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How can transit spectroscopy reveal gases in an exoplanet’s atmosphere?

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Habitable zone The habitable zones (green) for stars that are like the Sun (middle), hotter than the Sun (top), and cooler than the Sun (bottom). The red areas are those in which liquid surface water would be lost as a result of a runaway greenhouse effect, and the blue areas are those in which liquid surface water would be completely frozen.

Transit spectroscopy is a powerful technique used in astrobiology and astronomy to identify the chemical composition of an exoplanet’s atmosphere. When a planet passes in front of its host star, it causes a measurable drop in the star's brightness.1

As the planet transits, some of the starlight filters through the planet's upper atmosphere. Gases present in that atmosphere absorb specific wavelengths of light, leaving behind a unique "fingerprint" in the resulting spectrum. By observing variations in the depth of the transit across different wavelengths, astronomers can detect the presence of specific gases, such as hydrogen, sodium, and methane.1

This method is particularly effective for studying the atmospheres of large, close-in extrasolar planets, providing critical data on their composition and potential habitability.1