The scale of the universe cannot be fathomed with the human eye, but the night sky offers a start. If you live in or near a city, you only ever have seen a few stars. We need to travel away from built-up areas to see the heavens in their glory.Footnote ¹ Someone with good eyesight (or glasses) would then be able to see around nine thousand stars, at least if she travelled to observe the sky first from one hemisphere of the Earth, then from the other. Binoculars would increase the tally of stars to maybe two hundred thousand, while even a cheap portable telescope would expand even that ten- or twentyfold. That, however, takes us only a tiny fraction of the way towards apprehending the whole. Our best scientific telescopes, pitched on mountains in Chile or Hawaii, or launched into space, allow us to estimate that there are around one hundred billion stars in our Milky Way galaxy: 100,000,000,000 stars.Footnote ²

Until we observed planets around other stars, we could not be sure that there were any. Planets could have been common, or extraordinarily rare. According to one theory for how solar systems form, planets would be routine, coalescing alongside stars from the same cloud of dust, or nebular.Footnote ³ The rival theory envisaged that planets are formed by the collision of one star with another, or of a star with a comet.Footnote ⁴ That would make planetary systems vanishingly rare, since the immense size of stars dwindles almost to nothing compared to the distance between them. Collisions would happen a great deal less often than the mid-air encounter of one ball with another on a golf course.Footnote ⁵ By the 1960s, the consensus among scientists was shifting towards the nebular hypothesis, but it took observation to settle the matter.Footnote ⁶ The epochal moment in recent science, which has done so much to provoke further research, was the announcement by Michel Mayor and Didier Queloz in 1995 they had uncovered a planet orbiting another star like our own sun. Since then, the number of these ‘exoplanets’ in our human catalogue has grown apace. On 5 October 2021, The Extrasolar Planets Encyclopaedia listed 4,846 planets, in 3,582 planetary systems (with 798 of those solar systems known to possess more than one planet).Footnote ⁷ Those numbers are the provocation for this book, even more so once we extrapolate them to the galaxy as a whole, or even to the entire observable universe.Footnote ⁸

The discovery of a first planet outside our solar system ranks among the most momentous feats of science, and a good deal has been written by theologians in response, chiefly with the prospect of extraterrestrial life in view. While theological interest has intensified, however, it would be a mistake to think that it is entirely new. As we will see, thinking in Christian theology about the implications of life elsewhere in the cosmos goes back almost six hundred years.

For a conservative estimate of habitable planets, we might concentrate only on the solar systems of sunlike stars. That can be defined in a few different ways, but we might end up classifying around 4 per cent of the stars in the Milky Way that way. That gives us 4 billion sunlike stars in our galaxy. Observations over the past twenty-five years suggest that most stars are encircled by planets, but many of those planets are not likely sites for life: some are composed of liquified or solid gas, like Jupiter, Saturn, Uranus, and Neptune; others are rocky but burnt to a crisp, like Mercury, or partly rocky but a very long way from the star, and cold, like Pluto (demoted to status of a ‘dwarf planet’ in 2006). To work out the proportion of stars with habitable planets, the two main criteria are a rocky composition (rather than gas), and a temperature at which any water present on the surface would be a liquid. Earlier estimates had about one in five sunlike stars with habitable Earth-like planets. Recently, that has been revised upwards, to between 0.37 and 0.60 such planets per sunlike star.Footnote ⁹ These figures will no doubt shift. For instance, we are beginning to make progress in understanding the capacity of a star to have several habitable planets in its orbit.Footnote ¹⁰ Habitable moons are also possible. However, if our estimate of the proportion of sunlike stars with Earth-like planets remained at about a half, that gives us two billion such planets in the Milky Way.

Two billion is an incomprehensibly large number, but even that is only a start. Our galaxy is not alone. In fact, by a strange coincidence, the number of galaxies in the observable universe seems to be about two hundred billion (200,000,000,000): almost identical to the number of stars in our galaxy.Footnote ¹¹ If our galaxy is more or less average in terms of the number of stars it contains, that puts the number of stars in the observable universe at something like 2 × 10²²: twenty thousand billion billion stars, or two followed by twenty-two noughts. That in turn would suggest around 8 × 10²⁰ sunlike stars, and perhaps 4 × 10²⁰ rocky planets of the right temperature circling them: four hundred billion billion.

Of course, water will not be present on every rocky planet capable of harbouring it in liquid form, and all sorts of factors may be particularly conducive to the evolution of life, which may or may not apply to this or that planet. For instance, the presence of a moon may be significant, if life evolved in tidal pools, or underwater hydrothermal vents, if not. Other features may be significant for protecting any life that does evolve: a giant neighbouring planet, like Jupiter, may be useful for hoovering up asteroids that would otherwise collide with an inhabited planet. Even lowering the figure of four hundred billion billion by several orders of magnitude, that still leaves us with an astonishing number of potential cradles for life, and that, to my mind, changes everything. The number of places where life could evolve and take hold seems to be extraordinarily large. Those calculations, moreover, say nothing about the billions of billions of habitable planets that may already have been and gone, or are yet to be.

