How do scientists measure the size of a planet without visiting it?

For Solar System planets, astronomers can use direct imaging, radar, occultations, and spacecraft flybys or orbiters to measure size precisely. For planets around other stars, they often measure how much starlight is blocked during a transit, which reveals the planet’s radius. Combined with gravitational data or Doppler measurements, they can also estimate mass. These methods allow scientists to compare planets even when they are millions or trillions of kilometers away.