Some people might argue that the question whether aliens exist or not is not something which is relevant to the faith. However, the existence of intelligent aliens would fundamentally change our worldview: Are they fallen, too? If yes, are they saved? If they can be saved, how? Did the Son of God become incarnate as another alien, too?
Let me begin by saying that there is no official, dogmatic Catholic teaching on this subject. However, there are strong reasons against their existence. In this article, I present quotes from the Bible, the Church fathers and other sources which suggest that there are no aliens.
The Bible
There are some Bible passages to consider here.
“The heaven of heaven is the Lord’s: but the earth he has given to the children of men.” (Psalm 113:16)
“It is he that sitteth upon the globe of the earth, and the inhabitants thereof are as locusts: he that stretcheth out the heavens as nothing, and spreadeth them out as a tent to dwell in.” (Isaiah 40:22)
“For I think that God hath set forth us apostles, the last, as it were men appointed to death: we are made a spectacle to the world, and to angels, and to men.” (I. Cor 4:9) Here, angels and men are named as intelligent creatures.
Letter of Pope Zachary
Although this is a private papal letter, but it still shows that the belief in non-human rational beings was opposed by the Church. In the 8th century, Pope Zachary sent a private letter to St. Boniface, a British bishop in Germany, about Virgil, who was an Irish priest and astronomer.
He wrote: “As for his perverse and abominable teaching, which he has proclaimed in opposition to God, and to his own soul’s detriment – if the report of his having spoken thus be true – that is, that there are another world and other men beneath the earth, or even the sun and moon (‘quod alius mundus, et alii homines sub terra sint, seu sol et luna’): take counsel and then expel him from the church, stripped of his priestly dignity.” (here, M. Tangl, ed., “Die Briefe des heiligen Bonifatius und Lullius”, MGH EppSel 1:178-79; quoted in John Carey, PhD, Ireland and the Antipodes: The Heterodoxy of Virgil of Salzburg, published in Speculum Vol. 64, 1989)
It is unclear what happened to Virgil, but since he was canonized, we can assume that he abandoned this belief of his.
Church Fathers
St. Augustine (p.149) was one of the most well-known of the Church fathers. In his work City of God, he rejects the idea of “innumerable worlds” (Book XI, ch. 5) proposed by the Greek philosopher Epicurus. Later on in his book, he condemns those “who, though they do not suppose that this world is eternal, are of opinion either that this is not the only world, but that there are numberless worlds […]” (Book XII, ch. 11)
St. Jerome, the translator of the Vulgate, which was for a long time the official Latin bible of the Catholic Church, counted the idea of “innumerable worlds” as one of the heresies of Origen. (link)
Doctors of the Church
St. Thomas Aquinas clearly states in his Commentary on the Book of Job that there are three kinds of rational creatures:
“Moreover, these three things already discussed can be metaphorically referred to the three kinds of rational creatures. Thus when he says, ‘She is hidden from the eyes of all the living,’ he refers to men; when he adds, ‘she is concealed from the birds of heaven,’ he refers to the angels. When he further continues, ‘Ruin and death said: ‘We have heard of his fame with our ears,’’ he refers to the demons who are removed from God by damnation, in that they have knowledge of divine wisdom only by its reputation from afar.” (The Commentary of St. Thomas Aquinas on the Book of Job, translated by Fr. Brian Mullady, O.P., p. 360, here)
So, according to St. Thomas, there are only people, angels and demons (fallen angels).
Popes
Multiple popes used the phrase “men and angels” when referring to intelligent creatures. Here are some examples:
“[T]he Blessed Virgin is infinitely superior to all the hierarchies of men and angels, the one creature who is closest of all to Christ.” (Pope Leo XIII, Magna Dei Matris 9, here)
“[S]he [Mary] is above all men and angels in glory.” (Blessed Pope Pius IX, Ineffabilis Deus, here)
“From this it follows not only that Christ is to be adored by angels and men, but that to him as man angels and men are subject, and must recognize his empire; by reason of the hypostatic union Christ has power over all creatures.” (Pope Pius XI, Quas Primas 13, here)
Catechism of the Catholic Church
The Catechism of the Catholic Church also contains multiple passages implying that human beings are the only visible rational creatures. We can look at the following passages:
“[…] Man occupies a unique place in creation: (I) he is “in the image of God”; (II) in his own nature he unites the spiritual and material worlds; (III) he is created “male and female”; (IV) God established him in his friendship.” (CCC 355)
“Of all visible creatures only man is ‘able to know and love his creator.’ […]” (CCC 356)
“God created everything for man […]” (CCC 358)