You know, now that I was replaying it, it occurred to me that there should be some way to create even bigger planets. Clearing more than the ~10 tiles you usually clear per planet could be very satisfactory, on some sort of time-attack mode.
I'm thinking of some mechanism so that when the 9-tiles-rule is satisfied, the blocks don't disappear right away but allow you to keep on adding pieces for a short period of time that gets renewed which each new piece added to the "planet" (although then I see nothing that could prevent you from just keep pressing space to get all the pieces as the game is now, but I guess that's a matter of being more strict on what qualifies as a planet, or adding more rules for what can be attached to what - maybe based on the color of the tiles or something...)
Anyway, just sharing my thoughts, I hope it says something about how good this game is that I keep coming back to it! ;)