|MadSci Network: Astronomy|
Before we get to why planets and moons are mostly shaped like spheres lets first talk about bubbles. You know the kind I mean, the ones you make by dipping the little plastic wand into the soapy stuff. Although it's true that you can get special wands that can make big giant ones that are long and floppy, the ones I'm talking about are the regular ones that come out as perfect spheres. Because spheres are special, like no other shape.
If you imagine yourself to be really really tiny and sitting at the exact center of a bubble, then the distance to the surface of the bubble is the same in all directions. That wouldn't be the case for a cube, or a shoebox shape, or a potato shape or any other shape you can think of--only a sphere. All places on the surface of a sphere are the same distance from the center. The soap bubble takes this shape because no part of the soap bubble is any different from any other part and all parts are trying to do the exact same thing--get smaller. Just like a balloon, the surface of a bubble really wants to be smaller than it is. It's only because there is air inside pushing out that makes it big. Since each piece of the bubble's surface is trying to get smaller it's kind of like a race between all the little sections of the bubble. But because each part is the same the race comes out as a tie. And when it's a tie no one piece is any closer to the center than any other, in other words you get a sphere. A balloon doesn' t blow up into a sphere for the reason that not all parts of a balloon are the same so the race doesn't come out as a tie.
The shapes of planets are spheres because the sphere is the shape you get when the race is a tie. Each piece of the planet tries to get to the center. A really really long time ago the heaviest pieces of our planet which are made of metal, not rock, won this race and they are now at the center of Earth, metal wins over rock. But each piece of rock that makes up the rest of the planet is still trying to win the race and because most of them are pretty much the same it comes out a tie, and the shape of a tie is a sphere. So planet's are spheres because the sphere is the shape you get when no one piece of the planet is much different from any other and all the pieces are trying to reach the center. Why do the pieces of a planet try to reach the center, I'm sure you know, it's the force of gravity.
Some little moons, like the moons of Mars, and most asteroids don't have enough gravity to start the race to the center. In those cases the shapes can be very bumpy, even like a potato.
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