Really like the game idea. You need some time to get the hang of it, especially to know which projectiles to shoot at which planets, but you learn it (the hard way :D). The music and sound was good, but i have some general improvement ideas:
- change shooting from spacebar to left mouse button
- but the power indicator in the middle. Maybe color the circles of your aim line which would look cool.
- implement a zoom function to hit planets better that are further away.