Consists of measuring the change in brightness of a star when one of its planets passes in front of it, thereby obscuring part of it. If this decrease is perceived periodically, it can be inferred that an opaque object is orbiting the star. The amount of light lost depends on the sizes of the star and the planet. The duration of the transit depends on the distance from the planet to the star and on the stellar mass. Since the mass and size of the star can be determined by spectroscopic observations, the size of the planet and its distance can be determined from the light curve.
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