Interplanetary weaponry?

That's actually a reasonable way to balance them. Give the other guy plenty of time to move out of the way.

That's actually really true. I hadn't thought of it in that regard. I mean, so long as you actually had an advanced enough radar/sensor to notice things going on with other planets/nearby space, you should be able to figure out the trajectory of munition to a fair degree of accuracy and know where it's going to land at. With the further along its course the better the accuracy of knowledge so you can be like, "Oh crap, I'm getting sniped from another freaking planet, alright where is it coming in at? Okay, round about this area... so I'm going to go ahead and move units out of there, start building up some replacement factories over here, and maybe go ahead and disassemble the ones I had built there so I don't lose the resources completely."

we need a giant laser to destroy planets, we need nukes too as a bonus. check out my post, we need a death star!!!