transit method
Hold a small ball in front of a distant lamp and the lamp dims a touch as the ball crosses it. Now do that across light-years. If a planet's orbit happens to be edge-on to us, the planet passes directly in front of its star once each orbit and blocks a tiny sliver of its light. The star does not move, does not change color — it just dims, very slightly, for a few hours, and then brightens back. The transit method finds planets by catching these faint, repeating dips in a star's brightness.
The dip is small. A Jupiter-sized planet blocks about 1 percent of a Sun-like star's light; an Earth-sized planet blocks only about 0.01 percent — one part in ten thousand, like noticing a single dimmed bulb in a stadium of ten thousand. But the dip carries gold: how deeply the star dims tells you the planet's size, because the bigger the planet's disk, the more light it covers (the transit depth equals the ratio of the planet's area to the star's). The interval between dips gives the orbital period, and the same dip seen again and again, perfectly on schedule, confirms it is a planet and not a passing fluke.
The transit method is how we found most of the thousands of known exoplanets, through missions like Kepler and TESS that stared at huge fields of stars at once. Its power is that it measures a planet's radius and, with follow-up, opens the door to studying its atmosphere. Its built-in limits are real: it only works for the small fraction of systems tilted edge-on toward us, and a lone dip can be faked by a passing star or instrument glitch, so confirmation usually needs the radial-velocity method or repeated, identical transits.
Kepler stared unblinkingly at one patch of sky for years, watching about 150,000 stars at once for these tiny dips. A planet that blocks just 0.01 percent of starlight, repeating like clockwork every few hundred days, betrayed thousands of unseen worlds — most found this way, never glimpsed directly.
A planet's shadow dims its star by a tiny, repeating amount — and reveals its size.
The transit method gives a planet's size, not its mass, and works only for orbits seen nearly edge-on; combining it with radial velocity yields both size and mass, hence density.