One more comes to mind, maybe a bit whack:   
      
   Maybe two gigantic planets collided and formed jupiter made out of gasses from   
   this enormous explosion.   
      
   So some kind of planet collision might also explain the missing planet.   
      
   Bye for now,   
     Skybuck.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)