During a visit to the Los Alamos National Laboratory in 1950, Enrico Fermi, one of the architects of the Manhattan Project, was having lunch with other physicists when the conversation turned to flying saucers. At one point during the discussion, Fermi blurted out: “Where is everybody?” – or, as another participant remembered it, “Don’t you ever wonder where everybody is?” If aliens exist, in other words, why have we never encountered them?
The episode has become famous, marking the moment when extraterrestrial life went from a theme for philosophical speculation to a serious scientific problem. For centuries, people had wondered whether life exists on the moon or Mars, but they accepted that we would never know for certain, since making contact with another planet was impossible. By the 1950s, those limits no longer seemed so fixed. In addition to space flight, researchers were developing tools to detect the radio signals that reach us from stars many light years away. These developments were still in their infancy, but it was already clear that humanity was no longer bound to Earth the way it always had been.
This expansion of human horizons raised an awkward question. Since the 16th century, when Copernicus showed that Earth is not the centre of the cosmos, modern science had learned to embrace the idea that there is nothing unique about our planet. And if Earth is statistically average, there is no reason why it should be the only home of intelligent life. Given the age of the universe – about 13.8bn years, according to current estimates – it should be full of species with technology far more advanced than what we have come up with in a few thousand years of civilisation. Yet we can’t find any trace of them.
That was the case in 1950, and it is still the case today, despite well-publicised claims to the contrary. Since 2017, when the New York Times reported that the US military possessed video recordings of pilots’ encounters with UFOs, aliens have gone mainstream. The House of Representatives even held hearings on what are now called unidentified anomalous phenomena (UAP) as a potential national security threat. But the blurry and inconclusive “Pentagon UFO videos” have not been followed by any clear images of nonhuman craft, just as earlier waves of UFO sightings never produced any unambiguous evidence.
For most scientists, the real mystery about aliens isn’t their presence but their stubborn absence.
***
The first serious effort to detect radio signals from alien civilisations in deep space was made in 1960, when the American astronomer Frank Drake conducted an experiment he named Project Ozma. Using a telescope at the National Radio Astronomy Observatory in West Virginia, Drake looked for signals at a precise location on the electromagnetic spectrum: 1420 MHz, known as the “hydrogen line” because hydrogen atoms emit photons at that frequency.
Because hydrogen is so abundant in the universe, the line is easy for radio telescopes to detect. In a 1959 paper, astronomers Giuseppe Cocconi and Philip Morrison argued that this fact would be discovered at an “early stage in the development of radio astronomy” by any technological civilisation, as it was by human beings. That made it likely, they reasoned, that any species looking to broadcast radio signals to attract the attention of cosmic neighbours would choose to do so at 1420 MHz. But though Project Ozma spent 150 hours observing two relatively nearby stars similar in size to our sun, it detected no unusual activity at the hydrogen line.
In 1971, Nasa convened a panel of experts to study the best ways to search for extraterrestrial life. Their report, Project Cyclops, was published the next year, and has remained a kind of bible for the search for extraterrestrial intelligence (usually shortened to Seti) ever since. The report called for the construction of the Cyclops system, “an ‘orchard’ of antennas 10km to 10 miles in diameter and containing 1,000 to perhaps 2,500 antennas”. Such an array could scan the sky 200,000 times faster than Ozma.
The cost of building Cyclops was estimated at $10bn to $25bn, the equivalent of $77bn to $192bn today, making it about as expensive as the Apollo programme. It was never built, and Congress ended Nasa’s modest funding for Seti projects in 1993. Since then all US efforts have been privately funded, mainly by billionaires such as Paul Allen, cofounder of Microsoft, and Yuri Milner, a Soviet-born entrepreneur. Advances in computer technology mean that today’s Seti efforts are vastly more sophisticated than half a century ago, able to examine millions of radio channels from tens of thousands of galaxies. But the result remains the same: no artificial signal from an alien civilisation has ever been detected.
That doesn’t discourage true believers. Seth Shostak, an astronomer at the California-based Seti Institute, writes that Seti’s “failure is tempered by the fact that the number of star systems sampled is still quite small”. In 2010, astronomer Jill Tarter, one of the leading figures in the field for decades, calculated that so little of the universe had been searched that it was like dipping a single glass into the ocean to determine whether it contains fish.
