I think you should be able to blow up planets at the highest level. Sort of an endgame special. Once a planet has been blown up the chunks of the planet can have easily obtainable exposed ore or other special things. Also it would be fun. =) -benamation
I suppose by this point we'll have planet scanners too so there'll be stuff like 'There is a [race] city here, are you sure you wish to proceed?'