Needs some difficulty
So I played this game for about 5 boring minutes (waaay too slow, too few asteroids, fire rate too fast, difficulty ramping way to slow) and then tried to lose by letting the planet die.
THIS IS WHERE I FOUND THE TRUE CHALLENGE
For you see, the only way for the planet to die, is for every building/tree to be hit, and the planet itself, which seems Impossible. Apparently on top of the asteroids not aiming for key targets, they seem to aim Away from key targets, thus making this game an "avoid the flying objects" game.
On a more serious note, this game, though it looks good and I didn't find any problems with the controls at all, would Really benefit from a difficulty increase. I noticed slight difficulty ramping (though I noticed it was in number only, and not speed) but it was entirely too slow.
Powerups that actually Power You Up might be nice too. In such a cliché genre, you really need to stick with conventions. Fire increase, weapon upgrades, secondary fire, etc.
Perhaps some form of boss would be of benefit too (unless there was one, but I didn't stick around long enough to find out). All in all, I think this was a good try. I might check out your other submissions too.
If this is your first soirée into this genre, I hope it acts as a learning experience for you. Take what you did right, fix what you did wrong, and come back to us with a much improved Planet Protect 2.
Good luck in all your future flash endeavors.
P.S. I just wanted to note you made an excellent choice of music. It's very simple, fits well with the game, and doesn't get stale. The change in speed/beat varies it just enough to make it not annoying.