The evolution of life is not impossible. I am sufficient evidence of that, as are you. Life can evolve, and there look to be billions of billions of planets where that might have happened. The emergence of life is a remarkable thing, and perhaps not at all common. But is it so uncommon as to happen only once in, say, four hundred billion billion opportunities? Extrapolation from one example is a perilous business, but we do have one more piece of information. Life got started on Earth surprisingly early: it stretches back maybe 3.8 billion years. The planet is 4.5 billion years old, and it spent around 0.5 billion years in the Hadean eon: the literally Hades-like first period, during which it was bombarded by meteorites, covered in volcanoes, and bathed in the radiation of elements with short half-lives. Life began only a short time later – 0.2 billion years later – and it has been going for 3.8 billion years since. That rapid arrival may offer one suggestion that life is not too difficult to get going.Footnote ¹²

At present we are far more able to estimate whether a planet might be broadly habitable than we are at assessing whether it is inhabited, but we stand at the cusp of a significant leap. Already to some extent, and soon with much greater accuracy, we will be able to analyse the atmospheric composition of planets around other suns. The James Webb space telescope is a significant addition here. Even the meagre absorption of a star’s light by a planet’s atmosphere (microscopic in comparison to the star) as the planet passes in front is enough to yield information as to which gases are present. If the combination of gases we see is thermodynamically anomalous – if the combination is not likely to form a stable mixture – that may serve as a sign of life. The Earth would appear anomalous in just this way if seen from elsewhere, since it contains a highly reactive combination of methane alongside oxygen.Footnote ¹³ This capacity to analyse atmospheres is set to change the stakes when it comes to detecting other life. Up to now, the emphasis has been on waiting for signs from an advanced civilisation (in radio transmissions, for instance, or by detecting the traces of how an advanced civilisation might engineer an entire solar system). Soon, however, we will be able to look for signs of life before it has reached an advanced state (if it ever does), given away simply by how it perturbs the chemistry of the planet it inhabits and shapes. That expands the range of living planets we could detect enormously. In terms of Earth, it would mean being able to detect life as it had been present for perhaps three billion years, not as it has been present for one hundred.

In recent decades, scientific study in the area of this book has shifted in the direction of thinking about the universe as a whole as a place where life can evolve and flourish. We see this in a shift of terminology. For a period, the language of ‘exobiology’ predominated, as the name for scientific speculation about life beyond (‘exo’) Earth (or on ‘exoplanets’). Today we more often talk of ‘astrobiology’, which is the scientific study of the place of life in the cosmos. The shift is from concentrating on life other than terrestrial life (of which, as yet, we know none) to thinking about place of life in the cosmos per se, of which terrestrial life is part. That goes hand-in-hand with the integration of ‘planetary science’ (previously seen as being about other planets) with ‘Earth science’, with each discipline enriching the other. In this book I will use ‘exobiology’ when addressing other life, and ‘astrobiology’ when I am thinking about the place of life in the universe.

Astrobiology and Christian Doctrine

Little in recent science outshines the discovery of planets around other stars. Results pouring in since the mid-1990s have transformed our understanding of the universe, which turns out to be strewn with planets, a fair proportion of them potentially habitable. That makes astrobiology – the scientific study of life as a phenomenon of the cosmos as a whole – a discipline de jour. Renewed theological discussion of other worlds, and life elsewhere, has followed, although as a topic for Christian theology per se, that is not new. Theologians have been writing about the theological implications of biological life beyond Earth since the mid-fifteenth century.Footnote ¹⁴ The attitude of those early Renaissance theologians was typical of much that would follow: they were unphased by the prospect of other life, but also brisk in their discussions, leaving us only a paragraph at most on the topic. They acknowledged the possibility of life elsewhere, thought that it posed no particular problem for Christian belief, and moved on to some other topic. Examples of this sort of response into Early Modernity could be multiplied at length. We see it, for instance, in John Ray’s classic combination of biological survey and theological wonderment, in which he throws off, more or less in passing, a single, unthreatened mention of life on other planets in relation to God. In this period, theology often features in works approaching life beyond Earth from a scientific perspective (such as one from John Wilkins, Bishop of Chester).Footnote ¹⁵ Again, however, those theological comments tend to be as notably brief as they are unruffled.

