There is a common belief among astrophysicists and other scientists that studying the universe has revealed our own planet as something less than special. The reasoning is as follows: Earth, long assumed to be stationary and unmoving, is just one of many planets orbiting our sun. Our sun is nothing more than a regular, nondescript star, one of hundreds of billions found within the Milky Way. The Milky Way itself is just one of an estimated 2 trillion galaxies strewn across the expanse of our observable universe. As our own insignificant home, Earth, is teeming with life, including intelligent and technologically innovative human beings, wouldn’t it be reasonable to infer that whatever is common here is plentiful throughout the universe?

According to this default assumption, the same ingredients found here—elements, molecules, and various favorable conditions—can be found practically everywhere we look. The same physical rules that apply here are no different elsewhere in the universe. Given all the stars, planets, and chances for life that surely exist within our galaxy and beyond, we’ve mostly stopped asking whether life exists beyond Earth. Instead, we now ask how common it may be.

But for all this impressive theorizing, the best evidence hasn’t matched expectations. Despite decades of searching, we haven’t detected even a single robust signal that indicates the presence of intelligent aliens. This conundrum is commonly known as the Fermi paradox, after the famed physicist Enrico Fermi. It goes like this: If the ingredients for life are everywhere, and there are astronomically large numbers of stars and planets where it’s possible for life to have arisen, then we’d expect many instances in which intelligent aliens rose to prominence well before the advent of human life on Earth. Such beings should have had plenty of time either to have colonized the galaxy or designed a broadcasting system that would be unmistakable as a sign of intelligent life. Yet we haven’t discovered a shred of credible evidence favoring the existence of intelligent extraterrestrials. 

If the universe is teeming with life, then where is everybody?

While we certainly owe it to ourselves to look for their presence with all the resources we can muster, we must confront the possibility that perhaps we’ve got it all wrong about just how common life in the universe is. Perhaps the ingredients and conditions on Earth don’t inevitably lead to life arising on a potentially habitable world beyond our planet. And even if life does arise elsewhere, it may be the case that it frequently fails to thrive. Maybe it’s the case that even successful life only rarely becomes complex, differentiated, or intelligent as we understand those terms. Or, quite possibly, it’s exceedingly rare that even intelligent life becomes technologically advanced. In all of space, as far as intelligent life goes, perhaps humanity is truly alone.

The first scientific estimate concerning the number of intelligent, spacefaring, communicative extraterrestrials came from the American astronomer Frank Drake. His method of constructing estimates for the number of intelligent extraterrestrial civilizations—developed in 1961—gave rise to what’s now known as the Drake equation.

Although his estimates—and even his framing of the problem—are outdated today, we no longer rely on the degree of guesswork we once did. In the decades since Drake first set about his task, we’ve surveyed the vast abyss of the distant universe and discovered many important things. We’ve learned the size of the observable universe and the duration of time since the hot Big Bang. We know that there are at least 2 trillion visible galaxies. We now understand star formation, stellar populations, and how stars burn through their fuel and die. We know that over the entire cosmic history of the observable universe, there have been approximately 10²⁴ unique stars.

That’s our starting point for estimating the number of chances that the universe must have produced Earth-like life. 

If we assume that life like us requires a planet like ours, we need a star that’s Sun-like in nature and a planet with a rocky surface and thin atmosphere orbiting that star. But that’s just the beginning. We also need that planet’s size and mass to be similar to those of Earth. Additionally, this Earth-like planet must orbit its Sun-like star at a distance that allows for liquid water to exist on the planet’s surface. And the planet must have a sufficient number of certain atoms and molecules—the raw materials of life.

