When an exoplanet's orbit is aligned edge-on to our line of sight, it periodically passes directly in front of its star and blocks a small fraction of the starlight. Telescopes like Kepler and TESS watch a star's brightness for years and flag these tiny, periodic dips — a "light curve" — as candidate planets. The geometry of the dip tells you the planet's size; its timing tells you the orbit.
depth ≈ (Rp / Rs)²
T14 ≈ (P/π)·asin( √((1+Rp/Rs)² − b²) / (a/Rs) )
P(yr)² = a(AU)³ / M★(M☉) — Kepler's third law
- Planet / star radius — bigger planets block more light, deepening the dip roughly as the square of the radius ratio.
- Orbital distance — sets the orbital period (via Kepler's third law) and, at fixed star radius, how sharply the star's disk curves under the planet's path.
- Star mass — heavier stars pull the planet around faster at a given distance, shortening the period.
- Impact parameter (b) — how far off-center the planet's chord crosses the star's disk (0 = through the middle, →1 = grazing the limb). A grazing transit is shallower and shorter.
- Limb darkening — real stellar disks appear dimmer near their edge; toggling it rounds the light curve's bottom instead of leaving it flat, which is what real Kepler/TESS light curves actually look like.
Real relevance: this exact depth/duration geometry is how missions like Kepler, TESS and PLATO catalogue thousands of exoplanets purely from ground- and space-based photometry, without ever directly imaging the planet.