Woah ...
Earth, as we now know it, formed from the collision of two similarly sized planets, called Heaven and PreEarth (called Heaven, so that, the collision can be viewed as the mythical marriage between Heaven and Earth). These two, once comprised a double planet system. Heaven orbited PreEarth, and they both orbited the Sun (just like the Earth and Moon today, except that Heaven, with a radius some ninety percent that of PreEarth, was much larger than today's Moon).
Do you have any idea how hard and unlikely that is to have happened?
Do some basic gravity calculations and try to get orbital stability for both the pre and post crash ... have you tried?
Forget everything else at the moment you would have to show me that you could get stability for both configurations which is very very very unlikely.
Examples 6,7 and 8 on here will show you how ahrd this is going to be
http://faculty.ifmo.ru/butikov/Projects/Collection1.html 
