The transit method is based on the observation of a star's small drop in brightness, that occurs when the orbit (dashed line) of one of the star's planets passes ('transits') in front of the star. The amount of light lost -typically between 0.01% and 1%- depends on the sizes of the star and the planet; and the duration of the transit depends on the planet's distance from the star and the star's mass. Since the star's mass and size can be determined from spectroscopic observations, the planet's size and distance can be determined. Of course, a transit has to occur once for every orbital revolution of the planet around the star. This repeated occurrence of transits is the major diagnostic tool to determine if an observed transit is really from a planet - it has to appear once in each of the planet's 'year'.
The repeatability of transits gives also the opportunity to observe a known transiting planet in the future with improved instrumentation - one day it may be possible to examine a planet's atmosphere (by spectroscopy of absorption lines from its atmosphere while it is transiting) and check it for indicators of life - such as the presence of free oxygen in the atmosphere.
