Thanks for the extensive feedback.
The speed of all Planets is actually fixed, just the direction of the rotation is random. As far as I know, there are no bugs related to the movement, but it just looks like some Planets aren't really moving because of the relative movement of their "parent".
I already had some ideas on getting more depth into the gameplay, for example having Planets with unique effects which could combo with other certain planet-types. And having multiple suns, also with their own stats and effects.
Maybe I'll come back to this game some time and expand it.