Are we alone in the heavens? Is there life, including intelligent life, elsewhere in the universe? What would be the implications for humanity and our relationship to God if there are other intelligent beings? And what are the implications if we truly are alone?

These questions have captured the human race for millennia. The second-century Assyrian  writer Lucian of Samosata, a city of the Roman Empire in modern-day Turkey, wrote what may be the first known example of a science-fiction novel, about a visit to some aliens on the moon and Venus. The third-century Christian theologian Origen believed that stars and planets were rational, spiritual, living beings far higher in the spiritual hierarchy than human beings. However, some of them fell due to sin, and thus endured the punishment of serving as the souls of human bodies. Later, the fifteenth-century French monk Guillaume de Vaurouillon argued that “Infinte worlds, more perfect than this one,” each with their own life forms, “lie hid in the mind of God.”

The modern history of the idea of extra-terrestrial life coincides with the Copernican revolution, the gradual discovery that the Earth is not the centre of the universe. As throughout most of human history, in sixteenth-century Europe, people thought that the whole of the universe revolved around the Earth as its centre, while religious orthodoxy drew a sharp distinction between the earth and the heavens, believing the heavens to be perfect and eternal, while the earth was corrupted and temporal.

This ideal began to change after the astronomer Nicolaus Copernicus’ book De Revolutionibus Orbium Coelestium was published posthumously in 1538. Copernicus became the first major astronomer to challenge the orthodox view when he suggested that the Earth and the other planets orbited the Sun, not the other way around. Though it took more than a century for this view to be widely accepted, the 16th-century priest and monk Giordano Bruno latched onto the current of thought that he had set in motion, and proposed an even more radical and heterodox vision of the universe. According to Bruno, the planets were other Earth-like worlds, and the earth and the heavens were equally sacred. He even suggested that the stars were in fact suns orbited by their own earth-like objects. This was more than 400 years before the first exoplanet – a planet orbiting another star – was discovered. Furthermore, Bruno postulated that these planets contained living creatures, just as Earth does.

Bruno’s and Copernicus’ ideas about the solar system were proved correct less than ten years after Bruno’s execution by Galileo’s observations through his telescope. (Bruno was burnt at the stake by the Roman Inquisition for deviating from Catholic doctrines such as the Trinity, transubstantiation and the virginity of Mary; whether his scientific views about the universe played a part is a matter of debate among historians.) As well as proving beyond reasonable doubt that the Earth and the other planets did indeed go around the Sun, Galileo also showed that Jupiter had four moons of its own (today we know that there are at least 67); the Moon was rough, not smooth as people had thought; and there were many more stars than were visible to the naked eye.

As the reality of Galileo’s discovery gradually sank in, the old religious orthodoxy of the Earth as the center around which the universe revolves gave way to a new religious orthodoxy, which held sway for the next three centuries: If God has indeed created other Earth-like worlds, he must have put other intelligent beings on them – otherwise, why would he have created them?

Belief in man-like extra-terrestrials was promoted by thinkers on both sides of the Atlantic. The French author Bernard de Fontenelle posited that the extra-terrestrials had personalities consistent with their natural environment. Inhabitants of Mercury, the closest planet to the Sun, were hot-tempered; those of Venus, the next planet from the Sun, were “full of wit and fire, always in love, writing verse, fond of music, arranging festivals, dances and tournaments every day”; while the residents of far-flung, freezing Saturn possessed dull, lethargic minds and bodies. In the United States, Benjamin Franklin suggested that there were many alien races both superior and inferior to humans, while John Adams believed that every planet in the universe contained some form of intelligent life.

Belief in extra-terrestrial intelligence was still common when, in 1877, the Italian astronomer Giovanni Schiaparelli made what appeared to be a startling discovery: Schiaparelli claimed to have glimpsed a network of long, straight, dark lines around the equator of Mars which he called canali, a word which, unlike its English equivalent, does not necessarily suggest an artificial origin. Nevertheless, some people, including the American author Percival Lowell, suggested that they could be irrigation canals built by intelligent Martians, used to transport melting water from Mars’ polar ice caps to the equator during the summer. Others disputed the existence of the canals, arguing they were an optical illusion, while the British naturalist Alfred Russell Wallace, co-discover of the theory of evolution, argued Mars was too cold for liquid water to exist on its surface.

