I've just discovered this jam and I love It! I'm an enthusiast of planet and sci-fi, and I'm making a mini-game for the jam

I've just released a planet generator tool, that I'm using for my own jam game. Feel free to use it for the jam if you like! 

https://plasmator-games.itch.io/pixel-planet-creator

Also a question, is the restrictions of the jam related to only ONE planet or more than one?

Your game can have as many planets as you want, as long as at least ONE of them is one of the central pieces of your game