If extraterrestrial signals are out there and it’s just a matter of looking in the right place, it’s entirely possible that Seti could find one tomorrow. But it hasn’t happened in more than 65 years of searching and, until it does, alien signals remain as hypothetical as alien spaceships. Not only is there no evidence that extraterrestrials are trying to contact or visit us; there is no evidence that they exist at all. And this non-appearance presents a more serious challenge to modern scientific assumptions than any UFO sighting. It turns out that there really is something unique about Earth: as far as we currently know, it is the only place where life exists. But why?
***
In October 1961, a year after Frank Drake failed to detect any alien radio signals with Project Ozma, he invited a dozen scientists to a conference to discuss the future of Seti. To clarify the problem, he tried to calculate how many extraterrestrial civilisations could be broadcasting radio signals in our galaxy. The answer, Drake reasoned, depended on seven variables, including “How many planets in an average system are capable of supporting life?” and “On planets where life exists, how often does an intelligent species like humanity evolve?”
Multiply all of the variables and you get the number of potential targets for Seti in the Milky Way. Drake wrote out the formula, which has been known ever since as the Drake equation. At the time, none of the variables could be estimated with any confidence. Drake’s back-of-the-envelope calculations yielded an estimate of 50,000 transmitting civilisations, but this was essentially just a guess. The real purpose of the Drake equation was to establish a research programme, a set of questions for astronomers to try to answer. In the 21st century, they have made significant progress, thanks to new space-based telescopes that make it possible to see the cosmos in more detail than ever before.
For the first variable – how fast are new stars formed in the Milky Way – it’s now estimated that 10 to 20 new stars are born every year (a far slower rate than when the galaxy was young and richer in star-forming gas). The total number of stars in our galaxy is on the order of 100bn. In the visible universe as a whole, there are an estimated 2tn galaxies, yielding a total of about 100 sextillion stars. So much is still unknown about the universe that this figure is certain to be revised, but it’s sufficiently mind-boggling to get the Drake equation off to a promising start.
It is much easier to detect stars than the planets that orbit them, and in 1961 the answers to Drake’s second and third questions – how many stars have planets, and how many of those planets are potentially habitable – were completely unknown. As telescopes gained in power, it became possible to detect exoplanets – planets outside our solar system – by measuring the tiny variations in a star’s light caused by a planetary mass passing in front of it. The first exoplanet was detected in 1992, the second in 1995. The pace of discovery exploded in the 2010s, thanks to observations from the Kepler space telescope and the Transiting Exoplanet Survey Satellite.
As of 2025, some 6,000 exoplanets have been identified in the Milky Way, including at least three orbiting Proxima Centauri, the closest star to the sun. This is just the tip of the iceberg. While it’s not yet possible to conclusively answer Drake’s second question, a study published in Nature in 2012 suggested a minimum of 100bn planets in our galaxy alone.
To answer Drake’s third question – how many of those planets are capable of sustaining life – a new scientific discipline has emerged: astrobiology, the study of life among the stars. This is a paradoxical endeavour, since there are no examples to study. Instead, astrobiologists think about the basic conditions that make life possible, and how we might determine whether distant planets meet those conditions.
Exoplanets are far too distant to see what is happening on the surface. But it’s possible to measure the size of the planet and its distance from the star it orbits, and make deductions about its atmosphere and temperature – and, more indirectly, about its magnetic field. Since Earth is the only planet where we know life exists, it’s natural to assume that exoplanets that resemble it in these respects are the most likely to be habitable.
Astrobiological research thus also involves investigating life on our own planet, to understand the range of environments in which it can develop. The discovery of “extremophiles”, primitive organisms that thrive in extreme conditions, has shown that living things can flourish under two miles of ice and in hot springs that reach 208F (98C). This suggests that life of some kind could develop even on inhospitable planets. For instance, while there is no liquid water on Mars today, frozen remnants of ancient oceans are thought to exist under its surface; it’s possible that some kind of extremophilic life could be detected by future probes.
In 2025, Nasa announced that a rock sample analysed by the Perseverance rover included two minerals, vivianite and greigite, that on Earth are byproducts of bacterial metabolism. The minerals may be a sign that primitive microorganisms existed on Mars several billion years ago. If confirmed, this would be a revolutionary discovery. As the astrobiologist David Catling has written, “Even the simplest microbes native to Mars … would change the balance of probabilities that life exists elsewhere in the galaxy, for they would demonstrate that life can originate twice within one solar system.” In fact, proof of life on Mars would not necessarily mean that it originated independently on two planets. It’s also possible that life could have emerged on Earth and travelled to Mars or vice versa, carried through space in meteorites or dust. That would be evidence for the “panspermia” theory, which holds that life originated in just one or a few places in the cosmos and then spread naturally over eons.
