Gravitas: interactive astrophysics sandbox

Transit Photometry

This chart shows the total brightness of the system as seen by a distant observer. When a planet passes in front of a star along the observer's line of sight, it blocks a fraction of the starlight, producing a characteristic dip called a transit.

The dip depth is proportional to (R_planet/R_star)². A Jupiter-sized planet produces roughly a 1% dip, while an Earth-sized planet produces about 0.008%. The rounded U-shaped bottom comes from limb darkening: stars are brighter at their center than at their edges, so the depth varies as the planet crosses the disk.

A subtle secondary eclipse occurs when the planet passes behind the star, briefly hiding its reflected and thermal light. Between eclipses, the smooth phase curve rises and falls as more or less of the planet's illuminated hemisphere faces the observer.

This is the primary detection method used by NASA's Kepler and TESS missions, responsible for discovering thousands of exoplanets.