Most of the Arkadian Ring is air of course.  It wraps around an entire star, and there are not enough rocks left in the entire binary system to pave all the way around.  So Arkadia is made up of islands of rock floating in the air while they orbit around the star.  The center has air thick enough to breath unaided, and while it thins towards the edges, all aircraft are limited to atmospheric speeds when traveling between islands.  It can take a while to get anywhere.