Introduction: Contemplating Alien Worlds and Life Throughout Human History

The question of whether life exists beyond Earth is an old one. Long before the modern development of astrophysics and astrobiology, human imagination envisioned alien worlds and life. Even in antiquity, Epicurus wrote that “there are infinite worlds both like and unlike this world of ours … Furthermore, we must believe that in all worlds there are living creatures and plants and other things we see in this world” (Bailey 1926). “Worlds” for Epicurus are what we would more likely call “universes,” but even so we can see in his words the vision of life flourishing beyond Earth. Such envisioned life from antiquity (and today) also includes nonphysical spiritual beings such as angels who can navigate seamlessly across space and time, at times becoming apparent on Earth to interact with humans. More recent science fiction has proposed the idea of life unattached to a particular locality. The television series Star Trek: The Next Generation (Paramount Television 1987–94), for example, introduced “Q” as a manipulative and powerful character freely navigating across and between dimensions, appearing in any desired form and place.

Most speculation of extraterrestrial life in recent centuries, however, involves planets and moons, with physical life living on, or at least originating from, particular planetary locales. The Copernican revolution energized a new era of speculation about the possibilities of other inhabited worlds, including the inherent theological and philosophical challenges these present. In literature, life has been imagined on every planet in the solar system as well as Earth’s moon. In the nineteenth and early twentieth centuries, now-debunked “observations” of extraterrestrial life—from the “Great Moon Hoax” published in the New York Sun falsely ascribing observations of diverse lunar life to astronomer Sir John Herschel (Figure 1), to the infamous canali (channels) on Mars described by astronomer Percival Lowell as bands of planted vegetation—built upon societal openness to the possibility of life thriving on other worlds. Thomas Dick, a minister and popular writer on Christianity, philosophy, and astronomy wrote in 1838 that “the Moon is inhabited by rational creatures, and . . . its surface is more or less covered with vegetation not dissimilar to our own Earth.”

Figure 1

Lunar animals, as falsely reported to be observed by astronomer Sir John Herschel. Alien worlds imagined to host life can be as close as the moon. (From the Finding our Place in the Cosmos collection, Library of Congress 2025.)

The idea of life beyond Earth has also inspired theological reflection for centuries, spawning multiple profound questions: If life exists beyond Earth, was it created by the same Creator as life on Earth? Were the same processes for life creation and diversification used, e.g., instant appearance or long-term evolution through natural selection? How did God relate to these creatures, and vice versa? Do they have a sense of morality, and do they have moral failings? Do they understand good and evil? For Christians, the issues take on more intensity due to the centrality of Jesus Christ to their faith—a God who came in human form to planet Earth. Would the redemption of humanity from the ramification of their sin, provided through the life, death, and resurrection of Jesus, extend to morally culpable beings beyond Earth? Would the promised return of Jesus and a new, or renewed, “heaven and earth” have any meaning for beings and civilizations on other planets? Had God become incarnate in other forms for other worlds (Crowe 1986, 2008)?¹ Musings on the spiritual state of life beyond Earth extend into science fiction. The famous space trilogy of C. S. Lewis (2011), for example, considers the moral state of, and role of Christ in the lives of, imagined beings on other planets in the solar system as a comparative lens through which to view the workings of God through Jesus Christ on planet Earth.

Then there are the fantastical imaginings of intelligent aliens from other planets visiting Earth. Science fiction films and books vividly portray both positive and dangerous possibilities of such encounters. Personal encounters with alien beings and sightings of unearthly spacecraft are claimed by people around the world, usually through sensationalist media and without mainstream credibility. Official studies of (mostly aerial) sightings are beginning to take place in a more public and scientific fashion. NASA (2023), for example, recently established an Unidentified Anomalous Phenomena Independent Study Team to recommend scientific approaches and data handling for reports of unusual sightings. It is important to note that most unidentified anomalous phenomena can be traced to a non-alien cause, such as human activity or atmospheric physics.

