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While the radial velocity method provides information about a planet's mass, the photometric method can determine the planet's radius. If a planet crosses (transits) in front of its parent star's disk, then the observed visual brightness of the star drops by a small amount, depending on the relative sizes of the star and the planet. For example, in the case of HD 209458, the star dims by 1.7%. However, most transit signals are considerably smaller; for example, an Earth-size planet transiting a Sun-like star produces a dimming of only 80 parts per million (0.008 percent).

A theoretical transiting exoplanet light curve model predicts the following characteristics of an observed planetary system: transit depth,  transit duration, the ingress/egress duration, and the period of the exoplanet. However, these observed quantities are based on several assumptions. For convenience in the calculations, we assume that the planet and star are spherical, the stellar disk is uniform, and the orbit is circular. Depending on the relative position of an observed transiting exoplanet to a star, the observed physical parameters of the light curve will change. The transit depth of a transiting light curve describes the decrease of the normalized flux of the star during a transit. This details the radius of an exoplanet compared to the radius of the star. If an exoplanet transits a solar radius size star,  a planet with a larger radius would increase the transit depth and a planet with a smaller radius would decrease the transit depth. The transit duration of an exoplanet is the length of time that a planet spends in front of the star. This observed parameter changes relative to how fast or slow a planet is moving in its orbit as it transits the star. The ingress/egress duration of a light curve transit describes the position of an exoplanet relative to the surface of the star. If a planet transits a star relative to the diameter of the star, the ingress/egress duration is shorter because it takes less time for a planet to fully cover the star. If a planet transits a star relative to any other point other than the diameter, the ingress/egress duration lengthens because the planet spends a longer time partially covering the star during its transit. As the exoplanet transits the star, the dip in normalized flux causes a change in the observed light curve.