Imagine stepping into a time machine, one that could traverse not only billions of years but also countless light-years of space, all in search of life in the universe. Where would you find most of it, and what would it look like? The answer—or at least scientists’ best guess—might surprise you.
You might think most life out there would be like what we see on Earth today: grasses, trees and frolicking animals all orbiting yellow stars on watery worlds under blue, oxygen-rich skies. But you would be wrong. Astronomers conducting a galactic census of planets in the Milky Way now suspect most of the universe’s habitable real estate exists on worlds orbiting red dwarf stars, which are smaller but far more numerous than stars like our sun. In part because of their immense numbers, such stars are in some respects easier for astronomers to study. Consider, for instance, the red dwarf star called TRAPPIST-1, just under 40 light-years away. In 2017 astronomers discovered that it is orbited by at least seven temperate Earth-size planets. A plethora of new observatories—chief among them NASA’s recently launched multibillion-dollar James Webb Space Telescope—will soon begin studying the planets of TRAPPIST-1 and other nearby red dwarf planets for signs of habitability and life.
In the meantime, no one really knows, of course, what you would see if you visited one of these strange worlds in your planet-hopping time machine, but if they are at all like Earth, chances are you would find a planet dominated by microbes rather than charismatic megafauna. A fascinating study published in January 2018 explores what this curious fact might mean for alien-hunting astronomers. Co-authored by David Catling, an atmospheric chemist at the University of Washington, the study peers deep into our planet’s history to devise a novel recipe for finding single-celled life on faraway worlds in the not too distant future.
Here on Earth most life is microbial, and a careful reading of the planet’s fossil and geochemical record reveals it always has been. Organisms such as plants and animals—as well as the oxygen the plants produce and the animals breathe—are relative newcomers, having arisen only in the past half a billion years or so. Our planet seems to have spent the first two billion of the remaining four billion years of its history as a “slime world” ruled by methane-belching microbes for which oxygen was not a life-giving gas but rather a deadly poison. The emergence of photosynthetic cyanobacteria (named for their verdant hue, which comes from chlorophyll) defined the next two billion years and banished the “methanogen” microbes to dark places where oxygen could not go—subterranean caverns, deep muds and other smothered environments where they still exist to this day. The cyanobacteria gradually greened our planet, slowly filling the atmosphere with oxygen and setting the stage for today’s familiar world.
Touching your time machine down on Earth at a random point in the planet’s history, roughly nine times out of 10 you would find only single-celled life or algae and would risk suffocation in the oxygen-starved open air.
This creates a quandary for scientists hoping to use the Webb telescope, rather than a time machine, to seek out other life-bearing worlds. Molecules in a planet’s atmosphere can absorb passing starlight, imprinting barcodelike signatures on the light that astronomers can then detect. Plentiful oxygen is one of the most obvious barcodes to look for in a planet’s atmosphere across the light-years because it is one of the hardest things to make sans biology. In the parlance of astrobiologists, the highly reactive gas is a potent “biosignature” because in large concentrations it tends to be “out of equilibrium” with its surroundings. That is, oxygen tends to fall out of the air as a component of rust and other mineral oxides rather than lingering as a gas, so when it exists in abundance, something—photosynthetic life, in Earth’s case—must be constantly replenishing it. But with our planet as their guide, astrobiologists are forced to acknowledge that oxygen may be the least likely thing they will ever see—genetic evidence suggests the complex oxygen-producing photosynthetic pathway pioneered by cyanobacteria is an extraordinary evolutionary innovation that appeared only once throughout the entire multibillion-year history of Earth’s biosphere. One might expect, then, that any living “Earth-like” world scientists will ever gaze on through telescopes is likely to be anoxic, or lacking in oxygen. What other biosignatures should they seek instead?
Right now the best way to find an answer is to hop back into the time machine. Not a real one, of course, but rather a virtual voyager, a computer model that plumbs the otherwise inaccessible depths of Earth’s anoxic past (or an alien planet’s present), exploring the possible chemistry of gases in the atmosphere and ocean that could have occurred there. Use data from old rocks and other models to dial in your best guess for the environmental conditions of, for instance, the Earth circa three billion years ago, let the computer crunch the numbers, and see whether any obvious imbalances—potential biosignatures—pop out. This is exactly what Catling did, working with his colleagues Joshua Krissansen-Totton and Stephanie Olson.