Exploring Planets and Looking for Life in our Own Solar System

Speculation about imagined life on other worlds was the only option until scientific space exploration became a reality. Exploiting the new technology of the telescope, Galileo Galilei set his sights on the moon, Venus, and Jupiter, recording systematic observations that were good enough to explain what had hitherto been mysteries and even challenge long-held paradigms about cosmology. As optics improved in subsequent centuries, observations of planets and the moon only fueled curiosity about the possibility of life beyond Earth.

The dawning of the space age in the twentieth century began a new chapter in looking for life. Finally, humans could actually send probes, and sometimes themselves, to investigate alien worlds. Early probes to the moon sent back close-up photos, with subsequent human missions investigating, and even returning, moon dust and rocks. The moon turned out to be a dusty, lifeless world with no atmosphere. In more recent years, interest in the moon as a place that could support life has been re-fertilized by the detection of water in molecules across its surface and in ice reservoirs at its shaded poles. The question now is not whether life exists on the moon but rather whether human life from Earth could be sustained there for a time using lunar water resources.

The 1960s and 1970s saw the first mechanical probes sent to study other planetary bodies. Of these, perhaps most famous are the Viking missions to Mars and the Voyager probes to the outer planets. These were joined and followed by hundreds of missions designed to study something of the character of solar system bodies from a close-up vantage point. Their findings made it clear that advanced life simply does not, and cannot, exist beyond Earth in our solar system, and likely never did. Given the centuries of human speculation otherwise, this was a significant disappointment. But it was quickly displaced by other intriguing discoveries and possibilities. For one, it became clear for the first time that the moons of other planets were numerous, diverse, and in many ways more interesting than their planets. Jupiter’s Io, for example, is a tumultuous hot and hellish world of volcanoes, while Saturn’s Titan has a thick atmosphere and liquid hydrocarbon seas. Intriguingly, Jupiter’s Europa and Ganymede and Saturn’s Enceladus have evidence of liquid water oceans underneath icy crusts (NASA 2025a). Now, wherever there is liquid water on Earth, we find microbial life. So, could microbial life exist in the oceans of these icy moons? We do not yet know, and we will not until we find ways of exploring deeply embedded water on faraway bodies.

Mars offers further mysteries. Probes and robotic rovers on Mars have been numerous since the 1970s, but they have not made any definitive discoveries of even single-celled life. What they find is a frozen desert of red dirt and rock. And riverbeds. That’s right: Mars shows clear signs that it once had flowing rivers and robust lakes. And when these robots dig just a little under the dirt, they find a whole layer of frozen water. Mars was apparently once a water-rich, habitable place. A massive change in climate accompanied the loss of atmosphere and both the evaporation and freezing of its water.

So far, scientific exploration of our solar system beyond Earth has turned up neither advanced life nor signs of even simple life. But it has drastically changed the questions we as humans are asking. Why does life thrive on Earth but nowhere else in the solar system? Why did Mars once have lakes, rivers, and a comfortable climate but now has only vestiges of mostly hidden frost? Was there ever microbial life on Mars? Is there microbial life in the oceans of Europa or other icy moons? And while it is clear life evolved in complexity on planet Earth, how did life get started in the first place? Was monocellular life a product of processes on Earth or was it transported to Earth via comets, dust, or asteroids (the panspermia concept)? And however life got started on Earth, could it have started similarly on other planets within our solar system if their conditions had been right? What about beyond the solar system?

Planets Beyond the Solar System: A New Era of Discovery

With the backdrop of centuries of contemplation and speculation, and the more recent solar system exploration, a monumental leap in human understanding of and encounters with worlds beyond Earth occurred in 1992. Astronomers Aleksander Wolszczan and Dale Frail discovered two planets orbiting the remains of a dead star known as a pulsar, not by imaging the planets but by detecting motion in the star caused by the gravitational tug of the orbiting bodies. Their phenomenal discovery and innovative technique led to the first detection of a planet orbiting an ordinary star, by astronomers Michel Mayor and Didier Queloz in 1995, leading to a Nobel Prize they shared with James Peebles in 2019.

