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Why are all planets and stars round?

Why are all planets and stars round? I realise the answer is likely to be gravity but that doesn’t explain why they should all be perfectly round, especially if material has been attracted to them from different quarters assuming it was attracted material that formed them in the first place!
Derek Gouldsworthy from Kent (Age 55+)

Why are planets spherical?
Richard Glover from West Midlands (ge: 35-44)

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One Response

  1. You’re right – it’s to do with gravity. And the strength of the material that your planet is made of. Rocks are only so strong – you can’t build a mountain arbitrarily high, because the rocks are not strong enough to support themselves under gravity. The bigger the planet, and the stronger the gravity, the closer to a perfect sphere. If you think of asteroids as little planets – the very small ones are all sorts of shapes, but as you gradually get bigger you get closer to a sphere – Ceres and Vesta, the largest, are pretty close to spherical. But nothing is a perfect sphere. On Earth, we have topography on the order of 10’s of kilometers. Even on a neutron star, with much higher gravity, there will be some topography.

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