The Science and Theology of Extraterrestrials

Adapted by the author for Sed Contra from the latest version of a lecture developed for chapters of the Thomistic Institute.

ONE OF the great discoveries in the past two decades is that planets are common, very common. We have gone from only knowing the planets in our own solar system, to finding thousands around other stars, and in the process, discovered that close to all stars have planetary systems around them. In our galaxy, the Milky Way, there are around three hundred billion stars. That means that we have hundreds of billions of planets within our galaxy alone. What we do not yet know is if these planets are only astronomical bodies, or if they are worlds in their own right, that is: if they host life.

If we’re thinking about the likelihood of life outside the solar system, we’re mostly interested in planets that are not too different from the Earth—not too far and not too close to their stars and therefore temperate enough to sustain liquid water. (I will come back to why liquid water is so important.) A few percent of stars in our galaxy may have at least one of these temperate planets, so even if they might not be the most common planet in the galaxy, there are still many of them.

Why do we focus on water? You might think astronomers are being very Earth-centric and focus on water because Earth’s life is water-based. However, water is a really good solvent for chemical reactions, and we think that life originated out of chemistry. If you need complex chemistry, you need a place for the chemicals to get together and build up.

But how often do these temperate planets actually end up with water on them? To answer that question, we have to look at interstellar clouds, which are over-densities of particles from which stars and planets form. Star and planet formation begins when such a cloud starts to collapse in on itself. As the collapse proceeds, most of the mass goes into the center, forming the star, but about one percent gets spread out in a spinning disk. This disk is where planets form. To form an Earth-like planet, dust particles come together in the disk to form the bulk of the planet. Water and other chemicals of interest to origins of life can be delivered in three different ways: You can trap them followed by outgassing from the rock, you can accrete gas that contains water vapor, or you can have impact from comets and asteroids that contain water ice. This all presupposes, of course, that you have water molecules in the planet-forming disk to start with.

If we turn our telescopes towards interstellar clouds, water is one of the most common things we see. We have a lot of “heavy water” in our solar system, and the only way we can explain its existence is that it was inherited from interstellar clouds. If our solar system formed with all this interstellar water, we think that other disks and planets should as well. In fact, we do have more direct evidence of water in disks from turning our telescopes directly towards them. Perhaps surprisingly, we see so much water in these disks that many exoplanets may form with too much water, rather than too little.

How about organics—that is, the kind of chemicals that are needed for origins of life? How often do we expect to form planets that also have these substances available? After all, the whole reason we were interested in water in the first place was to provide a solvent for chemical reactions. We have been using telescopes to take chemical images of different planet-forming disks, and it turns out that organic molecules are also incredibly common. First glance would tell us that planets should not just form in a way that is water-rich, but also organic-rich. There is one caveat, however: we don’t always see these molecules spread out evenly across the disk. It might be that planets form within quite different chemical environments, but overall, we expect a large number of them to be chemically hospitable to life.

When we put all this together, we expect most planets to form in disks that are rich both in water and organic molecules. If we add the observation that temperate planets are quite common, habitable planets (planets that are the right temperature, have water, and have buildings blocks for origins of life chemistry) should be present in considerable numbers as well.

“It might be that planets form within quite different chemical environments, but overall, we expect a large number of them to be chemically hospitable to life.”

This all sounds very promising, but it does not mean that you can assume that extraterrestrials are common. We don’t know exactly how life came into existence here on Earth, and that makes it difficult to predict how often life might originate in other places, even if the basic physical and chemical pre-conditions are met. It might be that habitable planets are common, but inhabited planets are not. Here is where facts end, and we enter into more speculation.

When carrying out such speculations we need to first consider what kind of life we are talking about. We will consider three different categories: single-cell bacteria-like life, animals, and rational animals. In popular discourse, it is often assumed that if there is any kind of extraterrestrial life, you should also expect intelligent extraterrestrials, but this goes against both good science, and good philosophy and theology.

EXTRATERRESTRIAL BACTERIA

Let’s start with the simplest kind of life we know of: single-cell organisms, perhaps similar to terrestrial bacteria. Bacteria are actually extremely complex creatures, but are relatively simple compared to the other two categories. One way to try to estimate how often you get these creatures on other planets is to look back at our own history and think about how they might have come into existence here on Earth.

