my granddaughter ask me why are planets round,and other space objects are not (asteroids etc)?

Q:

A: According to the universal law of gravitation, there is a force of attraction between every two particles of matter that is directly proportional to the product of their masses, and indirectly proportional to the square of the distance between them. That means, the more mass there is, the stronger the force of gravity. Roundness is the most natural shape for a liquid to take in space because when all the particles are attracted to each other, a round object has the lease amount of potential energy. Smaller objects, like astroids, do not become round because their hardness is not overcome by the force of gravity between the parts. Larger objects, like planets, become round because there is so much mass, that the force of gravity is able to overcome the hardness of the planet itself, and it acts similar to a liquid. But even planets are not PERFECTLY round and smooth. Earth, for example, has mountains. But if you'll notice, the ocean is a lot more smooth than the land. That's because water flows more easily than rock, and is able to reach a lower state of potential energy.

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