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How do scientists use reflected light to study the atmospheres of extrasolar planets?

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Planet TOI 700e — extraterrestrial life An illustration of the Earth-size planet TOI 700e orbiting within the habitable zone of its star. The habitable zone is the area around a star where liquid water, and thus possibly life, can survive on a planet's surface. Its Earth-size sibling, TOI 700d, can be seen in the distance.

Scientists study the atmospheres of extrasolar planets primarily through techniques that analyze how light interacts with the planet, rather than relying solely on reflected light. While direct imaging can utilize starlight reflected off a planet, this method is extremely challenging because stars are significantly brighter than their orbiting planets.12

Instead, researchers most commonly use transit photometry. When a planet passes between its host star and a telescope, the planet's atmosphere filters the starlight. By performing spectroscopic studies that measure variations in the depth of the transit across different wavelengths, scientists can identify the chemical composition of the planet's upper atmosphere, detecting gases such as water, methane, hydrogen, and sodium.1

Additionally, the concept of albedo—the fraction of light reflected by a body—is used in broader astronomical contexts to describe reflective properties, though it is more frequently applied to characterizing the surfaces of satellites and asteroids or the energy balance of planets rather than direct atmospheric profiling of distant exoplanets.3