In answering this question it is important to recognize that we have no evidence for life other than what is present on the Earth. We do not know of any other place beyond Earth where an indigenous life form exists --- either microbial or multicellular. All life that has been studied on the Earth exhibits such profound commonalities in genome and the core polymers of living processes that essentially all biologists accept a single, common origin for life on the Earth. We know of no alien life forms extant on the Earth.
That said, modern astronomy has established that planets appear to be as common as stars throughout our Galaxy and by extension the Universe as a whole. And given this abundance of planets, it would be surprising were there not literally billions of such planets capable of supporting life like that on Earth --- the only kind of life we know. In our own solar system, no fewer than three worlds other than Earth appear capable of supporting life. Our own planet’s geologic record establishes that life began early --- within the first 10 to 20% of our planet’s history. And so it would not be unexpected to find that simple life forms have arisen on many worlds.
The discovery of life on other worlds would not at all be inconsistent with Catholic belief, since it reflects the ability of the Creator to establish creatures wherever and whenever he wishes. Indeed, just as far more species of higher organisms have existed on the Earth in the past than exist today, we might imagine a universe populated with an enormous variety of life forms as an expression of the infinite power of God’s creativity and his desire to give being to many wondrous forms for which there simply is no room in Earth’s ecosystems.
But what about intelligent, self-aware extraterrestrial life --- what we might abbreviate as ETI’s? Here we cannot automatically assume we are common. Only one such life form has appeared on Earth --- us --- and it has taken evolution four billion years to produce our physical forms. Indeed, some planetary scientists estimate that we have appeared in the final 10% to 20% of the total duration of habitability of the Earth, as the Sun brightens and we move to a climate state in which our oceans will begin to boil away. Perhaps in most planetary systems there is insufficient time for intelligent life to arise --- we just do not know. And we do not even know whether self-awareness, rationality and free will automatically accompany intelligence. We cannot rule out that these powers are unique properties of human beings. For example, might it be possible to have intelligence without rationality or free will? As Catholics we hold to the conviction that God must implant in each human being a soul for these to exist. Science has nothing to say on this.
Suppose we find that ETI’s do exist? What happens to our uniqueness as being formed “in the image of God” if other self-aware material beings exist? From the standpoint of Catholic theology, the following points are clear and uncontroversial: if other embodied beings possessing reason and free will exist in the universe, they would have rational and immortal souls and be made in “the image of God.” God would love them and desire them to be in communion with him. If they were fallen, he would desire their redemption. How would redemption work for them? Would it involve God becoming “incarnate” among them in their own forms? There is no Church teaching on this question, and theologians are divided in their opinions. That such multiple incarnations of the Son of God are not impossible is suggested by the fact that St. Thomas Aquinas held that it was possible for God even to have multiple human incarnations. (See Summa Theologiae, Part III, question 3, article 7: “It is plain that … it has to be said that the Divine Person, over and beyond the human nature which he has assumed can assume another distinct human nature.”) This takes us, however, into the realm of theological speculation and debate.
If it exists, ETI might turn out to be so alien that we cannot comprehend it, which in turn might render moot some of the theological issues outlined here. In the forward to his novel “2001: A Space Odyssey”, Arthur C. Clarke warned his readers about speculating in this regard: “The truth, as always, will be far stranger”.
Resources for Further Study
Christianity and Extraterrestrials? A Catholic Perspective, Marie I. George (iUniverse Publ, 2005).
Would you Baptize an Extraterrestrial?, Guy Consolmagno and Paul Mueller (Image Books, 2014), chapter 6.
Jesus Christ: Fundamentals of Christology, 3rd ed., R. Kereszty, Pt. III, Chapter VI. Alba House, 2002.