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Why Are All The Planets Round?

The planets are all the same shape because they're so MASSIVE.

Big thing like planets have incredibly strong gravity, which pulls everything into the centre. If you did have a cube-shaped planet, gravity would eventually pull the corners in, making it round, like a ball.

So, were the planets once cube-shaped?

No. When our solar system was forming, gravity pulled gazillions of bit of dust and gas into clumps which grew bigger and bigger to become planets. But because everything on the surface was pulled evenly into the centre, the planets have always been round - right from the start of the universe.

Are very small planets round, too?

Tiny heavenly bodies, such as asteroids, have odd shapes because they've got less gravity. But as planets get bigger, their gravity gets stronger. On a big planet like ours, gigantic objects on the surface would be crushed under their own weight. That's why we don't have mountains 50 miles high or skyscrapers that are 3,000 stories tall. (Although very clever people are working on it!).

PS: Planets and stars are not really perfectly round. They spin, so they bulge out a bit around the equator.

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