Good game overall !
I would suggest you to add an hit indicator, it could be simple color flash. Because i'm not sure to hit the planet, and it is even more disturbing for the boss, i wasn't sure to be able to beat it. Plus the bullets disappear a bit far from the boss (like if the sprite have some border).
Also about the UI i would integrate more level, heat, fuel, score who seems a bit far and like outside the game.
But really nice project, keep up the good work :)