Hi!
Very cool entry, I like the take on the theme, and overall the game mechanics is very easy to understand, which makes it enjoyable to play.
I had some trouble understanding how to move the camera at first, and the background not following the camera yet zooming in and out made me have a strange feeling when starting to play. Also, it's a shame that everything is so small on screen, like I could almost not read the prices for upgrades, and the ships/bullets was also very small on screen was zooming out to see the full planet. The game seriously looks good, so I wish everything would be bigger to enjoy the artstyle to its full potential!
The defense gameplay was fun, the only thing that frustrated me a bit it is that there's no aim prediction; your ships will fire at an enemy ship's position, but that ship will move, so your bullet will not touch it. Either an aim prediction or faster bullets would fix the issue, I think (faster bullets are easier to implement and would add more juiciness to the game).
Anyway, pointing out what's negative is always easy, but don't get me wrong, I liked playing your game and I think it's really well made.
Congrats!