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Some visitors to the wonderful planetarium at the Audubon Louisiana Nature Center wondered how we weigh planets. It's not quite as easy as reading the weight on a scale, but in some ways, it's the same basic idea. When you stand on a scale, the scale measures how hard our planet Earth's gravity pulls on you. But how hard the planet pulls depends not only on your weight but also on the planet's weight: The heavier the planet, the greater its gravitational pull. So scientists can weigh Earth by measuring how hard it pulls on something. The first person to weigh a planet was one of my heroes, a scientist named Henry Cavendish who figured out all the details needed to find how much Earth weighs. He and I both were born on October 10, but he's a little older than I am—about 225 years older. How much do you think Earth weighs? It turns out it's a little over 100 trillion trillion pounds, or about 200 billion trillion times as much as you weigh!
To weigh other planets, we can observe how hard they pull on their moons or on spacecraft we send to visit them. The planet's weight determines how fast a moon moves as it orbits the planet. So astronomers measure how far the moon is from the planet and how long it takes to go around in its orbit, and that allows them to weigh the planet.
Gravity and the masses of planets and related topics are so important that they show up in many ways in space science. To learn more about orbits, try shooting a cannonball into orbit!