Why are planets round

A.� Because their mass is constantly being pulled to the centre by their gravitational field. A planets gravitational field behaves as if it were at the planets core. Over time the planets, which behave like a fluid due to the effects of internal radioactive elements that crate an enormous amount of heat, fall under the influence of the pull of their own gravity.

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The best way for all the planets mass to get as close to the centre as possible is to form a round ball. The technical term for this process is 'isostatic adjustment'.

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Additionally when most planets form they're partly molten which makes gravity's job easier.

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Q.� If everything is being pulled inwards, why are there mountains on Earth

A.� Although there are mountains on our planet, none of them is higher than 15,000 metres. However on Mars, which is a smaller planet and therefore subject to a weaker central gravitational pull, some mountains are as high as 25,000 metres.

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Q.� Why aren't asteroids perfect spheres

A.� Because they're not big enough. To form a sphere a body has to be large enough to generate a sufficiently strong gravitational pull. An asteroids gravitational pull isn't strong enough to overpower its internal strength and they remain irregular in shape.

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As rocks, asteroids are subject mainly to their internal electromagnetic forces. These forces can be in any direction since the positive and negative charges cancel each other out, giving rise to their irregular shape.

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Q.� So erosion has nothing to do with it

A.� No, although erosive forces do shape our visible world the general spherical shape of planets and stars from the long view is determined by their size and the related forces that act on them.

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Do you have a question about our universe Click here to ask.

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by Lisa Cardy