With these discoveries, a frenzy of search and discovery for more “exoplanets”—planets orbiting stars outside our solar system—began worldwide (ESA 2024). In just three decades, humanity has gone from knowing of no planets orbiting any normal star other than our sun to knowing of more than 5,000 such planets in over 4,000 star systems in our region of the galaxy (some stars have more than one planet). Nothing as revolutionary has been known in astronomy since the discovery of galaxies a century ago.

Exoplanets are generally discovered and studied by indirect means. They are small compared to their parent stars and do not emit their own visible light. They only reflect starlight, so they can be a billion times dimmer than their parent star and get lost in the glare of the starlight for any astronomer trying to see one. The common analogy is of trying to see a firefly next to a lighthouse, though the actual contrast is even greater. The technique Wolszczan and Frail used, measuring the periodic “wobbling” of a star to infer the gravitational pull of an orbiting planet, has been used to find hundreds of exoplanetary systems, especially in the initial years of the quest.

Other techniques have subsequently been developed for exoplanet detection and study. Transiting exoplanets have offered the most powerful tool for both detection and characterization. A “transit” occurs if a planet just happens to orbit its parent star in an orientation where the orbital plane is along the line of sight of an observer (or telescope). As the planet passes in front of its parent star from the viewpoint of the observer, a small fraction of that star is occulted. This means that even though the observer likely cannot “see” the tiny exoplanet, the total amount of light seen from the star will temporarily and periodically be reduced because of the foreground orbiting exoplanet. The amount the observed starlight is reduced is directly related to the size of the occulting planet. As such, the transit method reveals the existence and size of exoplanets, while the amount a parent star wobbles due to the gravitational tug of a planet can yield that planet’s mass. If you know a planet’s size and mass, you can then deduce its average density. Furthermore, if a transiting planet has an atmosphere, light from the parent star will pass through the outer limbs of that atmosphere on its way to our telescopes. Certain wavelengths of light may be absorbed by atoms and molecules in the planet’s atmosphere, and thus the final “rainbow” of light received at the telescope may be missing certain wavelengths of light. The technique of spectroscopy matches the pattern of “missing” light wavelengths to the types of atoms and molecules that must have absorbed it. Thus, “transit spectroscopy” and related “transmission spectra” are used by astronomers to determine the atmospheric composition of an exoplanet. Its orbital velocity is also easily determined, and this implies the distance between the star and the planet, which influences the planet’s temperature (and therefore its habitability).

Astronomers are like detectives, using these indirect techniques to discover and crudely characterize exoplanets, and they now have done so for thousands of systems. They also study the parent stars, which vary widely in their nature. The nature of the star (its temperature, flares, radiation colors, etc.) will affect the nature of the exoplanet, including whether it has conditions that could be suitable for life to exist or thrive.

What Are Exoplanets Like? Could They Harbor Life?

Even the first few discoveries of exoplanets made it clear that other star systems may be very different from our own solar system. Many systems harbor what are called “hot Jupiters,” that is, large, presumably gaseous planets analogous to Jupiter in our own solar system. The difference is that these giants orbit much closer to their parent star than Jupiter does to the sun—so close, in fact, that by the law of orbits, they must move very fast to keep from falling into their parent star, which can be closer than one-tenth the distance between Earth and the sun. These planets sometimes make their entire orbit in the matter of a few days (Jupiter takes nearly twelve years to orbit the sun).

Hot Jupiters were the easiest planets to first detect in exo-solar systems. After decades of searching, the full range of exoplanet masses and orbits now have been detected. We know they range from terrestrial planets smaller than Earth to super-giants bigger than Jupiter. But we have found that the most common size of planet in other star systems is not found in our own, and that is what we call a “super-Earth” or “sub-Neptune,” that is, a planet a bit larger than Earth but a bit smaller than Neptune (Figure 2).

Figure 2

The several thousand planets so far confirmed to be in orbit around other stars—exoplanets—fall into four broad categories: large gas giants, Neptune-like worlds, “super-Earths” bigger than Earth but smaller than Neptune, and terrestrial planets in Earth’s size range. Within these categories, however, scientists find even more variety. Among the gas giants, for instance, are “hot Jupiters,” infernal worlds with tight, star-hugging orbits. These images are artist conceptions of the many indirectly detected exoplanets, most of which we cannot yet image (NASA 2025b).