Over the past few decades, advances in exoplanet sciences, buoyed by the deluge of data from NASA’s Kepler mission, have enabled us to estimate all of these cosmic unknowns. Approximately 20 percent of all the stars out there are Sun-like, as opposed to red dwarfs (which tidally lock their planets and likely strip their atmospheres away) or the hot, blue stars whose stellar lifetimes are too short. At least 80 percent of stars have planets or planetary systems around them, and approximately 10 to 20 percent of those planets are Earth-like in size and mass. Well over 90 percent of them have enough of the necessary heavy elements—created in earlier generations of stars—for life to have possibly arisen. And finally, approximately 20 to 25 percent of the star systems we know of appear to have at least one planet in their star’s so-called habitable zone, which is the right location for an Earth-like planet to possess liquid water on its surface.

All told, we expect there are nearly 10²² potentially habitable, Earth-like planets containing the right conditions and ingredients for life. More than a billion such candidate planets exist in our Milky Way alone. 

But purchasing a large number of lottery tickets is no guarantee of winning the jackpot. The odds depend on the overall probabilities of victory. Even though an enormous number of chances for the advent of intelligent, spacefaring life within our observable universe exists, there are still three big sequential hurdles to overcome.  

First, life must arise from nonlife. The raw ingredients associated with organic processes must actually become what we recognize as life, through a vague, speculative process called abiogenesis.

Second, once life arises, it must not only survive and thrive for billions of years but also develop features such as multicellularity and specialized organs and functions. It must become complex and differentiated, and evolve to have the quality we recognize as intelligence.

Third, this intelligent life must then achieve technological advancement, either gaining the ability to announce its presence to the universe, to hear and respond to other intelligent broadcasts, or to venture beyond its home world and explore interstellar destinations.

In all the universe, Earth is our only example of a planet where any one of these three steps have occurred. The ease—or difficulty—of getting over these three hurdles represents the last major unknown in determining how common, or rare, intelligent extraterrestrials actually might be. It’s an unknown that has long been neglected by those who hope that the discovery of alien life is just around the corner. Thus, a closer examination of each is necessary if we’re to be realistic about the chances of finding other beings like ourselves. 

From NonLife to Life. The ingredients for life really are ubiquitous in a galaxy like the Milky Way. Organic compounds, including sugars, amino acids, molecules with carbon-containing rings, and even ethyl formate—the molecule that gives raspberries their smell—are found throughout space. They appear everywhere from interstellar gas clouds to the outskirts of young star systems. These chemical precursors to life are found throughout our solar system, showing up in lunar and Martian samples. Even analyzed meteorites that have fallen to Earth have been found to contain the 20 amino acids essential to life processes (and more than 60 additional amino acids with no known biological applications).

But flour, sugar, butter, and eggs are not the same as a cake. Similarly, there’s a big difference between the raw ingredients for life and life itself. Organic molecules may be everywhere, but what about actual life? To qualify as a living organism, these four criteria must be met:

1. Life must have a metabolism, harvesting energy, resources, or both from its external environment, to be used for its own self-sustaining purposes.
2. Life must react to external stimuli outside of its own existence, and alter its behavior in response.
3. Life must permit some sort of growth, adaptation, or the ability to evolve from its present form into a different one.
4. Life must be able to reproduce, creating viable offspring that arise from a process entirely internal to itself.

Proteins, despite having a metabolism and the capability of reproduction, are not alive, as they neither respond to stimuli nor alter their behavior. Snowflakes and other crystals, on the other hand, can grow and reproduce, but they have no metabolism. Even viruses can reproduce only by infecting a successfully living cell and are not considered alive as a result. 

These four qualities have never been found, together, on any world other than Earth. 

Earth likely possessed copious levels of raw, organic ingredients from its inception. In laboratory experiments that attempt to mimic the atmosphere of Earth’s early days, those precursors have been exposed to external energy and have given off protein fragments, lipid layers, and individual nucleotides. It’s not so difficult to imagine that life could spontaneously, under the right conditions, emerge from these molecular progenitors.

It clearly did on Earth. And we have some sense of when it happened. While the first microbial fossils we have date back some 3.5 billion years, there are graphite inclusions found in metamorphosed rocks that date back to 3.8 billion years ago. Certain carbon-based crystals, discovered in zircon deposits, push the suspected origin of life on Earth back to more than 4.3 billion years ago: nearly as old as the Earth itself.

