When an exoplanet passes directly in front of its star (a transit), it blocks a tiny fraction of the star's light — and that dip in brightness is how most known exoplanets have been discovered.
When an exoplanet passes directly in front of its star (a transit), it blocks a tiny fraction of the star's light — and that dip in brightness is how most known exoplanets have been discovered. The size of the dip depends only on how big the planet is relative to its star.
A Jupiter-sized planet (radius ≈ 69,911 km) transits a Sun-sized star (radius ≈ 695,700 km). Depth = (69911/695700)² ≈ 0.0101, or about 1.01% — a dip large enough for ground-based telescopes to detect.
It's the small dip in a star's observed brightness caused by a planet passing in front of it (transiting) from our viewing angle.
Transit depth ≈ (planet radius ÷ star radius)², since the dip in brightness is proportional to how much of the star's disk area the planet blocks.
Even though the planet itself isn't visible, the tiny, periodic dimming pattern it causes reveals its presence, size, and orbital period.