The Church has no direct teaching on extraterrestrials, and so Catholics are free to speculate. But, using Catholic and scientific principles, some of the following parameters and distinctions may be interesting or helpful.
In the first place, we ought to define “extraterrestrials.” If by this term we mean “persons,” then it is clear that there are myriads of extraterrestrials since angels are persons. We are human persons, but they are angelic persons. And, since it is the teaching of the Church that God makes use of His angels to guide all His creation, then angels are everywhere in the universe, assisting in God’s governance of all things.
If by extraterrestrials you mean biological life of any sort, all the way from simple paramecia to more complex life forms, it seems likely that there is some of this type of life out there, especially less complex forms of life.
If, however, you mean humanlike creatures with physical bodies and rational souls, the likelihood drops significantly, but is not zero. Nowadays, some argue that there are thousands or even millions of places where humanlike life could emerge. Others say such places are exceedingly rare. Both sides use statistics.
Those who think there are many Earth-like planets capable of sustaining humanlike life-forms point to the vast size of the universe with billions of galaxies, each with billions of stars and presumably a lot of planets. They argue that, statistically, there must be many stars with planets like our own.
Others, however, point out that for the Earth to be what it is, and to be able to support our kind of intelligent life, is not just the result of a few factors. It is the result of hundreds, even thousands of factors. The sun must be stable, and just the right kind of sun without too much of the wrong radiation. The planet must have an orbit that is closer to a circle than a steep ellipse. (Earth is only three degrees from a perfect circle, whereas most of the other planets have steep ellipses that bring the planets relatively closer to the sun and then farther.) The atmosphere must have just the right kind of gases. It must be a volcanic planet that generates those gases and also generates a magnetic field that deflects the most harmful rays of the sun. The Earth also benefits from Jupiter and Saturn nearby, which catch comets and other space debris. The Earth also has a good balance of land and water, and the continents are rather evenly distributed. This limits deserts and other weather extremes. Earth also has a steady rotation on a good axis that permits seasons, and the proper distributions of rain, and an almost perfect relationship with our moon so that tides are steady and ocean currents are assisted. The list could go on for a long time. However, the statistical probability of all these same sorts of things coming together in this or a similar combination on distant planets is rather low. There is just a kind of “God-given” perfection to Earth that is hard to replicate statistically. Scientifically, this is called “Rare Earth Theory.”
All that said, there are no theological problems with holding that there is intelligent humanlike life on other planets. But even if there is, we would still have to ask: Do they have free will? Do they have a conscience? Has God revealed himself to them? How? Did they sin and need redemption? Did God join their race or family by becoming one of them? Will they go to heaven? If so, why has no one who has a vision of heaven ever seen them there?
At any rate, the speculations are numerous. As for us now, God has not revealed anything specifically about life elsewhere. Evidentially, He doesn’t think we need to know too much about this. Thus we do well to spend only a little time wondering of it.
Rev. Msgr. Charles E. Pope is a priest of the Archdiocese of Washington, D.C.