But if we can approximate when Earth’s earliest organisms first arose, we still don’t know much about where it happened. Was it in the oceanic tidepools that formed along the edges of continents, triggered by sunlight and shadow, evaporation and fluid flow, and gradients of water activity? Was it near the volcanically energetic, hydrothermal vents at the bottom of the oceans? Or was it in hydrothermal fields, where freshwater and volcanic hotspots on continental land came together in the presence of minerals and organic molecules?

Not only don’t we know the answer, we don’t know whether life arose just once or many times. We don’t know whether an organism arising in one environment outcompeted all the others, or whether it was the ancestor of everything that’s ever lived. We don’t know whether the conditions that gave rise to life required a rare confluence of circumstances or whether they happened easily.

While many scientists are optimistic that it may be easy to create a simplified form of life, we’ve never successfully done so, nor have we witnessed it happening. We have yet to detect any life-form that didn’t originate on Earth. And as far back as we’re capable of tracing it, all life on Earth goes back to a single, universal common ancestor. Life might be common in the universe, but until we detect a second example where life arose from non-life, we cannot know. Thus, we must accept that we might be all alone in the galaxy, and, perhaps, beyond.

From Life to Complex Life. For life to achieve multicellularity, complexity, and the differentiation required for intelligence, it must persist and thrive for billions of years. For perhaps the first 2 billion years of life on Earth, such life was single-celled and prokaryotic (lacking a cell nucleus or other internal organelles), and only reproduced by copying itself and dividing. It is said that the only source of genetic variation came via random mutation, which is an extremely slow pathway for evolution. In a stable environment, where the organisms that are currently successful face few challenges to their survival, there are no pressures that favor the selection of a novel organism that might rise to prominence.

The journey from simple life to complex life requires a changing planet. When there’s a change in resource availability, competition, or the survivability of the environment, species can easily go extinct. Many a successful organism that thrived for millions of years on Earth was destroyed by a changing climate, a volcanic eruption, an asteroid strike, or even its own metabolic waste products. Whenever an organism can no longer occupy an ecological niche, it leaves open the possibility for new life-forms to rise to prominence. While we expect similar processes all across the universe, all of our inferences flow solely from Earth’s biological history.

Consider the following example from our own planet. On Earth, organisms have been taking advantage of photosynthesis for more than 3 billion years. In photosynthesis, light of a particular wavelength strikes a molecule and excites it, and the Sun’s energy gets put to biological use. Hydrogen, sulfur, and numerous acids initially provided the electrons that early photosynthetic organisms used in their life processes. Hundreds of millions of years later, the cyanobacteria (or blue-green algae) arose, using the oxygen molecules in water as electron donors. Unlike other photosynthetic organisms, the cyanobacteria produce molecular oxygen as a waste product.

After hundreds of millions of years, that oxygen accumulated in the atmosphere. It reacted with early Earth’s methane, producing carbon dioxide and water, which greatly reduced our planet’s greenhouse effect. Thus the cyanobacteria’s success translated into disaster for the planet, causing a mass extinction as the planet froze over entirely. Simultaneously, the corrosive, toxic oxygen killed off most of the other, non-oxygen-using life-forms.

Yet this disaster was enormously beneficial for accelerating evolution. The cyanobacteria thrived, while other organisms—facing selection pressures and changing environments—evolved in myriad directions. Separated organelles arose inside cells, and creatures accumulated larger numbers of genes and new combinations of abilities. The organisms that were more resilient to change survived, passing on their genes to a new generation. These eukaryotes (cells containing separated, independent internal structures) developed specialized internal systems that functioned independently of others. Eventually, multicellularity enabled further differentiation, and sexual reproduction allowed offspring to express vastly different traits from those of their parents. Nearly 4 billion years passed between life’s first moments on Earth to the Cambrian explosion, when complex, differentiated life became dominant. 

If we located an alien planet and found starfish, sharks, crustaceans, and insects, we’d be delighted.

