Managed to get 22 planets! It was simple and fun. I was going to see if I can hop back and forth between planets to cheese the game but I'm glad you disallowed that :) There's definitely potential to expand upon this with some more sense of progression and variations in planets. Maybe you could just randomize the planet size a bit for example or have some planets that are spinning so your speed is different.
One improvement with regards to level generation is that the volcano planets should generate to the top-right of you or if they do, they should be rotated. It's possible to still make it but it's pretty tight since you can easily jump straight into the volcano.