This method only works for star-planet systems that have orbits aligned in such a way that, as seen from Earth, the planet travels between us and the star and blocks some of the light from the star.
A planet does not block much light from a star, (only 1% or less) but this can be detected. Like the radial velocity method, this method has a bias towards discovering large planets orbiting close to their stars, because larger planets block more light and transit more frequently so they are easier to detect. This method will not work for all systems, however, because only about 10% of super-Jupiters are aligned in such a way that we see them transit. Smaller planets in larger orbits are even less likely to be aligned in such a way that we can observe transits. For planets that do transit, astronomers can get valuable information about the planet's atmosphere, surface temperatures and size. Over 180 planets have been discovered by this method (as of 7 December 2011).
