How do scientists measure a planet's mass without landing on it?

A planet’s gravity depends directly on its mass, so astronomers can infer mass by observing motion. For planets with moons, the moon’s orbit reveals the planet’s pull; for planets visited by spacecraft, tiny changes in trajectory provide precise measurements. This method has been used for centuries and became especially accurate with radar, radio tracking, and modern spacecraft navigation. For example, the motions of Jupiter’s moons helped confirm its huge mass long before any probe arrived. The same principles are used today for planets, dwarf planets, and exoplanets.