Their “time machine” is essentially just a numerical approximation of an immense volume of air trapped in a large, transparent box with the open ocean at the box’s base; the computer simply calculates how the constituent gases in the box should react and mix with one another over time. Eventually the interacting gases use all the “free energy” available in the box and reach equilibrium—a point where no further reactions can occur without more energy from outside, a bit like soda water that has lost its fizz. Comparing the cocktail of exhausted gases with the livelier mixture originally trapped in the box reveals precisely where and how the world’s atmosphere was initially out of equilibrium. From first principles, this approach can replicate the most obvious example of atmospheric disequilibrium today on our planet—the presence of oxygen and traces of methane (the latter being the gentle exhalations of the once mighty methanogen biosphere). Simple chemistry shows these gases should not co-exist for long, and yet on Earth they do, offering a clear sign for any watchful alien astronomers that something lives and breathes on this particular pale blue dot. But for the ancient, anoxic Earth, the modeling shows something different.
“Our research provides the answer” to the question of how to find anoxic life on an Earth-like planet, Catling says. “Most life elsewhere is probably simple—like microbes—and most planets have probably not advanced to a stage of an oxygenated atmosphere. The combination of relatively abundant carbon dioxide and methane (absent carbon monoxide) is a biosignature of such a world.”
Krissansen-Totton explains in more detail: “Having methane and carbon dioxide together is unusual because carbon dioxide is carbon’s most oxidized state, and methane (composed of a carbon atom linked to four hydrogen atoms rather than any oxygen at all) is its least,” he says. “Producing those two extremes of oxidation in an atmosphere at once is challenging to do without life.” A rocky, ocean-bearing planet with more than 0.1 percent methane in its atmosphere should be considered a potentially inhabited planet, the researchers say. And if the atmospheric methane reaches levels of 1 percent or more? In that case, “potentially” doesn’t cut it—such a world would “likely” be home to alien life.
Jim Kasting, an atmospheric chemist at the Pennsylvania State University, who is not affiliated with the study, says its results are “on the right track,” even though “the idea that methane might be a biosignature in an anoxic atmosphere is not exactly new.”
What is new, Catling and his co-authors say, is their robust treatment of how a methane-based biosignature would manifest itself and how it could be discerned from nonliving sources. According to their models, methane in the atmosphere of an anoxic Earth-like planet would typically react with the carbon dioxide that still filled the air, mingling further with another ubiquitous gas, nitrogen, as well as with water vapor to ultimately rain out as heavier compounds. Further calculations by Catling and his team concluded that no abiotic methane sources on a rocky planet could produce enough of the gas to counteract this process—whether it is volcanic outgassing from a planet’s interior, chemical reactions in hydrothermal vents or even asteroid impacts. Only a thriving planetary population of methane-belching bacteria could account for the gas. And, most crucially, even if abiotic sources could come up with enough methane, they would almost inevitably produce a great deal of carbon monoxide as well—a gas poisonous to animals but positively delicious to many microbes. Thus, methane and carbon dioxide together, unaccompanied by carbon monoxide, on a rocky, ocean-bearing world would best be interpreted as an airtight sign of anoxic life.
This is good news for astronomers. The Webb telescope, it turns out, will be hard-pressed to directly discern oxygen in any potentially habitable planet it surveys during its mission. Just as your eyes can see visible light but not radio waves or x-rays, Webb’s vision is tuned for the infrared—a portion of the spectrum ideal for studying ancient stars and galaxies but where oxygen’s barcodelike absorption lines are rather slight and sparse. Consequently, some researchers have feared the search for life will have to wait for other, more capable telescopes many years or decades in the future. But although Webb cannot easily see oxygen, its infrared eyes might excel at glimpsing signs of anoxic life. According to Nikole Lewis of Cornell University, who worked on Webb as a project scientist at the Space Telescope Science Institute in Baltimore, the telescope can perform the simultaneous detection of methane, carbon dioxide and carbon monoxide in the atmospheres of some planets around red dwarf stars. And not just big, bloated gassy planets where we wouldn’t expect life to exist anyway. “Webb can achieve the required precision to detect the molecules in the atmospheres of planets like those in the TRAPPIST-1 system,” Lewis says.
Even so, Lewis and others note that Webb may still struggle to fulfill the most crucial part of Catling’s criterion—determining the relative abundance of each gas to pin down whether methane on some distant planet is the result of an erupting volcano or burping microbes. Consequently, Catling isn’t holding his breath for Webb to find an anoxic biosphere on some red dwarf world.
“Webb probably has to get lucky to find life, but you never know, so this is potentially exciting for astrobiology,” Catling says. “We want to make more people aware that there’s more to looking for life than looking for oxygen.”