Very cool!
Currently the journey between station is a bit annoying. I guess you could introduce a new game mechanic for it in a bigger game. Anyway, very addictive tower defense style.
You need to put something that show player is interacting with planets and aliens. A destruction animation or a score, because currently you shoot but don't really notice if you are right.
Maybe that animation is the one in your weapon (is it a vacuum cleaner?), but it is very subtle.
The idea is good.
Wow, that Gameboy speakers sound very well! I don't remember Gameboy sounding with so quality. (I have not problem with HTML and music. It sounds)
It is required a way to skip introductions, please.
I feel physics a bit weird. I understand it is the gameplay you choose, but I prefer a bit more control.
Nice game.