It has been suggested that a planet called Nibiru, which orbits the Sun in the opposite direction to the other planets and takes several thousand years to complete it orbit, hit a planet. The result is the Earth and the Moon. On a previous orbit it is supposed to have struck a different planet and the result of that is the Asteroid ring. I do not know if this is true but the ancient Sumarians wrote of this and said they had been visited by the inhabitants of Nibiru, the Annunaki. This is why some fear a return of Nibiru and wonder if this is what the Maya anticipate in their calender. Hope this is of help.