On Earth, any life form more advanced than bacteria requires liquid water to survive. Thus the most important criterion for a habitable exoplanet is that it can’t be too close to the star it orbits, where intense heat would evaporate water, or too far from the star, where cold would turn it into ice. If an Earthlike planet is inside the “Goldilocks zone” where the temperature is “just right” for liquid water to exist, it can be considered a candidate for life.
The first such planet to be discovered was Kepler-452b, seen by the Kepler space telescope in 2015. It is a bit larger than Earth and takes 385 days to complete an orbit of its star, very similar to our year of 365 days. By 2025, the Habitable Worlds Catalog, a database maintained by the University of Puerto Rico, included 29 Earthlike exoplanets, and the process of discovery has only just begun. A study published in the Astronomical Journal in 2020 estimated that the total number of Goldilocks-zone planets in the Milky Way is at least 300m, and possibly as high as 6bn.
Astronomy has made enormous progress in filling in the first three variables in the Drake equation. The difficulty, as the astronomer Sara Seager has written, is that “the first three factors … are measurable; the other four are not, and arguably never will be”.
The closest we can come to answering Drake’s fourth question – how many planets actually develop life – is to look for biosignatures, chemical compounds in the atmosphere of an exoplanet that on Earth are associated with the presence of life. In 2025, researchers announced they had found dimethyl sulfide in the atmosphere of K2-18b, an exoplanet in the habitable zone of a star 124 light years from Earth. Because that compound is produced on Earth by ocean plankton, it could be a sign of the presence of organic life. But the finding was quickly challenged by other scientists, who argued the data did not prove the presence of dimethyl sulfide.
K2-18b is a good example of the limits of astrobiology. To find an exoplanet is very difficult; to find one in the Goldilocks zone is even harder; to detect potential biosignatures is harder still. And even if biosignatures were to be definitively confirmed, it would only show that life may exist. There is no way to know whether it actually does, much less what form it takes.
The search for exoplanets is radically expanding our understanding of the cosmos. “We have learned that the universe is teeming with a fascinating variety of planets, more types than we could have imagined,” writes the astrobiologist Lisa Kaltenegger. But so far, it has found exactly as many extraterrestrials as radio astronomy has: zero. The more we learn about the universe, the more acute the Fermi paradox becomes.
***
To explain this “great silence”, researchers focus on the last three variables in the Drake equation: on planets where life exists, how often does an intelligent species like humanity evolve? Of intelligent species, how many develop the technological ability to send radio signals into space? And finally, how long does a transmitting civilisation last before it disappears?
These questions go beyond the limits of astronomy; they can’t be answered, even in principle, by developing more sensitive telescopes. The only way to shed light on them would be to assemble a catalogue of species across the cosmos and study their history. But the reason we have to ask Drake’s questions in the first place is that we don’t have such a catalogue.
In practice, then, thinking about extraterrestrial intelligence means thinking about the only intelligent species we do know – ourselves. What conditions had to exist for Homo sapiens to evolve and develop spacefaring technology, and how long can we expect technological civilisation in its current form to survive?
Of the seven variables in the equation, Drake and his colleagues found that last one, which they called “L” (for “length of time”), the hardest to calculate, offering a range of estimates from 1,000 to 100m years. This uncertainty reflects the fact that L is essentially a prediction about the future of humanity. As long as ours is the only technological civilisation we know about, we have no way to estimate the longevity of such civilisations except to think about how long ours can be expected to last. And there is a good reason to think this may not be a long time. The birth of radio astronomy and space flight coincided with the invention of nuclear weapons; the same rocket technology that sent astronauts to the moon was used to build intercontinental ballistic missiles. Human beings have managed to avoid destroying ourselves in a nuclear war for the past 80 years, but it would be a bold prophet who would guarantee that we will not do so in the next 80, not to speak of the next 800. It is possible that technological progress is self-limiting: any species powerful enough to escape its planet is powerful enough to destroy it.
Drake thought that multiplying all the variables in his equation would yield about 50,000 transmitting civilisations in our galaxy. If the true number turns out to be just one, it follows that at least one of those variables must have a much lower value than expected. In other words, there must be one or more steps in the evolution of advanced civilisation that are very difficult to surmount.
As the economist Robin Hanson wrote in an influential 1998 paper, “There is a ‘great filter’ along the path between simple dead stuff and explosive life.” For some reason, “the vast vast majority of stuff that starts along this path never makes it. In fact, so far nothing among the billion trillion stars in our whole past universe has made it all the way along this path.”