It was not until NASA’s Mariner 4 spacecraft flew past Mars in 1965 that the idea that any sort of complex, intelligent life existed on Mars was buried for good. The probe photographed a barren, dead surface and measured temperatures as low as -148 degrees Fahrenheit. Today we know that there is almost no liquid water on the surface of Mars. Water is an essential ingredient of life, and the Earth is the only planet in our solar system located in what scientists call the ‘habitable zone’, or colloquially the ‘Goldilocks zone’: the area that is the right distance from the Sun for water to exist as a liquid, not as ice or steam. Every planet on either side of Earth is either scorching or frozen. Observation of Venus, once considered a lush, life-nurturing environment, has revealed poisonous sulfur dioxide clouds, temperatures of up to 800 degrees Fahrenheit and air pressure that exerts 90 times the force of that of Earth on every square centimeter of your body. Study of any planet beyond Mars reveals temperatures approaching absolute zero, the lowest possible temperature, at -460 degrees.

Not surprisingly, scientists have given up hope that any intelligent life exists elsewhere in the solar system. Nevertheless, the discovery of microscopic creatures on Earth collectively known as ‘extremophiles’, which can live in extreme conditions previously thought completely incapable of sustaining life, has led to the idea that simple, microbial life could exist beyond what we once thought of as the ‘habitable zone’. Some of these weird and wonderful creatures thrive at the bottom of the sea or up to three miles underground, with negligible light or energy from the sun; others in extremely hot, cold, acidic or alkaline environments; still others with exposure to intense nuclear or ultraviolet radiation. One species of extremophile called the water bear has even survived being taken into the vacuum of outer space. If some life forms can survive, even thrive, in such extreme conditions on Earth, could other life forms live in equally extreme conditions on other planets?

Deposits across the surface of Mars of the mineral gypsum, whose chemical formula dictates that it can only be created from water, show that liquid water used to exist on Mars, meaning that simple microbial life could have existed. The presence of methane in Mars’ atmosphere, a gas that on Earth is only created through biological or geological processes such as mud volcanoes (all of Mars’ volcanoes have been inactive for millennia), suggests that there could be life in caves below the Martian surface, where large amounts of ice could melt into liquid water. Just in the last month, scientists have even uncovered evidence that a small amount of liquid water does still exist on the surface of Mars, though this would probably not be enough to sustain life.

Jupiter’s moon Europa – one of the four Galilean moons – is another possible habitat in our solar system. Underneath Europa’s icy surface is thought to lie a huge, hundred-mile-deep ocean of water, kept liquid by heat energy from the gravitational pushing and pulling of Jupiter and the other moons. Titan, one of Saturn’s moons, is cold enough to have liquid methane: methane lakes and methane clouds producing methane rain; this could act as a substitute for liquid water.

Although the chances of actually finding complex, intelligent life in these places is effectively zero, the discovery of the sheer, unimaginable vastness of the universe has shifted our attention to beyond our own backyard. Since the invention of the telescope, we have gradually discovered that our green and blue planet is a tiny droplet in an indescribably vast ocean. Our Sun is one of around two hundred billion stars in our galaxy the Milky Way. The Milky Way is so huge that it would take light – the fastest thing in the universe – 100,000 years to cross it, and what’s more: our galaxy is just one of an estimated 170-200 billion galaxies in the observable universe.

It is estimated that the observable universe contains 10,000 stars for every grain of sand on  every beach or desert on Earth. Moreover, we now know that, as Bruno suggested all those centuries ago, some of these stars do have their own planets orbiting them. Even if the chances of life developing on an individual planet, or in an individual solar system, are minute, surely there must be some planets with intelligent life out there, somewhere?

In 1960, the British astrophysicist Sir Frank Drake formulated the Drake equation, a formula to work out the number of alien civilisations in the Milky Way that are trying to communicate with us. The formula tries to work this out by multiplying together factors such as the how many stars have planets, including planets with life, on how many of those planets life has developed intelligence and civilisation, and how many of those intelligent civilisations have developed communications technology that they are using to send communications out into space.