Planets can range in density from diamond-like solids to foamlike balls. Some show signs of water vapor or carbon dioxide in their atmospheres, like exoplanet WASP 39b. Others disappointingly show no signs of an atmosphere at all, like several of the terrestrial planets in the habitable zone of the red dwarf star TRAPPIST-1. The “habitable zone” is the range of distance from a star where it is possible for water to exist in liquid form (not all frozen or boiled away) on an orbiting planet. Since liquid water seems to be a requirement for life on Earth, it is reasonable to deduce that the exoplanets with the highest probability of sustaining life would be in the habitable zone of their parent stars. Habitable zones are closer in for cooler stars, like the plentiful dwarf stars, than for larger, warmer stars like the sun. However, being in the habitable zone does not necessarily mean a planet can support life; other factors also come into play. Take, for example, Venus and Mars in our own solar system. They are in our sun’s habitable zone, along with Earth, but Venus and Mars are unsuitable for life because of their atmospheres and resulting extreme environments. On the other hand, even planets outside the habitable zone might sustain life, especially on their moons, if those bodies have thermal properties that enable liquid water (possibly under an icy crust, like on Europa). Furthermore, the most common stars in our galaxy, red dwarfs, generally produce frequent large flares and ejections of charged material that likely would make stable life on planets in their nearby habitable zone impossible.

So, on the one hand, thanks to the bountiful discoveries from recent telescopic surveys and statistical extrapolation, we now know that planets are very common; on average, every star has at least one planet, which means there are hundreds of billions of planets in our galaxy alone. But on the other hand, it appears that most of these planetary systems could be inhospitable to sustaining life, especially advanced life requiring long-term environmental stability. Studying the nature of exoplanets and their interactions with their parent stars is now one of the highest priorities of astronomical research worldwide.

The Nature of Life beyond Earth

If we want to look for life beyond Earth, we need to know what we are looking for and how we would recognize it. That may seem obvious, but the task of understanding possible life in alien environments and its detectable signs is a rich interdisciplinary challenge. The relatively new scientific field of astrobiology addresses issues related to the origins of life, the definition of life, the environments where life can thrive and evolve, and the observable signs of biological activity, including the “biosignatures” we might detect on a distant exoplanet with observations from our own telescopes nearby Earth. If we were to observe Earth from a distant star system, for example, a good telescope might tell us that Earth’s atmosphere contains oxygen, methane, and water vapor. None of these on their own are proof of life on Earth, but together they provide strong evidence of habitability and biological activity. While our best professional telescopes are not quite yet capable of detecting conclusive biosignatures from Earthlike exoplanets, it will not be too many years before the advancing technology will enable such detections, and astrobiologists will be telling us what we should be looking for in the light spectra from these alien worlds.

This might be good enough for finding simple life (e.g., microbes) on other worlds, and that discovery would be profound. But what about finding intelligent, advanced, or even communicative life and civilizations? That is harder. Much harder. Not only because of the difficulty most star systems would have in providing long-term stability for advanced life (see earlier) but also because even if these life forms exist, the likely distances involved between such planets harboring advanced life would be daunting for any likely detection, and even more prohibitive for meaningful communication.

The most famous attempt to quantify the likely number of advanced civilizations in our own Milky Way galaxy was developed by Frank Drake in the 1960s and expressed in his famous “Drake Equation.” In this formulation, the number of technologically advanced civilizations detectable today is a product of several terms, each of which can be determined with lessening uncertainty as scientific methods improve. These terms include factors like the rate of formation of stars, the fraction of stars with planetary systems, the fraction of life-bearing planets on which intelligent life emerges, and, most difficult to estimate, the length of time such civilizations release detectable signals into space (or, rather, how long they survive). A modified version of the Drake equation seeks to deduce only the number of technological species ever formed over the history of the universe (Frank and Sullivan 2016). The conclusion from these approaches seems to be that although technologically advanced species must be incredibly rare, we are likely not the only one. Yet, detecting (or visiting) and communicating with other advanced civilizations is practically impossible given the vastness of time and space, our current technologies, and the limits of the speed of light and electromagnetic communication. (A big disappointment for Star Trek science fiction fans.) Nevertheless, the uncertainties in all the measured quantities of the Drake Equation (Figure 3) compel us to keep studying alien worlds and the history of the cosmos because the possibility of finding life, especially simple life, beyond Earth is real, and its consequences profound.

