Question: Where did the asteroid belt come from? How was it formed?

Susan Cartwright answered on 15 Jun 2015:

We know that young stars are surrounded by large discs of gas and dust – we can see these in star formation regions like Orion or Taurus. We believe that large planets form when dust grains collide and stick together to make first pebbles, then boulders, and finally planets. In the asteroid belt, this process did not get as far as making full-sized planets, but stopped after making objects with sizes of up to a few hundred kilometres across (the largest asteroid, 1 Ceres, is about 950 km across). This is because the asteroid belt is too close to Jupiter – Jupiter’s gravity keeps distorting the orbits of the asteroids, so that gentle collisions causing the colliding objects to stick together become more difficult. In fact, collisions between asteroids do occur, but they do so at rather high relative speeds, and are more likely to break one or both of the colliding objects into pieces than to result in the formation of a bigger object. (We see the results of such collisions as “asteroid families” – sets of asteroids with very similar chemical composition and orbital parameters, each set representing the fragments of a larger body that has been broken up.)

Jupiter’s gravity also probably explains why Mars is such a small planet compared to Venus and the Earth: much of the rocky material that would have gone into the formation of Mars was swept out of that part of the solar system by Jupiter, resulting in a shortage of raw material and thus a rather undersized planet.