Christian rumination on other life remained a significant topic in the centuries that followed, even if it was not explored in any detail. When Anthony Trollope, for instance, wanted to depict a group of characters talking about a modish topic of the day in Barchester Towers (1857), he had them talking about life elsewhere in the solar system, and its theological implications. In 1920, Frank Weston (1871–1924), Bishop of Zanzibar, saw his contribution on the topic as joining an already lively scene, in which ‘it is often argued that if other planets are dwelling-places of rational beings the incarnation with its atoning work cannot be true’. (He found this conclusion ‘unwarranted’.)Footnote ¹⁶

A succession of familiar theological names commented on extraterrestrial life in the twentieth century, if only in passing, among them Yves Congar, Hans Küng, Eric Mascall, Wolfhart Pannenberg, Karl Rahner, and Paul Tillich. Some of that writing expanded the bundle of theological discussions of the theme, but they still rarely exceeded a few paragraphs in length. Towards the end of the century, and into the twenty-first, chapter-length surveys of themes in theology and astrobiology became common, and were published in multidisciplinary edited collections.Footnote ¹⁷ That genre allows only a limited scope, however, and writing for a non-theological audience tends to restrict the author’s capacity to go into detail. Nonetheless, these contributions are witness to the enduring place of theology among the arts and humanities, and to recognition of the role that religion plays for many in interpreting the world, not least if life elsewhere were to be confirmed.

Over the past two decades, edited collections have appeared, devoted to theological discussions of other life in the universe, plus a few single-author volumes.Footnote ¹⁸ Nonetheless, room remains for development in several directions, to which I hope this book will be a contribution. One would be to move writing further from commentary upon what I described above as the ‘bundle of theological discussions’, accumulated from historical sources. However distinguished those authors might be thought to be, their insights are typically sparse and offered – as I have said – for the most part in passing. Far more valuable is attention to existing theological writing that bears upon questions raised by astrobiology, for all the implications for other life would have been entirely absent from the author’s mind. Chapter 14 of this book provides an example. Down Christian history we find discussion about what difference (if any) the Incarnation has upon the relation of the Word to the rest of creation. In the Reformation, and debates stemming from it, this typically had to do with Christ’s presence in the Lord’s Supper. There is obviously much in that material that bears upon the question of multiple Incarnations elsewhere in the universe, even though that topic was not historically in view. In this fashion, a central task for theological consideration of life elsewhere in the universe is to expand the range of the historical material, such as this, that can be brought to bear on the topic.

Another expansion would address the range of doctrinal topics under discussion, whether in turning to topics previously little considered at all (such as eschatology), or in bringing topics together that have otherwise mainly been treated separately, with the hope of cross-fertilisation. Alongside such expansions of breadth, there will also be value in an increase in academic depth or focus. Much that has been written theologically about life elsewhere in the universe, historically and to some extent today, has been conceived with a wide or popular religious readership in mind. The motivations for that are often admirable, but a degree of technical precision can be lost as a result. That is particularly to be seen in discussions of Christology. For instance, where the idea of more than one Incarnation has been denied, what is meant by ‘Incarnation’ is often difficult to pin down, or else set out very much at variance with the sort of formulations of Christological thinking that are foundational to Catholic, Orthodox, Anglican, and Protestant traditions. Any attempt to engage astrobiology with greater theological precision will likely also entail a deeper grounding of our discussions in a specific theological tradition: Augustinian, Bonaventurian, Calvinist, Thomist, or whatever.

Addressing questions of purpose, one motivation for a book such as this is a desire to help the human community (and specifically, the Christian community) to be more ready to receive, process, and respond to any future signs of life elsewhere. Detection might come in a decade, centuries hence, or perhaps never, but if it does, it will be useful to have thought through the implications in advance. Large numbers of people will turn to their religious traditions for guidance – as to what other life means for the standing and dignity of human life, for instance – and work done in advance will surely pay off.

A second motivation aligns with a theme familiar from literature and poetry in the twentieth century, namely that after a journey – physical or intellectual – in unfamiliar territory, one can return home with fresh eyes.Footnote ¹⁹ As an example, while theology has previously been carried out with human beings in view, it has recently found itself (and its reflections on humanity) enlivened by attention to non-human animals. In the same way, our theology can find useful provocation, even invigoration, by having life beyond our planet in mind for a spell. I have already quoted Eric Mascall (1905–1993) in the frontispiece, writing – indeed – on our topic:

The text quoted alongside Mascall is a now quite famous poem by Alice Meynell (1847–1922), which offers an example of what invigoration can look like, in which several aspects of Christian belief shine in new ways once placed in different light.

