In general one should not take Agadic texts at their simple level as there are profound messages hidden inside the words of each Agadah. In this case, Rabeinu Bachye (Bamidbar 10:35) understands the worlds to be time periods of 50 years, in which the world will renew itself, but he warns not to think about this matter as it concerns what will be after the destruction of the present world and one may not ponder this matter (see Chagigah 11b).

Rabbi Yosef Albo writes that "since the universe was created for the sake of man, no other creature can exist possessing free will. Since any extraterrestrial life would neither have free will nor be able to serve a creature having free will (as terrestrial animals and plants serve a terrestrial man), they would have no reason for existing and therefore be totally superfluous" (Ikarim 4:8).

However, others are willing to contemplate extraterrestrial life which exists without free will (Sefer ha'Bris 1:3:3).

There is a source that says that there are 18,000 Tzadikim around Hash-m (Sukah 45b), and since every Tzadik is like a world in his own right, the worlds could be referring to Tzadikim who are closer to Hash-m than the angels (Rashi ibid.).

Yoel Domb