An Existential Problem in the Search for Alien Life

In 2020, a team of researchers found something surprising in the high clouds of Venus. Earth-based telescopes detected the spectral signature of phosphine, a simple molecule that should have no business persisting in those extremely acidic clouds. Cautiously excited, the researchers wrote that the phosphine could be the result of “unknown photochemistry or geochemistry”—or, they noted almost coyly, “possibly life.”

It was a thrilling possibility. “Signs of Life Found in the Clouds Surrounding Venus,” one headline blared; another, “Aliens Were on Venus This Whole Time?!” It was also, it turned out, a false alarm. The phosphine not only wasn’t a signal of life, but probably wasn’t even there at all, a swing-and-a-miss of data interpretation. The clouds of Venus were still, as far as anyone knew, as uninhabited as they’d always seemed.

Scientists took the false alarm in stride. Back to the drawing board, they seemed to say, shrugging—or back to their telescopes at least. This is how science works, after all: gradually, in small steps, in announcements and skepticism and reconsideration of the data. It’s even harder when the subject of study is extraterrestrial. Venus is the closest possible home to alien life, but there’s no way to go and scoop a sample of its atmosphere to put under a telescope. The search for alien life is done remotely, by interpretation and inference. The suspected phosphine—considered a “biosignature” by astrophysicists, because phosphine on Earth is only ever abundant when it’s a product of life—wasn’t even observed directly. Instead, the researchers detected it by analyzing wavelengths of light that could hint at what molecules might be in Venus’s atmosphere. The researchers were searching for a sign of a sign of life. There was a lot of room for error.

The search for extraterrestrial life is not the kind that is likely to yield an aha moment—not in the sense that, with the tools currently available, scientists are going to look at data brought in from the cosmos and instantly declare, “Yes, this is life.” There are too many technical hurdles, too many variables that will need time to be sorted out. And even accounting for those issues, another obstacle exists—an enduring puzzle that tests the limits of science. The fact is, we still don’t know what life is.

Hold a rock next to a flower and you’re probably confident you know the difference. But since the days of Aristotle, scientists and philosophers have struggled to draw a precise line between what is living and what is not, often returning to criteria such as self-organization, metabolism, and reproduction but never finding a definition that includes, and excludes, all the right things. If you say life consumes fuel to sustain itself with energy, you risk including fire; if you demand the ability to reproduce, you exclude mules. NASA hasn’t been able to do better than a working definition: “Life is a self-sustaining chemical system capable of Darwinian evolution.” It’s a decent way to describe life on Earth, but it lacks practical application. If humans found something on another planet that seemed to be alive, how much time would we have to sit around and wait for it to evolve?

The problem is that, in any attempt to define life, we’re inherently constrained by human intuition and the one example we have so far that informs it. The only life we know is life on Earth. Some scientists call this the n=1 problem, where n is the number of examples from which we can generalize. We have no idea if earthly life is average in the cosmos or some sort of freak outlier. With all the varied chemistries of other planets, all the contingencies that drive evolution, all the ways that matter and energy interact—who knows how strange life on another world might be? What if life as we know it is the wrong life to be looking for?

What we really want is more than a definition of life. We want to know what life, fundamentally, is. For that kind of understanding, scientists turn to theories. A theory is a scientific fundamental. It not only answers questions, but frames them, opening new lines of inquiry. It explains our observations and yields predictions for future experiments to test. Consider the difference between defining gravity as “the force that makes an apple fall to the ground” and explaining it, as Newton did, as the universal attraction between all particles in the universe, proportional to the product of their masses and so on. A definition tells us what we already know; a theory changes how we understand things.

In recent years, the potential rewards of unlocking a theory of life have captivated a clutch of researchers from a diverse set of disciplines. “There are certain things in life that seem very hard to explain,” Sara Imari Walker, a physicist at Arizona State University who has been at the vanguard of this work, told me. “If you scratch under the surface, I think there is some structure that suggests formalization and mathematical laws.” It has long been presumed that although theories can explain physics and chemistry, biology is too messy, too contingent, to be boiled down to math and formulas. In 1997, the renowned biologist Ernst Mayr wrote that although the molecules that compose living organisms obey the laws of physics just as all molecules do, “organisms are fundamentally different from inert matter.” There is a threshold that matter can cross, beyond which the laws of physics do not explain or predict what happens; on the other side of that threshold is life.

But Walker doesn’t think about life as a biologist—or an astrobiologist—does. When she talks about signs of life, she doesn’t talk about carbon, or water, or RNA, or phosphine. She reaches for different examples: a cup, a cellphone, a chair. These objects are not alive, of course, but they’re clearly products of life. In Walker’s view, this is because of their complexity. Life brings complexity into the universe, she says, in its own being and in its products, because it has memory: in DNA, in repeating molecular reactions, in the instructions for making a chair.

