As a Catholic who enjoys reading and writing science fiction, one dilemma I've had to consider is theologically, is the existence of non-human sentient beings possible? At first blush, it may seem problematic because there is no mention of life outside of Earth in the Bible or traditional theology. There is the fact that Jesus Christ (not only God, but a human being) is the saviour for the entire universe -- so where would that leave non-human sentient creatures? It is interesting to consider how they might fit into God's Plan and how we might explain same to nonbelievers or fellow Catholics who might be shaken (some could think the existence of "aliens" means that evolution is true, or that Adam and Eve did not exist, &c.).
I've discussed this topic a number of times with fellow Catholics and there seem to be a few lines of thought. The first question is whether they are ensoulled creatures or not?
One line of thought is that if sentient creatures had souls, then God would have to have a different salvation plan for them than for humans, since they would not be descendents of Adam. They could be more like angels (not fallen) or I suppose they could be fallen and in need of redemption but this raises further issues as Christ is the redeemer of all yet how can he redeem non-human creatures who are not descendents of Adam? However, those who hold this view would say that God might have a totally different salvation plan for life on other planets.
I prefer the "simpler" solution that non-human sentient life could not be ensouled life. I don't really see how ensouled life that didn't descend from Adam and Eve could be compatible with the Creation narrative in Holy Writ. However, if I may quote Steve Skojec, another Catholic writer who I used to correspond with frequently: "[t]here is nothing explicit in our understanding of Christ's redemptive sacrifice for us that would exclude the possibility of other races with immortal souls that could follow alternate, or even similar, paths of redemption." He uses angels as an example of ensouled creatures with a different path from humans. Or maybe there are hints in the "And other sheep I have that are not of this fold: them also I must bring. And they shall hear my voice: And there shall be one fold and one shepherd." (John X,xvi); could the sheep "not of this fold" be ensouled creatures from another world and of different parents than Adam and Eve?
Let me return to what constitutes ensouled life? The soul is rational but this could be a necessary but not a sufficient condition for ensoulment. My view is that, to be ensouled, one must be infused with the faculty to know and love God (even if that faculty, like the rational one, is never actuated by the development of some individuals). It is clear to me that rationality itself does not equal ensoulement because some apes, parrots, mynah birds, and porpoises are supposedly self-aware but not ensouled and not able to comprehend the concept of God.
I think it could therefore be possible to have highly intelligent, sentient, even civilized and technologically advanced beings than nevertheless have no souls. I think an excellent speculative example of this are the "Moties" in the novel, The Mote in God's Eye. Unfortunately, the book only has the ship's chaplain pondering the question of whether they're ensouled in one scene and I really thought they should have developed that theme more (I'm not sure why they even bothered with that one scene).
While the question is never answered in the novel, it is my opinion that the aliens encountered by humans in the book (the "Moties") are indeed soul-less creatures yet highly intelligent (in fact, their technology is superior to humans' and they can develope new technology at a terrifyingly fast rate). I say they are souless because they really have no ability to choose between right and wrong -- everything they do in the novel is dictated by their biological imperatives. I don't want to ruin the book for any of you if you've not read it.