At one time, astronomers thought that two planets orbited between Mars and Jupiter. The planets collided and smashed apart. Their pieces formed the asteroid belt. Most scientists now think those two planets never existed. The gravity of Jupiter is so strong, no planet could have formed between it and Mars. Jupiter's gravity would have kept the pieces from coming together.
When the solar system formed, there were large chunks of material already in the belt. They were held in place by the gravitational pull of