The trouble is that we can make informed, educated estimates about only the first three values in the equation: the rate of star formation in our galaxy (R* in the equation as above), the fraction of stars that have planets (fp) and the average number of habitable planets per solar system (ne).   The first we know to be about 7 per year; evidence increasingly suggests that a star having planets is the norm rather than the exception; and it is thought that about 1 in 5 stars has an Earth-sized planet in the star’s habitable zone, though one has to take into account the other factors that make a planet habitable as well.

As for the remaining four, we have very little to go on. The fraction of habitable planets that actually do develop life (fl) is very difficult to say. The fact that life could develop on a planet does not necessarily mean that it will, and life only began once on Earth – so far as biologists know, every life form on Earth can be traced back to the same evolutionary source. If life was found to have begun independently elsewhere in our solar system – which, as we saw earlier, is possible – then that would suggest a high number, though we would still be working from a very small sample size.

The fraction of planets with life whose creatures go on to develop intelligent civilizations is equally difficult. Some argue that intelligence is an inevitable outcome of evolution on planets like Earth, though it should be remembered that on Earth, simple microbial life forms can survive a far wider range and variety of conditions than complex, intelligent life forms, and again, we know of only one planet – Earth – that has life, and on which that life has happened to develop intelligence. We can’t really draw any general conclusions. Estimates of the fraction of civilisations that send communications out into space (fc) and for how long those civilizations send the communications (L) are equally speculative.

Despite all of this, given the vastness of the Milky Way, even conservative estimates of each of the factors in the Drake equation suggest that there may be hundreds of advanced civilizations trying to communicate with us just in our own galaxy. And given that there are over 100 billion galaxies in the observable universe, that means that, give or take the factors making a galaxy hospitable or hostile to life, there could be trillions – yes, trillions – of advanced civilizations in the universe.

A different but equally important question is: If aliens exist, will we ever find them? Nearly 2000 exoplanets –planets orbiting other stars — have been discovered since 1995, including a few that could support life. However, it is very unlikely that finding exoplanets will help us to find life. The light-year distances of these planets from the Earth and the huge difference between the brightness of a star and the brightness of its planets make direct imaging extremely difficult. Therefore, most exoplanets have been detected indirectly, either by detecting wobbles in a star’s motion that must be caused by the gravitational pull of a planet, or a slight drop in the brightness of the star caused by a planet moving in front of it. Because exoplanets are so hard to detect, most of the ones we have found are giant gas planets like Jupiter that could never sustain intelligent life; so far, we have found only one Earth-sized planet in her star’s habitable zone. Nevertheless, finding potentially habitable exoplanets can help direct our search.

Since the 1960s, scientists have been monitoring electromagnetic waves to check for alien transmissions, and sending transmissions from their own planet – including every television and radio programme – out into space, in the hope that one day an intelligent civilisation will receive and respond to them. Astronomers measure distances by the speed of light, the same speed at which these transmissions will travel. A light year is the distance that light can travel in a year: about 6 trillion miles. Our nearest foreign star, Proxima Centauri, is 2.4 light years away. If there is intelligent life in our corner of the galaxy, we should be hearing something pretty soon and could, over years, decades or centuries, have two-way communication with them. If there used to be alien civilisations on the other side of the galaxy (with its diameter of 100,000 light years) or in our nearest foreign galaxy, the Andromeda Galaxy (2.5 million light years away), we may even receive a transmission from a civilization that lived thousands or millions of years ago.

Given the numbers, it is perhaps strange that, after half a century, we still haven’t found anything. The contradiction between the apparently overwhelming likelihood of the existence of extra-terrestrial life in the universe, and the fact that we haven’t found anything yet, is called the Fermi Paradox, and that is a whole other issue with an even greater variety of unknowable answers.

Yet, if we do discover intelligent, extra-terrestrial life, what will be the implications, for humanity and for Christian belief? It would certainly be yet another leap away from the pre-Galilean view of the universe, with the Earth and humanity at the centre. We know that Earth is one of eight planets orbiting a medium-sized star near the outer edge of our galaxy. That star orbits the centre of the Milky Way along with hundreds of billions of other stars, and the Milky Way drifts through the cosmos along with hundreds of billions of other galaxies. The idea that there is another race of intelligent beings, maybe even billions of other races, would be one step further.

