In order to figure out how heavy a planet is, scientists need to know two things: how long it takes nearby objects to orbit the planet and how far away those objects are from the planet. For example, the closer a moon is to its planet, the stronger the planet will tug on it. The time it takes an object (whether it’s a moon or spacecraft) to orbit a planet depends both on its distance from the planet and how heavy the planet is.
Why do scientists usually talk about mass rather than weight?
An object’s weight is dependent on its mass and how strongly gravity pulls on it. The strength of gravity depends on how far away one object is from another. That’s why the same object weighs different amounts on different planets. It’s sometimes easier to compare planets using a measurement that isn’t quite so complicated. That’s why scientists and engineers often measure an object’s mass—how much matter the object contains—rather than its weight.
Mass stays the same regardless of location and gravity. You would have the same mass on Mars or Jupiter as you do here on Earth.