The Urantia wasn't transmited by aliens but by celestials.


The Inhabited Worlds

(559.1) 49:0.1 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.

(559.2) 49:0.2 All inhabited worlds are basically grouped for celestial administration into the local systems, and each of these local systems is limited to about one thousand evolutionary worlds. This limitation is by the decree of the Ancients of Days, and it pertains to actual evolutionary planets whereon mortals of survival status are living. Neither worlds finally settled in light and life nor planets in the prehuman stage of life development are reckoned in this group...

[link to www.urantia.org]