The transit method of exoplanet detection is a photometric method, based on multiple measurements of the brightness of a star. Around 5 - 10% of exoplanet systems are aligned such that the exoplanet passes across the face of its parent star. This transit is not seen directly in images that we might take, but the light from the star dims in a measurable (and repeating) pattern (see Figure 1). For a perfect alignment, coupled with a faint star and a giant planet, this dip could be as high as 2% which is easily measurable using photometry software.

Figure 1: The characteristic dip in brightness as an exoplanet crosses the face of its parent star (note: this effect typically causes a drop in brightness of 0.5 - 2 %).
Credit: NASA Ames

This method lends itself to a survey type telescope such as the COROT and Kepler spacecraft. Kepler stared at a small region of the sky around the constellations of Cygnus and Lyra. A survey of this type was also successfully employed by the ground-based WASP (Wide Angle Search for Planets) consortium.

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