Sitchin's theory proposes the planets Tiamat and Nibiru. Tiamat supposedly existed between Mars and Jupiter. The planet's orbit was disrupted by the arrival of a large planet or very small star (less than twenty times the size of Jupiter) which passed through the planetary system between 65 million and four billion years ago. The new orbits caused Tiamat to collide with one of the moons of Nibiru. The debris from this collision are thought by the theory's proponents to have variously formed the asteroid belt and the current form of planet Earth.