Nice take on the theme and nice idea overall.
I didn't really like the sound effects and think the music doesn't fit but bonus internet points for composing it yourself (from my understanding)
I agree with one of the previous commenters that this kind of gameplay would have even more potential if it the game wasn't just scrolling. One idea would be to have enemies spawn from all around you converging on you (or maybe the planet you are supposed to defend?) with you moving all around the screen defeating them in all directions.