Around 4.2 billion years ago, the Earth was temperate. A common assumption is that the young Earth did have a deep reservoir of small organic molecules, which then reacted to form more complex molecules, which combined to form macromolecules, and then began to present the functions and structures that we associate with life. We don’t know the precise time at which life first emerged on Earth. What we do know is that around 3.6 billion years ago, life was present on the Earth, and it was very abundant, and it was already extremely complex. That suggests that it had been around for quite some time, and it may have taken only tens or hundreds of millions of years for life to originate here after the Earth first became habitable. If life originated this quickly, we should perhaps expect it to happen often on other planets as well, and I think that is the intuition of most scientists. However, expecting something is very different from knowing it, and we can’t say whether this theory of the origin or prevalence of life is correct until we construct a more complete theory of origin of life on Earth and find life on at least one other planet.

How should we think about the origin of life from a Christian perspective? Is it reasonable to posit that there is something in how chemistry functions that naturally guides it towards biology? On the one hand, it seems to superficially take God out of the process of creating life—if science explains more, then God is needed to explain less. It may seem a bit threatening to a Christian world view. But that isn’t a good way to view how God creates in the world. God typically interacts with the world through natural causes. He endows creation with the dignity to be able to change matter. A universe where created things bring forth new goods is, in a way, a much more impressive creation than one where new things come into existence only through a direct action by God.

Second, if the law of chemistry points to biology, then it suggests we live in a universe that was fundamentally created to bring forth life. That is powerful, because it brings us as living beings back to the center of the universe. We are what the universe is aiming towards.

“It may seem a bit threatening to a Christian world view. But God typically interacts with the world through natural causes. He endows creation with the dignity to be able to change matter. ”

So where that does leave us? We don’t understand origin of life well enough scientifically to make strong predictions about the prevalence of life in the Universe. It may be that we are alone, that we are the arc carrying with it all the life of the universe. This is not the universe I’m hoping for, but it does have a beauty to it: It shows how contingent we are, how precious the gift of the earth is. But, while we wait to see what kind of universe we live in, I will happily assume that extraterrestrial bacteria exist and move on to extraterrestrial animals.

EXTRATERRESTRIAL ANIMALS

On the one hand, bacteria and animals are not so different, in that they are both natural, material creatures. But it is useful to separate them, because it is not obvious that you will always get animals just because you have life. You may think that once you have life, Darwinian evolution will take over, and more complex animals will rapidly emerge. But if we look at the Earth, this is not what happened. We had origins of life maybe 4 billion years ago, and then it took 3.5 billion until animals appeared. This suggests that it’s not easy to evolve from single cell creatures into more complex ones. We might very well end up having many planets that have only bacterial life on them.

Why does it matter whether we have a small number of worlds with animals or many? St. Thomas says that all the living creatures show something of God’s goodness and glory that could not be shown without them. God is infinite, which means that no finite creature could point to him fully. The more instances of creatures we have, the more aspects of his goodness are revealed. That makes me hope that he has created a universe that has more interesting life forms than could ever exist on Earth alone. Perhaps equally important, only intelligent animals would be a fitting precursor for rational ones.

EXTRATERRESTRIAL RATIONAL ANIMALS

In the end, the main kind of extraterrestrial life that most people are interested in is the possibility of extraterrestrial rational animals, beings that resemble ourselves. It’s important to distinguish these from very intelligent animals, such as large monkeys or chimps. Here we’re talking about animals with a rational soul, with a will and intellect similar to our own.

On Earth, it seems like evolution produced intelligent animals rather quickly, at least compared to the lifetime of the Earth. Then starting around five or six million years ago, creatures that resemble us physically began to appear. These were ape-like creatures that walked on two legs and that developed and used some simple tools. Out of these, the body of our humanoid ancestors appears a few hundred thousand years ago. Is Darwinian evolution then sufficient to explain humans?

Even if we just stay with the science, uninformed by theology, a very intelligent animal is not necessarily recognizably human. Humans produce art, reason, language, and so on. That did not happen for all intelligent animals—as far as we know, it only happened for one. And even for homo sapiens, it may not have happened immediately. The biological homo sapiens were around for some time before they started to develop clear signs of culture about fifty thousand years ago. This is when beautiful cave paintings appear, where you can see creativity and joy in the interesting shapes and colors.