Figure 3

One form of the Drake Equation, a mathematical formula for estimating the number of technologically advanced civilizations (NOIRLab 2025).

The Significance of Life in the Universe

What is perhaps most profound regarding the universe is simply that there is any life at all. To state what is obvious yet in some sense astonishing: we know there is life in the universe, on at least one planet! Including self-reflective life (that’s us). And this life is intricately connected with the content and history of the entire cosmos.

Astronomers are now busy detecting and studying exoplanetary systems, but that is in some sense like plucking apples off an apple tree; studying the tree itself and its roots is the core of astrophysics and cosmology, and this tells us how the “apples” came to be. Perhaps the most profound discovery since Galileo confirmed a sun-centered solar system is the finding in the early twentieth century by astronomer Edwin Hubble that other galaxies outside our Milky Way exist, and the universe is expanding. This led to the study of distant galaxies, which is a venture back in time, because it takes time for light to travel from stars and galaxies to our telescopes. We are seeing objects in space as they were, and for galaxies, that generally means millions and sometimes billions of years ago. By comparing galaxies distant in space and time to those like our own, we can see that galaxies have changed over cosmic time. They have formed grand spiral structures (Figure 4), and often those structures have been disrupted and enlarged by mergers with other galaxies.

Figure 4

Galaxy NGC 5248, as viewed with the Hubble Space Telescope (ESA/Hubble and NASA, F. Belfiore, J. Lee and the PHANGS-HST Team). Spiral galaxies like this one, and our own Milky Way, contain hundreds of billions of stars. Stars are still actively forming from galactic gas, lighting up colorful nebulae along the spiral arms of the galaxy. Those stars, and the planetary systems that form around them, contain heavier elements like carbon and oxygen produced in previous generations of stars.

The stars inside galaxies are also not stagnant. Advances in stellar astrophysics have made it clear that stars are balls of condensed gas with atoms at the core undergoing fusion and releasing photons of light. These fusion reactions create elements heavier than the original hydrogen atoms that make up the bulk of stars. The heavier elements can include carbon, iron, oxygen, and more. When stars run out of a stable supply of hydrogen at their cores (after millions or sometimes billions of years, depending on the star’s mass), they release some of this heavier material they produced into the interstellar environment. Thus, heavier elements seed interstellar gas clouds, and future stars that form within these clouds have a larger fraction of heavier elements. Eventually, stars can and do form with orbiting disks of dusty material made of heavier elements orbiting around them. It is in these disks that planets form along with the star. With telescopes like the Hubble and the newer Webb space telescope as well as ground-based radio observatories like ALMA, astronomers are studying young stars with circumstellar disks and embedded planets.

The point here is that planetary systems like our own solar system required previous generations of stars to produce the heavier elements needed for solids and water to form. These in turn enabled disks and planetary systems to form around stars like our sun. Stars were not just created once. We know there were generations of massive stars (they have the shortest lifespans) coming and going before our solar system formed 4.5 billion years ago and that many other star and planetary systems have formed since.

Some would point out that none of this stellar manufacturing would be taking place were it not for the finely tuned fundamental physical constants and laws that govern the universe. It appears from many current physical models that if, for example, the gravitational constant or the electromagnetic constants that govern atomic structure were even slightly different, stars (and thus planets and life) could not form. That is indeed an eye-opening realization. But even if the universe were not “fine-tuned” and stars and life were inevitable, the fact the universe exists at all and produces life is remarkable.

