Could it be that you did not use Time.DeltaTime for rotating the planet? This could explain why the rotation speed was ok for you and is too high for me. Just an idea.
But having an option for the rotation speed sounds good. A slider would be cool.
Well, actually after your last comment I went there to have a look and I discovered that as long as I use
Input.GetAxis("Horizontal") I can be assured that it is frame-independent by default so I shouldn't be worried about Time.deltaTime in this case, I can leave it out. I guess the only option here is to have an option menu where the player could set the sensitivity, that should solve the problem, yes a slider in a settings menu would be good.