The discovery of possible signs of alien life on Venus was among the biggest news in astronomy this year, but within days, it faded out of the headlines. This was not a unique event in history: Flash back to 1996 when then-President Bill Clinton stood on the south lawn of the White House to announce the putative discovery of fossilized alien microorganisms in a Martian meteorite. Even as these and other possible discoveries are announced, they arrive with debate and quickly fizzle because we cannot confirm alien life has actually been discovered.
This year’s trip through the hype cycle of the detection of alien life was no different. Researchers announced the discovery of phosphine on Venus as possible evidence of alien life, with the caveat that phosphine might be made in the Venusian environment in the absence of life, though the researchers had endeavored to eliminate the possibility. However, critics quickly pointed out that the evidence for phosphine detection was itself weak, and the consensus by now is that the detection was likely a false alarm. But even if a positive detection of phosphine could be confirmed, the deeper debate of whether or not life produced it will ensue. This debate—or the one in the next iteration of the hype cycle—is unlikely to be resolved anytime soon, because the way we currently search for life is insufficient.
Astrobiologists primarily search for signs of life, or biosignatures, based on life as we know it on Earth. A familiar example is the search for oxygen in exoplanet atmospheres. The oxygen in Earth’s atmosphere is a product of photosynthesis, and our atmosphere would not have abundant oxygen in the absence of life. And, importantly for remote detection, oxygen is observable at interplanetary distances. For these reasons, astrobiologists thought detection of oxygen in the atmosphere of an Earth-like exoplanet would be a smoking gun sign of life. However, models simulating exoplanets under diverse conditions have repeatedly shown how oxygen could be abundantly produced on planets without life. Phosphine, too, is known to be produced in the atmospheres of Jupiter and Saturn in the absence of life; if its presence were confirmed in the Venusian atmosphere, we’d have no assurance that it wasn’t being produced by as-yet-unknown nonliving mechanisms.
The challenge presented by Venusian phosphine is a microcosm of the crisis astrobiologists will face in the coming decades, a crisis of contentious announcements and ongoing disagreement. The discovery of life on another planet should be a momentous event for humanity, but any announcement of a biosignature detection made right now will not be a milestone but a mess, because scientists will have no consensus that we’ve even made a discovery. Here on Earth, we don’t recognize life by its atmospheric byproducts. In fact, none of our current biosignatures address the central question: What about us makes us alive? Our biosignatures are not definitive signs of life because we don’t have a coherent theory of what life is.
To avoid this crisis, astrobiologists need to confront head-on the question that we have so far very stealthy averted answering: What is life? We can’t answer this by discovering alien life, because we can’t discover alien life without first attempting an answer. We need to answer it by understanding the abstract principles governing life here on Earth. Then, in turn, we can use our search for alien life to test our answer.
So far, it has seemed like there was more progress to be made by ignoring this question altogether and instead satisfying ourselves with token definitions. It might superficially appear like life should be easy to define, but for any definition proposed, there are always forms of life that end up excluded and nonliving objects that meet the definition’s criteria. Carl Sagan famously showed that adopting a definition that includes the ability to eat, metabolize, excrete, breathe, move, and be responsive to external stimuli—seemingly straightforward criteria—might lead any aliens encountering the Earth to assume automobiles are the dominant life form. A popular definition that “life is a self-sustaining chemical system capable of Darwinian evolution” excludes any organisms that can’t reproduce because they are not capable of evolution—therefore mules and many senior citizens are excluded if you read the definition too strictly. The challenges of defining life have led some to even declare that life does not exist, or at least that attempts to define it are completely useless to science. But while defining life is fraught, invoking problematic hard boundaries between what life is and is not, deriving a theory that explains life need not conform to our naïve expectations and can instead expand our understanding.
Other disciplines rest securely on this kind of theoretical foundation. In astrophysics, predictions based on explanatory mathematical laws and observed patterns have played an important role guiding our search for phenomena conjectured to exist but which did not yet have substantive empirical evidence. For example, Albert Einstein published his theory of general relativity in 1915, laying the foundation of a new theory explaining gravitational physics. He did so by making a counterintuitive and deeply insightful connection between the properties of light and gravity, in the process reshaping our notions of space and time. Among his theory’s many new predictions was the existence of gravitational waves, discovered a century later in 2015. That was a groundbreaking discovery, but it was only possible because of the theoretical foundation that Einstein devised a century earlier. It also demanded decadeslong development of highly sensitive instrumentation and sophisticated statistical analyses—the successful detection of extraterrestrial life is likely to demand the same. It could be that whatever theory we uncover to explain life may similarly have a counterintuitive foundation and lead to even more revolutionary ideas about how our universe works—after all, life is far more complex than gravitation. But right now, we are without theory to guide our search. Without such a theory, even the 100-year time scale for the gravitational wave discovery may be sorely underestimating how far we are out from discovery of alien life, if it is indeed out there.
What should a universal theory for life look like? It will have to account for features of living systems that no current laws of physics and chemistry can explain. That is, it will have to pinpoint processes in the universe that occur only because of life. One direction this approach can take is toward complexity. Physics alone, for example, can’t account for life’s ability to build complex objects via the processing of information—that is, objects that are statistically impossible to be produced in the absence of information or instructions for how to build them. If we found a screwdriver or other complex object—say, a protein—on Mars, it would be indicative of life because we do not expect the laws of physics alone to cause these objects to fluctuate into existence. These ideas are being formalized in new approaches to agnostic biosignatures that don’t require the same chemical hardware as life on Earth but instead focus on quantifying the properties of molecules that carry hallmarks of evolutionary history. Sets of molecules too complex to form by chance would require a physical system with information on how to assemble them—that is, complex molecules are statistical indicators of the presence of living systems. This would be more definitive than current biosignatures—while the best we can say of an exoplanet’s atmospheric oxygen might be “we haven’t figured out how this could exist without life as we know it,” agnostic biosignatures would allow us to assess the mathematical likelihood that what we’ve found was created by a living information processing system selected by an evolutionary process, leading to clearer and clearer hypotheses about the universal principles underlying life.
While theory-driven approaches to astrobiology remain in their infancy, they provide promise for realizing the answer to the age-old question, Are we alone?, in terms of both making the scientific discovery itself and how the international community will react to it. Until we can capture with our own imagination the breadth and depth of what life is, we will not be moved by claims of its discovery by biosignature indicators that give us no impression of the life that generates them. A discovery that confirms deep theories of what life is will be far more meaningful than announcing we have found a metabolic byproduct. The discovery of alien life is not just about an “aha” moment of finding a signal on a distant world: It is a process of discovering ourselves, and what we are in the cosmic story. As with other great advances in our understanding of the universe, it will require we dig deep and theorize about the nature of ourselves. But detection of life requires us to do this in a way no field of science has yet had to confront, because this is about more than trying to understand the universe—it’s about understanding what we are within it. Only when we can do that will we have any hope of definitive discovery of alien life, and of all the ways that discovery will change how we understand life on Earth and ourselves.