How do astronomers detect exoplanets by the transit method?

The transit method looks for a periodic drop in starlight caused by a planet crossing the face of its star. The amount of dimming tells astronomers the planet’s size relative to the star, while the time between transits gives the length of the orbit. NASA’s Kepler mission, launched in 2009, was designed to detect these tiny signals and found thousands of candidate planets. Follow-up observations can confirm the planet and sometimes measure its density when combined with radial velocity data. This method works best when a planet’s orbit is edge-on from our viewpoint.