Oh, I didn't realize it was possible to unlock the planets in the wrong order. Though, when I started playing, most of the planets weren't implemented at all, so it's possible I just unconsciously thought there was some order to them when there isn't. The walkthrough at least suggests there's an intended order.
Though, Urukai (at least currently) has the most difficult missions, so that kinda makes it feel like it's intended as a late game planet. Then again, Harmonia is definitely the easiest and it was implemented pretty late.