Lee Cronin, a chemistry professor at the University of Glasgow and Walker’s main collaborator, told me that when Walker first explained to him her ideas for a theory of life, “I was like, ‘I have no clue what you’re talking about. But it feels like we are saying something super similar.’” Cronin studies the origin of life, also a major interest of Walker’s, and it turned out that, when expressed in math, their ideas were essentially the same. They had both zeroed in on complexity as a hallmark of life. Cronin is devising a way to systematize and measure complexity, which he calls Assembly Theory. He measures the complexity of an object—say, a molecule—by calculating the number of steps necessary to put the object’s smallest building blocks together in that certain way. His lab has found, for example, when testing a wide range of molecules, that those with an “assembly number” above 15 were exclusively the products of life. Life makes some simpler molecules, too, but only life seems to make molecules that are so complex.

No one expects to find an alien cellphone in Mars’s Jezero Crater. But Walker’s whole notion is that it’s not only theoretically possible but genuinely achievable to identify something smaller—much smaller—that still nonetheless simply must be the result of life. The model would, in a sense, function like biosignatures as an indication of life that could be searched for. But it would drastically improve and expand the targets. Walker would use the theory to predict what life on a given planet might look like. It would require knowing a lot about the planet—information we might have about Venus, but not yet about a distant exoplanet—but, crucially, would not depend at all on how life on Earth works, what life on Earth might do with those materials. Without the ability to divorce the search for alien life from the example of life we know, Walker thinks, a search is almost pointless. “Any small fluctuations in simple chemistry can actually drive you down really radically different evolutionary pathways,” she told me. “I can’t imagine [life] inventing the same biochemistry on two worlds.”

Devising a universal theory for life is an ambitious project, to say the least. The scientists I’ve spoken with who are leading the search for biosignatures tend to welcome Walker’s unconventional approach, on the grounds that the more tools available, the merrier. Even so, no one is abandoning their search in the hopes that humanity will soon solve the mystery of life. After all, finding any examples of alien life—Earthlike or not, by whatever means possible—would radically advance our ability to understand the phenomenon.

Walker’s approach is grounded in the work of, among others, the philosopher of science Carol Cleland, who wrote The Quest for a Universal Theory of Life. But Cleland doesn’t share Walker’s ambitions that a theory may be within reach; instead she warns that any theory of life, just like a definition, cannot be constrained by the one example of life we currently know. “It’s a mistake to start theorizing on the basis of a single example, even if you’re trying hard not to be Earth-centric. Because you’re going to be Earth-centric,” Cleland told me. In other words, until we find other examples of life, we won’t have enough data from which to devise a theory. Abstracting away from Earthliness isn’t a way to be agnostic, Cleland argues. It’s a way to be too abstract.

It’s a knot easy to get tied up in: We don’t have a theory of life to guide the search for extraterrestrials, but we need to find extraterrestrial life before we can understand life with a theory. Instead of provincially looking for life as we know it, Cleland calls for a more flexible search guided by what she calls “tentative criteria.” Such a search would have a sense of what we’re looking for, but also be open to anomalies that challenge our preconceptions, detections that aren’t life as we expected but aren’t familiar not-life either—neither a flower nor a rock. It’s unsatisfying if you want a firm answer or a quick one, but it speaks to the hope that exploration and discovery might truly expand our understanding of the cosmos and our own world.

Cleland’s approach ends up sounding much like a lot of the work being done hunting for biosignatures today. A true discovery of phosphine in the clouds of Venus would be an anomaly for sure, and absolutely challenge preconceptions about the kinds of chemistry happening in those clouds. Other work in the field seeks similar surprises. The astrobiologist Kimberley Warren-Rhodes studies life on Earth that lives at the borders of known habitability, such as in Chile’s Atacama Desert. The point of her experiments is to better understand how life might persist—and how it might be found—on Mars. “Biology follows some rules,” she told me. The more of those rules you observe, the better sense you have of where to look on other worlds. In this light, the most immediate concern in our search for extraterrestrial life might be less that we only know about life on Earth, and more that we don’t even know that much about life on Earth in the first place. “I would say we understand about 5 percent,” Warren-Rhodes estimates of our cumulative knowledge. N=1 is a problem, and we might be at more like n=.05.

When I talk with people about a theory of life, which lately is my best attempt at small talk, I reach for the theory of gravity as a familiar parallel. Someone might ask, “Okay, so in terms of gravity, where are we in terms of our understanding of life? Like, Newton?” Further back, further back, I say. Walker compares us to pre-Copernican astronomers, reliant on epicycles, little orbits within orbits, to make sense of the motion we observe in the sky. Cleland has put it in terms of chemistry, in which case we’re alchemists, not even true chemists yet. We understand so little, and we think we’re ready to find other life?

Maybe we’ll never be ready. Yet how could we not search? Right now, the James Webb Space Telescope is peering into exoplanet atmospheres for spectral signatures. The Perseverance rover is bottling up soil samples on Mars for a future mission to bring to Earth to study. How could we not trawl the cosmos for anything that could help us understand our place in it, our kinship lines, and how this life, on Earth, came to be? And so we try. We scour other worlds, scan their clouds. And scratch beneath the surface, for the theory that might explain it all in equation and abstraction, to see the deeper truth beneath what we can see.