Are there other inhabited planets in the universe?

In the book's own words

ALL mortal-inhabited worlds are evolutionary in origin and nature. These spheres are the spawning ground, the evolutionary cradle, of the mortal races of time and space. Each unit of the ascendant life is a veritable training school for the stage of existence just ahead, and this is true of every stage of man’s progressive Paradise ascent; just as true of the initial mortal experience on an evolutionary planet as of the final universe headquarters school of the Melchizedeks, a school which is not attended by ascending mortals until just before their translation to the regime of the superuniverse and the attainment of first-stage spirit existence.