Transit method

The transit method watches a star's brightness and waits for a planet to pass in front of it, blocking a sliver of the light. The dip is tiny, often a fraction of a percent, but it repeats like clockwork once per orbit, which is how we know it is a planet and not noise. The depth of that dip gives the planet's size directly, from pure geometry, which makes transit one of the most trustworthy ways to measure a radius.

It only works when a planet's orbit happens to line up edge-on to us, so it misses most worlds, but the ones it catches arrive with a size and a year already attached. Space telescopes like Kepler and TESS turned it into the workhorse of exoplanet discovery: the large majority of the worlds we know were found this way.