How do scientists weigh a planet?

It would be a little difficult to plop Jupiter on a scale…

1st July 2018
How do scientists weigh a planet? © Getty

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.

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