TinWiki: Asteroid belt, page 1

The asteroid belt is a high concentration of space debris orbiting the Sun between Mars and Jupiter. The composition of the asteroid belt rocks are 92% silicates and the rest made of metallic elements. A question was posed to NASA about the asteroid belt, and they hypothesied that it was either a destroyed planet or planetary material that had failed to create a 'complete planet'. One hypothesis though is that the massive gravitational pull of the planet Jupiter prevents this material from being able to cohese into a stable body long enough to create a planetary form.

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Relevant discussion threads

Was The Asteroid Belt a planet???