Like the Drake equation, the great filter offers a structure for thinking about what makes technological civilisation possible. Any answer to that question involves astronomy, physics and biology. It also leads to questions about human nature and destiny that have traditionally been the province of philosophy and religion.
***
That certain physical conditions are necessary for the development of life has been recognised for a long time: distance from the sun, the presence of water, a breathable atmosphere. But there are a multitude of other cosmic coincidences that may be equally necessary for life to develop and survive. For instance, any planet that is regularly bombarded by asteroids would find it difficult to sustain life for long. When a single asteroid about six miles in diameter struck the Yucatán peninsula 66m years ago, it is thought to have extinguished three-quarters of all existing species, including the non-avian dinosaurs.
Such impacts would be much more common if not for the presence of Jupiter, a giant planet just the right distance from Earth to intercept comets and asteroids. A Jupiter-like neighbour may be a prerequisite for advanced life, since on a planet without one, the clock of evolution would be continually reset by mass extinctions.
Then there is the role of plate tectonics. The Earth’s surface is broken into large plates that drift very slowly over time. At their fault lines, old rock made of carbon is drawn into the planet’s hot interior and new rock is produced in the form of magma. This “conveyor belt” has the effect of stabilising the level of carbon in the atmosphere, preventing runaway heating or cooling that could extinguish life.
Geologists aren’t certain why Earth has tectonic plates – the current best theory is that the decay of radioactive elements in the interior of the planet produces heat that melts subsurface rock. But we know that ours is the only planet in the solar system to have them. Mars and Venus, which are rocky and about the same size as Earth, have rigid surfaces that don’t break into separate masses. So it’s possible that plate tectonics, too, belongs on the list of prerequisites for life.
The more conditions life needs, the easier it is to whittle down the galaxy’s 100bn planets as potential homes for it. The “rare-Earth hypothesis”, writes the Serbian astrophysicist Milan Ćirković, holds that “each of these requirements [for life] is unlikely and they are (or at least seem to be) causally independent, so that their combination is bound to be incredibly rare and probably unique in the Milky Way”.
Life on Earth seems to have emerged fairly quickly after the planet was formed; the oldest bacteria are only a few hundred million years younger than the solar system. This seems like an encouraging sign that, under the right conditions, it is easy for life to get started. But after those early beginnings, major evolutionary milestones came very slowly. It took about 2bn years for the first cellular nucleus to develop, enabling the transition from primitive prokaryotic bacteria to more advanced eukaryotes. Another billion years passed before the emergence of the first multicellular organisms.
Strikingly, genetic evidence shows that each of these crucial developments occurred only once in the history of life on Earth. This suggests that they may not be inevitable stages that life would pass through anywhere, but surpassingly rare accidents. Life may exist on other planets in the form of archaea – unicellular organisms that can thrive in extreme conditions – without ever evolving as far as sponges, much less plants and animals.
As for mammals like ourselves, our reign on Earth is also highly contingent. Dinosaurs were our planet’s dominant species for more than 150m years. By comparison, Homo sapiens has existed for only about 300,000 years, 0.006% of the history of Earth. When we look back on the history of humanity from this point of view, we seem to see ourselves perched on a toppling Jenga tower of incredible coincidences. So many factors had to line up perfectly for us to come into existence; change just one and we would not be here. The effect is vertiginous, reminding us that everything we take for granted about our world is in fact radically contingent. Not only do life and human life not need to exist; it would be immensely more reasonable for them not to exist.
***
In centuries past, some thinkers came to a similar conclusion and saw it as proof of providence, which shaped the finely tuned structure of the cosmos to foster the evolution of humanity. Today, rather than turning to old supernatural doctrines to explain human uniqueness, a new generation of cosmologists argues that we need to radically expand our idea of the forms life and intelligence might take, and how we should go about looking for them.
Seti operates on the premise that we will find aliens because they want to be found. It looks for radio signals sent deliberately into space with the goal of attracting attention from civilisations advanced enough to detect them. The Project Cyclops report argues that “the sending race will attempt to make the job of deciphering and understanding the messages as simple and foolproof as possible”.
This assumption reflects the optimism of the space age, when humanity landed on the moon and gazed into deep space for the first time. Having accomplished so much so fast, it was natural to believe that other intelligent species would be equally venturesome. Wouldn’t they want to end their cosmic loneliness as much as we do?
