Measuring planets by the light they block
When a planet passes in front of its star from our viewpoint, it blocks a small fraction of the star's light. A Jupiter-sized planet crossing a Sun-like star dims the light by about 1 percent; an Earth-sized planet dims it by 0.01 percent. Modern photometers (including the Kepler and TESS space telescopes) detect these tiny brightness dips in thousands of stars simultaneously. The transit method has discovered over 70 percent of known exoplanets because it is sensitive and systematic.
Transit depth reveals the planet's radius directly: a larger planet blocks more light. The transit period reveals the orbital period. Repeating transits allow precise measurements, stacking observations to detect planets dimming light by parts per million.
Orbital geometry and the detection bias
Transits only occur if the planet's orbital plane is nearly edge-on from Earth's perspective. An orbit perfectly aligned with Earth will show transits every orbit; a randomly-tilted orbit will never transit. This geometric requirement means the transit method preferentially detects planets in compact systems where orbits are aligned. As a result, known exoplanets are biased toward short-period, tight-orbit systems, not representative of all planetary systems.
Combining transits with radial velocity measurements yields planet density and composition. A transiting Jupiter detected by its light-blocking but showing tiny radial velocity signals is very different from a system where both measurements are large.