The transit method detects a planet by the small dip in a star's brightness as the planet crosses in front of it. The depth of the dip gives the planet's radius relative to the star, and the interval between dips gives the orbital period. It only works for systems whose orbits happen to be edge-on from our viewpoint, a minority, so surveys must watch enormous numbers of stars. Kepler and TESS have found thousands of planets this way, and starlight filtering through a planet's atmosphere during transit is how those atmospheres are analyzed.