Asked by: Paul Scanlan, Kent
We can weigh a planet (or, more correctly, find its ‘mass’) by using Newton’s Law of Gravitation. This law tells us that a planet exerts a gravitational pull that is proportional to its mass.
If the planet has a moon, then this gravitational pull must be balanced by the centripetal force that keeps the moon in orbit. In this case we only need to measure the moon’s orbital period and the distance between the planet and its moon to reveal the planet’s mass. For planets without moons, we can still use their effect on other planets to infer their masses.