Cool game ! The concept of switching planets work really nicely, and I like the fact that you have to carefully aim before shooting since the fire rate is so low. The graphics are clean, and hitting the missiles feels really good. As for the music... well, it's a bit repetitive, I must admit that I've muted the game after the 5 first minutes x) It's not that it's bad, but I think you should have made it more progressive, like it gradually gets more intense as the planets are getting bigger for example.
Adding powerups would have been a great way to fight the repetitivity, for example something that makes your ship move faster or that makes your porjectiles bigger. There seem to be a small issue concerning screen sizes as well, which is that missiles coming form the upper and lower sides of the screen are way harder to destory since the aspect ratio of the screen has more horizontal space than vertical space.
Overall it still was a great entry, and I really enjoyed playing it ! Congrats !