What I heard almost 60 years ago is that the asteroid belt, where most of the asteroids “live” may have been formed when a planet blew up or was smashed into and fragmented. If you look at a picture of the planets, you see that there is a noticeable gap between earth and mars.
The idea that some sort of lost planet used to be in that orbit has been around since Ceres (about 1/3 the total mass of the asteroid belt) and the other largest asteroids were discovered. I think the guy’s name was Oberth. There’ s more about this on the website of the late Tom Van Flandern.
There’s a mnemonic called “Bode’s Law” (or sometimes “Titius-Bode’s Law”) that includes the asteroid belt between Mars and Jupiter (that’s the first big gap) as a planetary orbit. But the entire total mass of all those asteroids wouldn’t equal Earth’s Moon.
http://milan.milanovic.org/math/english/titius/titius.html
Nope! Close, it is between Mars and Jupiter. ‘Bode’s Law’ is interesting:
http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1917Obs....40..346A&db_key=AST&page_ind=0&data_type=GIF&type=SCREEN_VIEW&classic=YES