THEOLOGY OF EXTRATERRESTRIALS

From a Christian point of view, we have an explanation for this. Within the Christian tradition, the answer is that what makes us truly human is a rational soul, which did not evolve naturally, but rather was specifically created by God. This means that rational aliens also cannot emerge naturally. If there are rational aliens, they exist in a very direct and specific way, because God wants them to exist. That should be comforting—we’re not accidentally going to run into an alien civilization. It also means that we cannot presume that there are alien rational animals, even if extraterrestrial life turns out to be common. Still such creatures are logically possible, and we should consider their theological implications.

One concern is that the existence of extraterrestrial rational animals seems to make humanity less central to creation. Their discovery would fit with a story that as history progresses, we have gone from being at the center of the universe to being more at the periphery. First our home, Earth, went from being at the center to orbiting the sun, then we learned that we all orbit the center of the galaxy, and shortly thereafter that we live in one of many galaxies. Now we know we’re one of many planetary systems. The possibility of extraterrestrials means that maybe we’re not even the only living planet. Why then would God care for us? What makes us so special?

But we do know that God cares. This is the place he chose to become Incarnate, and even without the Incarnation there is no reason why God’s friendship with humanity should depend on whether there are other rational creatures out there. If we end up having alien “siblings,” that does not mean that God cares or loves us any any less. On Earth, there are already billions of fellow rational creatures, and we do not think that we’re in competition with them for Christ’s love. Furthermore, the existence of the other kinds of intelligent being we know to exist—angels—has only been to our benefit.

The real potential controversy is instead linked to the Incarnation. We believe that we are saved through the Incarnation. If we are saved that way, how are our hypothetical aliens saved? Could there be a second Incarnation? St. Thomas Aquinas, surprisingly, says yes. Now St. Thomas did not believe that there had been or would be a second incarnation, but he was certain that God did not lose his power to become Incarnate by becoming Incarnate once. With this in the back of our mind, we can consider the logical possibilities for how this hypothetical rational alien race could be rescued from alienation from God.

“The real potential controversy is instead linked to the Incarnation. If we are saved, how are aliens saved? Could there be a second incarnation? St. Thomas Aquinas, surprisingly, says yes. ”

One possibility is that they don’t need it. We think of the fall as natural, because we are all children of the fall. But logically, falling away from God is the most irrational thing you might possibly do. We could have a universe of extraterrestrials who are looking at us and our disobedience in disbelief, as well as in wonder at God’s rescue.

Option two is that they exist, they fell, and they have not been saved. This is a logical possibility, but it does not seem consistent with how far God went to rescue us.

Option three is they have fallen, and that they’re saved through the same Incarnation that saved us. Paul’s letters say the Incarnation put the whole universe back in order and restored some of the directionality towards God. There are at least two problems with this idea, however. First, Paul also indicates that Jesus’s descent from Adam matters, and second, it would be difficult to fathom why we received the Incarnation and its awesome friendship with God while our alien brothers and sisters do not.

Option four is that they exist, they are fallen, and they were saved through some other means, for example their own incarnations. Even though God certainly has the power to such a thing, positing multiple incarnations makes many people, me included, uneasy. Somehow, that doesn’t seem like a thing that should happen over and over again.

In the end none of these options seem very satisfactory, and as a result there are Catholic thinkers who have concluded that it’s most likely rational extraterrestrials do not exist—that there might be many kinds of life in the universe, but no rational animals outside of Earth. I oscillate between what I think is the most likely option, but today, I’m enjoying contemplating the first possibility, and how it brings back into focus just how unnatural our fall was, even though we are right to call it our “happy fall” for the Incarnation that came to our rescue.

Professor Öberg obtained her B.Sc. in Chemistry from Caltech and her Ph.D. in Astronomy from the University of Leiden. She has taught at Harvard since 2013, where she is the Thomas Dudley Cabot Professor of the Natural Sciences. Her scholarship aims to uncover how chemical processes impact the outcome of planet formation, with special attention to the possible habitability of nascent planets. She has published over 250 refereed articles, including in Nature and Science. Professor Öberg has been awarded the Barry Prize by the American Academy of Sciences and Letters (2024), the Harnack Lectureship by the Max Planck Society (2022), a Simons Investigator Award (2019), the American Astronomical Society's Newton Lacy Pierce Prize (2016), a Packard Fellowship (2014), and a Alfred P. Sloan Fellowship (2014). She is the Vice President of the Angelicum Board, a Board Member of the Society of Catholic Scientists, a member of the American Academy of Catholic Scholars and Artists, and a frequent public speaker on questions of science and faith.