I wanted to blow up a planet, as I heard that mining the fragments created less server lag than mining the whole thing (I am playing in multiplayer). I completely exposed the core with salvage beams then fired a missile that does 10.5 million damage at it. This was a missile/beam/explosive 3500/3500/3500 module missile. I had the planet core selected in nav, the targeting reticule directly on the core and the targeting circle green. The missile appears to do nothing. The planet core reads as still having it's full 10 million HP. Most of the time the missile appears to miss entirely, but sometimes I get a brief flash of red text (too brief to read) that seems to indicate hit damage, but the actual displayed HP of the planet core remains unchanged. I have spent easily half an hour recycling the missile and refiring to zero effect. It looks to me like missiles are having zero effect on planet cores.