Great setting! I really liked the creative gameover text and the movement with gravity was certainly unique.
Some feedback:
- Better collision: I got stuck on the pixels of the planets a few times. Bullets can also get the player stuck, so they shouldn't collide with it at all.
- Camera: damp it slightly to avoid the background moving around jerkily.
- Bullets: make them faster so they reach the UFOs before you.
Overall, good game. I hope you'll work on it more in the future!