The recent detections of thousands of exoplanets and the realization that planets are common feed into an ancient question humans have had about ourselves: Are we significant? As we realize our planet is just one of hundreds of billions in our galaxy alone, does this make us insignificant? What if we do or do not find life beyond Earth—does that make life on Earth more or less significant? These questions do not have a consensus answer, because they are philosophical. If significance is based on position in the universe or lifespan relative to the cosmos, then we are indeed insignificant. But perhaps the significance of life in the universe comes not from position, lifespan, or rarity but rather from the fact that we are literally and physically connected with the universe and its processes from the very beginning of time as we know it. Our bodies contain atoms that were literally made in stars before the birth of our own solar system. And we are here to contemplate this. Whether or not there is life beyond Earth, the very existence of our life here speaks of profound significance.

Some Implications of Exoplanet Discoveries and the Search for Life

The aim of this article has been to summarize the incredible scientific advances of the last century in the search for worlds beyond Earth, and the related search for extraterrestrial life. This quest has profound implications for humanity. Initial theological implications of this contemporary scientific search have been addressed by, for example, theologians David Wilkinson (2017; Wilkinson and Wiseman 2024), Ted Peters (2018), Andrew Davison (2024), and their collaborators. Potential impacts for society more broadly have been explored by, for example, historian Steven Dick (2000; 2020).

Given the more recent explosion of exoplanet detections, which open up genuine possibilities of finding at least simple life beyond Earth in the coming decades, theology and philosophy must now engage with renewed vigor the implications of possible myriad life-bearing worlds. Are we, as humans, ready?

And what about the spiritual state of potential life on other planets? Centuries of thorough but inconclusive thought on this topic cannot be adequately summarized here; we simply do not know (but check out C. S. Lewis’s space trilogy for a fictional approach).

Contemplations of life on and beyond Earth lead to reflections, both corporate and personal, on the spiritual significance of life in the cosmos. Informed people of faith can no longer envision the heavens as static and sterile. As a scientist and a person of faith, my personal reaction to a lively universe is one of enriched appreciation of God—God as Creator, God Incarnate, and the God we worship. Here, I elaborate on each.

First, can the universe teach us something about the nature of God? Science is a good tool for addressing how the universe works but not for analyzing a possible “who” behind it. That said, for observers looking through a lens of faith, it is possible to infer some character qualities of God through what is seen in the universe, as per Psalm 19: “The heavens declare the glory of God; the skies proclaim the work of his hands.” Thinking about the cosmos, myriad exoplanets, and a universe that produces life, I conclude that God is powerful, creative, and a lover of life, including life that can contemplate beauty and meaning.

Next, in arguably the most profound passage in the New Testament, John 1 states that “[i]n the beginning was the Word” and that “[t]hrough him all things were made.” And then, the great declaration: “The Word became flesh and made his dwelling among us. And we have seen his glory, the glory of the one and only Son, who came from the Father, full of grace and truth.” The word of God is manifest in Jesus, and Jesus’s birth on planet Earth marks the incarnation of God among us. I reflect on this in light of the modern cosmology described earlier. According to this passage, an aspect of God, through whom the dynamic universe was created, was born in human flesh on our planet, with a body containing atoms forged in stars. If we are made of “star stuff,” as Carl Sagan mused, then so was Jesus. What does this mean for us? I conclude it means that God’s incarnation in Jesus must have relevance for the entire universe. It also means God cares about life on this particular planet.

Finally, as science and technology are quickly opening up a universe to us richer in content and history than ever imagined, I believe faith communities and their leaders would be wise to embrace with joy our new knowledge of a universe full of planets and the potential for a universe full of life. Worship of the Creator—including hymns, prayers, liturgy, and sermons—should honor the God of a fruitful, evolving universe, abundant in planets and the elements needed for life.

Perhaps a twentieth century hymn is a good starting point for envisioning science-revitalized twenty-first century worship. The 1955 version of “How Great Thou Art” (Carl Boberg, Stuart Hine) contains a prescient mention of worlds we can now envision as exoplanets and directs human response toward awestruck and exuberant worship:

Oh Lord my God

When I in awesome wonder

Consider all the worlds thy hands have made

I see the stars;

I hear the rolling thunder;

Thy power throughout the universe displayed

Then sings my soul; my Savior, God to thee:

How Great Thou Art! How Great Thou Art!