making the exoplanet appear bigger. A plot of the size of the exoplanet as a function of wavelength gives a transmission spectrum. The shape of the transmission spectrum itself then can indicate which components are present in the atmosphere. An example of a transmission spectrum is shown below.
Transmission spectroscopy has been used to extensively to study the atmospheres of HD 209458 b and HD 189733 b amongst many other transiting planets.