Some of these races may even be far more technologically advanced than us, if they and their planet are older than ours. Our Earth is predicted to survive for another 4 or 5 billion years, and the human civilisations a thousand years from now may be as incomprehensible to us as today’s world would be to someone from the Middle Ages.

Christianity would be able to cope with all of this. It eventually coped with Galileo’s discovery by recognising that science and religion operate in different, overlapping fields of inquiry. Church doctrine and scripture may be a reliable guide to theological and philosophical matters (the ‘why’ questions), but are not necessarily a good guide to empirical, scientific matters, which require observation or experiment to settle. The Copernican revolution did not undermine God; it merely undermined human ideas and interpretations about God, and the arrogant human presumption that we can know how God would create the universe – and because we know, we can expect everyone else to agree with us.

Perhaps other alien races are intelligent, but lack souls, lack the ability to distinguish between good and evil. Perhaps they do have the ability, but chose to do good rather than sin. Some, such as C.S. Lewis in his essay ‘On Religion and Rocketry’, have argued that there may be other races which are not fallen – in fact, as Lewis puts it, we may be the single lost sheep for which the shepherd has abandoned all the others. I would argue that it is the likelihood is always that at least one person will sin, and once sin enters a race through one person, the whole race becomes corrupted. Origen, the third-century Christian theologian whom we met earlier, saw the theme of fall and redemption as present throughout the universe and, if there are other beings with souls like us, I would agree: the fall because nobody is perfect like God and some beings will always choose sin; and redemption because God will always seek to redeem everyone who sins.

It could be argued that the idea that human beings are not the only intelligent species for which God has a plan is in conflict with the very anthropocentric focus of the Bible: the creation story of Genesis paints a picture of mankind being the crowning point of creation, and the story of the incarnation, crucifixion and resurrection is seen as a rescue mission particularly for humanity. Yet the story of creation as told in the Bible is told from a human perspective, and therefore naturally focusses on humanity. We may be the only creatures created in God’s image on earth, but that does not exclude the possibility of extra-terrestrial creatures created in his image too, who are equal to us before God.

Nevertheless, because they are anthropocentric, some Christian ideas would perhaps need some adjusting and clarifying. Does mankind entirely make up God’s image, or are we merely a part of his image that is completed by other races? Do Christians on Earth make up the entire body of Christ, or, if there are Christians on other planets, do we make up only an incomplete part of it?

Another question also pops up if there are other soul-endowed yet fallen creatures in the universe: If God saved humanity through Jesus, how is God saving the other species? For, as C.S. Lewis put it, “If we find ourselves to be but one among a million races, scattered through a million spheres, how can we, without absurd arrogance, believe ourselves to have been uniquely favored?” Perhaps Jesus’ incarnation, crucifixion and resurrection redeemed the entire universe. Or, perhaps more likely, especially if we are one species of millions, God has redeemed every intelligent species according to their particular needs. Part of the point of the incarnation is that God really knows what it is to be human. Since alien species will be different and have different experiences, and the nature and level of their sin might be different, different diseases may require different cures.

 C.S. Lewis wrote that, if redemption is so far unique to us, we one day encounter extra-terrestrial beings, it may be God’s plan that we spread the redemption that we have received by evangelizing to them. However, Lewis thanked God that this had not happened yet, for if we did reach them we would exploit them, treat them as inferiors and exercise the same “theological imperialism” that we had done on other human beings that differed from us. Some scientists argue that we are now ready to meet aliens, being far more open and less prejudiced than we were in, for example, Lewis’ time. Let’s hope they’re right.

The most exciting thing about the search for extra-terrestrial life is that we are on the cusp, the cutting edge of what we know about the universe. We have made enormous progress in understanding the universe in which we find ourselves, especially in the four centuries since the invention of the telescope. Yet in this area, for the moment, we are as in the dark as we were then, when we thought that the Earth was the center of the universe. Human beings have always wondered, “Are we alone?” We may be about to find the answer.