How do astronomers detect gases in an extrasolar planet’s atmosphere during a transit?

To detect gases in the atmosphere of an extrasolar planet during a transit, astronomers utilize spectroscopic studies.12

When a planet passes between its host star and a telescope, it causes a measurable drop in the star's brightness. By analyzing how the depth of this transit varies with the wavelength of light, researchers can identify the presence of specific gases in the planet's upper atmosphere. As starlight filters through the planet's atmosphere during the transit, different gases absorb light at characteristic wavelengths, leaving a spectral signature that reveals their composition.12

This technique has been successfully used to identify various gases, including hydrogen, sodium, methane, and water, particularly in the atmospheres of close-in giant planets.12