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What is the Transit Method?

The transit method of detection of exoplanets can only be applied to exoplanets whose orbit is oriented in such a way that for observers on Earth, the planet periodically eclipses its parent star. The motion of the planet across the disk of the star is called the transit, and during the eclipse, the star dims by a tiny amount. The dip in the light signal as a function of time is then used to constrain some physical parameters of the system. The dip is illustrated in the diagram below.

transit method diagram for observing exoplanets

See also this animation of a transit using real data.

The transit method is biased towards finding the largest planets. In transiting systems, the inclination angle of the orbit with respect to the observer is fairly well constrained simply from the fact that a transit is observed. In other methods the inclincation angle may be an unknown and introduces uncertainty in the planet's mass. However, the transit method does not yield the exoplanet mass because it is a purely geometrical method and does not involve measurements of dynamical parameters. On the other hand, combining the transit method with the radial velocity (RV) Doppler-shift method for a given system provides powerful complementary information because the inclination angle from the transit method can be used to remove the ambiguity in the mass obtained from the RV method alone.

Note that you do need to know the radius of the host star in order to get the planet's radius from the transit method, otherwise you will just get the ratio of the radius of the planet to that of the star.

Learn more about exoplanets with Exoplanets and Alien Solar Systems.

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