But is such a world common? On this front, we only know that Earth has been a cosmic success story; we have no idea what the probabilities are of simple life surviving, thriving, and evolving to produce something akin to our vast array of animals. It could be almost inevitable, given an Earth-like world, or it could come down to an ultra-rare confluence of circumstances—including DNA absorption, the rise of eukaryotes, multicellularity, and sexual reproduction—that led to our world’s winning the biological lottery. Without a sample size greater than ourselves, we cannot know the odds.

From Advanced Life to Advanced Technology. This final step is shrouded in the highest levels of uncertainty. If we judged our own planet by the criteria that we demand of extraterrestrial technology—that aliens either communicate or travel across interstellar distances—then Earth has been technologically advanced for less than a single century. We’ve certainly achieved some remarkable things in that time. These include sending radio signals out into the universe, broadcasting our presence to the stars; launching space probes and crewed missions beyond our own planet, and even (in the case of the Voyager, Pioneer, and New Horizons spacecrafts) out of the solar system; and monitoring the skies for other forms of intelligence out there in the universe. But it’s important to remember that, given all of the time that Earth has had complex, differentiated life, only 0.00002 percent of our history is marked by our being a technologically advanced civilization. Perhaps that says something about the difficulty of reaching such a milestone. 

It’s only by engaging in these recent endeavors that have we begun to ask why we haven’t found the signatures of other, similar civilizations. We assume, given so many chances, that someone may have reached this level of sophistication prior to us. Without evidence, however, we mustn’t assume that anyone else has been as successful, or as fortunate, as we have.

We also have no idea how far we’ll advance or how long we’ll last. We could drive ourselves to extinction rapidly in any number of ways. Alternatively, we could survive and thrive—overcoming the squabbles plaguing humanity today—thousands or millions of years into the future. While we’ve made it this far, it behooves us to ask some uncomfortable questions about the odds of our survival and how they apply to possible life-forms beyond our planet. If intelligent species arise, how often or how frequently will they annihilate themselves through a disaster like nuclear war? How often will intelligent aliens behave like the cyanobacteria, consuming resources well past the point of sustainability and poisoning their own environment to the point of uninhabitability? What are the odds that a species-threatening pandemic arises, and what are the probabilities of surviving such an event? Is it an inevitability that our development of automation and artificial intelligence will engineer our own demise? Will that transformation replace us with artificial life-forms that have no concern for our loftiest dreams and ambitions? Will our civilization collapse before we’re ever truly capable of contacting alien life? And do theoretical alien civilizations face the same challenges?

The odds may or may not be in favor of interstellar contact, as one catastrophic failure could bring our story—or the story of other beings—to an end. Even with more than a billion candidate Earth-like planets in our Milky Way, and 10²² within the observable universe, we cannot give a realistic estimate for how many intelligent alien civilizations should be out there today.

In the absence of evidence, all we have is speculation.

 For nearly 60 years, humanity has earnestly searched for life beyond Earth. We’ve attempted to quantify the odds of there being life elsewhere in the universe, and, more specifically, of intelligent, spacefaring extraterrestrials. Yet, for all of our efforts, we have yet to produce a meaningful estimate that’s anything more than guesswork. We do not know if there are millions of extraterrestrial civilizations thriving throughout the galaxy, or whether in all the visible universe, there’s only us.

When we ask the big question—“Where is everybody?” —it’s worth keeping a great many possibilities in mind. Aliens might be plentiful, but perhaps we’re not listening properly. Aliens might be plentiful, but they might self-destruct too quickly to maintain a technologically advanced state. Aliens might be plentiful, but they may choose to remain isolated. Aliens might be plentiful, but they might purposely choose to exclude Earth and its inhabitants from their communications. Aliens might be plentiful, but the problems of interstellar transmission or travel might be too difficult to overcome. 

But there’s another valid possibility that we must keep in mind, as well: Aliens may not be there at all. The probability of the three vital leaps, as described above, is enormously uncertain. If even one of these three steps is too cosmically improbable, it may well be that in all the universe, there’s only us.