My emphasis is largely on the repercussions for Christian theology of other intelligent life. Among any planets that do harbour life, intelligence would almost certainly be present on only a small proportion of them, but that is still naturally where most of the theological interest will lie: with the prospect of creatures that can know, love, pursue the good, fall prey to the curtailment of evil, know redemption, and stand in that remarkable relation to God in which, for all the infinite qualitative difference, creature and creator each address the other as a person. From time to time, I will consider life lacking memory, intellect, or will (or with only its first flickerings), but my interest is generally on the sort of life for which the categories of knowledge and revelation, sin and redemption, grace and the beatific vision, could be in view. With the launch of a new Leverhulme Centre for Life in the Universe at Cambridge, non-sentient life, and the transition from the non-living to the living, is set to feature prominently in my research in coming years. Those questions will be a topic for a further monograph.

Not every topic that might attract the interest of scholars of theology and religion concerning other life will receive attention here. That is in part because of the constraints of space, but also, and less arbitrarily, because they fall outside the areas in which I can claim even marginal expertise. Nor have I considered the question of ‘what next’ steps, following the imagined arrival of evidence of other life. There is nothing here, for instance, about evangelism, mission, or inter-planetary comparative theology (all popular themes for novelists). Indeed, situations of future ‘contact’ between us and other life are not in view at all. Nor is there anything about the Search for Extraterrestrial Intelligence (SETI), or its more recent counterpart, involving active broadcast of our presence (Messaging Extraterrestrial Intelligence, or METI). The latter has provoked discussion in recent years as to its ethics and advisability. Are we unwise to give our presence away? That theme is beginning to receive some theological attention, as are questions about what we should physically send into space, but the theological ethics of space exploration and messaging is not a topic for this book.

Also of interest to scholars of religion are traditions where extraterrestrial life features among the tenets of the faith. There is a galactic or inter-planetary dimension, for instance, to both Mormonism and Seventh Day Adventism, while religions such as Scientology and Raëlianism are even more fully grounded on extraterrestrial themes.Footnote ²¹ Also of interest to social scientists is the recent phenomenon of interpretating existing religions in terms of alien life and visits (these receive only a brief mention, in Chapter 2), and empirical studies, such as those of Ted Peters and Julie Louise Froehlig, to investigate what challenges might or might not be posed to members of religious communities by the prospect of other life.Footnote ²² The headline findings are that adherents of a range of religious traditions report that they can take the idea in their stride. Non-religious people also seem to over-estimate the challenges that religious people think they would experience if faced with evidence of alien life. The social sciences can also offer perceptive analysis of the work and world of space exploration, as itself a cultural phenomenon, as in the work of my colleague Timothy Jenkins.Footnote ²³

From time to time, I have drawn on writings and perspectives of other religions, not least from a paper by the rabbi Norman Lamm that remains fresh after several decades. I have also begun leading sessions on this topic at interfaith summer schools. By and large, however, I both recognise the potential for fruitful inter-faith dialogue around topics covered in this book, and confess to not having the knowledge, expertise, or space to pursue that here.Footnote ²⁴ Also standing largely outside the scope of this book would be detailed work with specialist Biblical scholars. I indicate in Chapter 17 that attention to what the Biblical writers might or might not have meant (or could or could not have meant) by terms such as ‘whole world’ is significant. Such questions would be useful topics for discussion across theological disciplines.

Theologically, the book largely comes from a particular perspective, namely Thomism. I offer that particularity as a strength, wishing to see conversations between theology and the sciences engage as much with the specificities of theology as with the specificities of science. Indeed, any deepening of theological engagement with science will, to my mind, more or less necessarily involve greater specificity in the theological position from which it is being addressed. Happily, work between theology and science today has by and large set aside generalised questions about the relation between theology-in-general and science-in-general, thinking instead about more particular scientific topics (here astrobiology), and doing so by drawing from the deep wells of particular traditions or schools of theological thought.Footnote ²⁵

Throughout the book, I have included footnotes to passages in the works of Aquinas. Sometimes that serves to identify the source of a text that I both quote and consider (usually for the sake of its strengths, but sometimes in relation to what I see as weaknesses). On other occasions, a citation points to a text that I do not discuss. Those are not offered as if the passage will answer or foreclose all further questions, but because I want to indicate where relevant material can be found in the works of this author, which – with all its strengths, and no doubt some weaknesses – could inform the reader in thinking further about the topic in hand.

Of all authors, I find Aquinas the most useful for fruitful theological consideration of the natural sciences. I hope that this book offers an illustration of that, and therefore a commendation of his writings, and of later Thomism, for the attention of those who engage in that sort of work. More specifically, I hope that this book will illustrate and commend the value of an approach to Aquinas and his legacy that was nicely summarised by Mascall:

Such a Thomism, seeking to be neither defensive nor revisionary, and valuing both openness and confidence, offers a form of theology that is both appealing on its own terms, and particularly open to conversations about scientific topics.