Gravity pulls from all sides
Why Is Every Planet a Ball?
Anything big enough pulls itself into a ball, because gravity tugs equally from every direction.
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Step 1 of 5
Gravity pulls towards the centre from everywhere.
Every part of a large body is dragged towards its middle, and that pull is the same strength in every direction around it.
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Step 2 of 5
A ball is the only shape that satisfies it.
If every point is pulled equally towards the centre, the settled shape has every point the same distance out. That shape is a sphere.
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Step 3 of 5
Small bodies are too weak to manage it.
An asteroid's gravity cannot overcome the strength of its own rock, so it keeps whatever lumpy shape its collisions left it with.
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Step 4 of 5
Spinning squashes the ball slightly.
A fast-spinning planet bulges at its equator because material there is carrying on straight while the planet turns beneath it.
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Step 5 of 5
Being round is part of the definition.
One of the official tests for planethood is whether a body's own gravity has pulled it into a round shape, which rules out the lumpy ones.
Rock cannot support a lump taller than about ten kilometres on Earth, which is why no mountain beats Everest by much.
The short version
Planets are balls because at that size gravity pulls equally from every direction and rock is not strong enough to hold any other shape.
Try it yourself
Watch a drop of water fall in slow motion. With nothing else acting on it, it pulls itself into a sphere for the same reason.
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