In preparation for a more detailed game loop, I've begun work on a new planet screen. The plan is that it will both become a planet selection screen and a shop where you can buy powerups. Instead of each level having a timer, your whole game session will have a timer. If you choose to move to a new planet, you will lose time but gain access to more valuable resources.
More to come on this - very far from done but for now it's functional in that you can, at least, choose which level you want to go to.