Such ideas about how aliens would act are fundamentally anthropocentric, imagining that any intelligent species would be driven by the same motives we are: love of knowledge and love of power. But there is no reason why intelligent life elsewhere in the universe should have evolved to resemble us, either physically or mentally.
In a 2016 paper titled The “Hard Problem” of Life, physicists Sara Imari Walker and Paul Davies observe that “both astrobiology and our assumptions about non-human consciousness tend to be biased by our understanding of terrestrial life”. We take for granted that the particular forms that evolved on Earth – “from the level of cells, to multicellular organisms, to eusocial and linguistic societies” – are the inevitable product of natural laws, so that something like them must emerge anywhere life exists. But Walker and Davies argue that, in fact, life as we know it is the product of “a combination of chance and necessity”, including many unlikely events that pushed the evolution of life on to new pathways.
Rather than expect all living things to look like us, with arms and legs and rocket ships and radio telescopes, Walker and Davies propose that we think about life in a new way, defining it in functional terms as “the actual physical mechanism that permits information to gain causal purchase over matter”. On Earth, information is encoded physically in DNA strands and synaptic impulses; elsewhere in the cosmos, it might take such different forms that we wouldn’t recognise it even if we found it.
This idea fascinated the 20th-century Polish science-fiction writer Stanisław Lem, who dramatised it in a number of novels and stories. In his best-known book, Solaris, humanity discovers a planet covered by a sentient ocean. The second chapter, The Solarists, is an elaborate imaginary history of scientific debates about this organism, which resembles a “syrupy jelly”. Is it a “primitive being” or a “highly organised structure”? Had it bypassed “all the terrestrial stages of development – that is to say, the emergence of protozoa and metazoa, plant and animal evolution” – yet still managed to become intelligent?
The novel develops into a kind of horror-thriller, as astronauts marooned on Solaris are stalked by bizarre apparitions. But these aren’t ghosts; they are attempts by the ocean-being to communicate with the visitors by reading their minds and adopting the shapes it finds there. The ocean itself seems to think by creating elaborate shapes and structures with its gelatinous waves. But the human visitors are totally unable to grasp the meaning of these movements. The real tragedy of Solaris isn’t the gruesome deaths of the scientists, but what one of them calls “the failure to make contact, the lack of response”.
More recently, theorists and storytellers – a distinction that often blurs when it comes to thinking about aliens – have begun to speculate that what separates us from extraterrestrials isn’t different biologies. Rather, biology itself may be a primitive phase in the development of mind, which truly advanced civilisations inevitably outgrow. After all, the most impressive technological achievements of the 21st century have not been in space exploration, but in computing, which allows us to create ever more detailed and powerful models of the world. If this progress continues, we have begun to worry, artificial intelligence and virtual reality could replace human intelligence and material reality.
Perhaps this is why we have so far failed to find traces of alien life. When intelligent species are sufficiently advanced, maybe their goal isn’t colonising planets, but escaping physical existence altogether. John Smart, a non-academic futurist who describes his work as “acceleration studies”, calls this the “transcension hypothesis”: aliens are invisible to us because they have transcended bodily existence as we currently understand it.
Presenting this idea in a 2012 paper, Smart speculated that as civilisations develop “more computation and simulation abilities”, they will expand not into outer space but into “inner space”, creating virtual realities more complex and interesting than physical reality. On Earth, computers have become ever more compact as they grow more powerful; so it makes sense to assume that the material substrate of alien virtual realities – the hardware on which the software of consciousness runs – will come to occupy less and less physical space. Ultimately, Smith argues, this process could yield a technology so compact that it will essentially disappear from the map of the cosmos.
It’s clear that the transcension hypothesis, like the cold war-era idea of galactic colonisation, is a kind of presentism, assuming that whatever is happening now will continue to happen in the future, only more so. It elevates the computer revolution of the 21st century to a universal process that every technological civilisation will follow.
But in thinking about extraterrestrial life, it’s hard to see how some degree of anthropocentrism and presentism can be avoided. In the absence of actual data about aliens, thinking about them is an act of imagination, and the only material our imaginations have to work with is our own experience. As our understanding of our species changes, so does our sense of what is probable or possible for our cosmic “others”. Just as accounts of UFO encounters are human testimonies, not scientific evidence, so all our thinking about aliens and how to find them is best understood as humanity’s evolving self-portrait.
Adapted from We Want to Believe: How Aliens Went Mainstream and Why It Matters, published by Columbia Global Reports on 25 August. To support the Guardian, order your copy from guardianbookshop.com. P&P may apply
• Listen to our podcasts here and sign up to the long read weekly email here.
