Super fun concept! Congrats on creating such a polished-looking game. I'd definitely play the hell out of a full version of this. I also loved the music. Here are a couple of notes that may be useful:
1. Personally, I'm not a fan of putting a timer on the shop phase.
2. I pressed the exit key a couple of times, expecting to pause the game, but that quit the entire game. Not ideal, especially for a jam, since that could be the difference between people playing the game or not.
3. The timer on the shop led me to make a few rushed mistakes at first, like trying a run without any attacking planets, so it would be nice to have a tooltip show up if you're about to make that mistake.
4. During the sell phase, I was expecting that hovering over a planet would tell me the planet's name