Photo illustration by Olena Shmahalo/Quanta Magazine

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Huddled in a coffee shop one drizzly Seattle morning six years ago, the astrobiologist Shawn Domagal-Goldman stared blankly at his laptop screen, paralyzed. He had been running a simulation of an evolving planet, when suddenly oxygen started accumulating in the virtual planet’s atmosphere. Up the concentration ticked, from 0 to 5 to 10 percent.

“Is something wrong?” his wife asked.

“Yeah.”

The rise of oxygen was bad news for the search for extraterrestrial life.

After millennia of wondering whether we’re alone in the universe — one of “mankind’s most profound and probably earliest questions beyond, ‘What are you going to have for dinner?’” as the NASA astrobiologist Lynn Rothschild put it — the hunt for life on other planets is now ramping up in a serious way. Thousands of exoplanets, or planets orbiting stars other than the sun, have been discovered in the past decade. Among them are potential super-Earths, sub-Neptunes, hot Jupiters and worlds such as Kepler-452b, a possibly rocky, watery “Earth cousin” located 1,400 light-years from here. Starting in 2018 with the expected launch of NASA’s James Webb Space Telescope, astronomers will be able to peer across the light-years and scope out the atmospheres of the most promising exoplanets. They will look for the presence of “biosignature gases,” vapors that could only be produced by alien life.

They’ll do this by observing the thin ring of starlight around an exoplanet while it is positioned in front of its parent star. Gases in the exoplanet’s atmosphere will absorb certain frequencies of the starlight, leaving telltale dips in the spectrum.

Filming by Tom Hurwitz and Richard Fleming. Editing and motion graphics by Ryan Griffin. Other graphics and images from NASA, the European Southern Observatory and Creative Commons. Music by Podington Bear.

In Theory Video: David Kaplan explores the best ways to search for alien life on distant planets.

As Domagal-Goldman, then a researcher at the University of Washington’s Virtual Planetary Laboratory (VPL), well knew, the gold standard in biosignature gases is oxygen. Not only is oxygen produced in abundance by Earth’s flora — and thus, possibly, other planets’ — but 50 years of conventional wisdom held that it could not be produced at detectable levels by geology or photochemistry alone, making it a forgery-proof signature of life. Oxygen filled the sky on Domagal-Goldman’s simulated world, however, not as a result of biological activity there, but because extreme solar radiation was stripping oxygen atoms off carbon dioxide molecules in the air faster than they could recombine. This biosignature could be forged after all.

The search for biosignature gases around faraway exoplanets “is an inherently messy problem,” said Victoria Meadows, an Australian powerhouse who heads VPL. In the years since Domagal-Goldman’s discovery, Meadows has charged her team of 75 with identifying the major “oxygen false positives” that can arise on exoplanets, as well as ways to distinguish these false alarms from true oxygenic signs of biological activity. Meadows still thinks oxygen is the best biosignature gas. But, she said, “if I’m going to look for this, I want to make sure that when I see it, I know what I’m seeing.”

Meanwhile, Sara Seager, a dogged hunter of “twin Earths” at the Massachusetts Institute of Technology who is widely credited with inventing the spectral technique for analyzing exoplanet atmospheres, is pushing research on biosignature gases in a different direction. Seager acknowledges that oxygen is promising, but she urges the astrobiology community to be less terra-centric in its view of how alien life might operate — to think beyond Earth’s geochemistry and the particular air we breathe. “My view is that we do not want to leave a single stone unturned; we need to consider everything,” she said.

As future telescopes widen the survey of Earth-like worlds, it’s only a matter of time before a potential biosignature gas is detected in a faraway sky. It will look like the discovery of all time: evidence that we are not alone. But how will we know for sure?

Victoria Meadows, an astrobiologist and principal investigator of the University of Washington’s Virtual Planetary Laboratory.

Scientists must quickly hone their models and address the caveats if they are to select the best exoplanets to target with the James Webb telescope. Because of the hundreds of hours it will take to examine the spectrum for each planetary atmosphere and the many competing demands on its time, the telescope will likely only observe between one and three earthlike worlds in the habitable “Goldilocks” zones of nearby stars. In choosing from a growing list of known exoplanets, the scientists want to avoid planetary circumstances in which oxygen false positives arise. “We’re looking at maybe putting our eggs, if not all in one basket, at least in only a couple of baskets,” Meadows said, “so it’s important to try and figure out what we should be looking for there. And in particular, how we might get fooled.”

Breath of Life

Oxygen has been regarded as the gold standard since the chemist James Lovelock first contemplated biosignature gases in 1965, while working for NASA on methods of detecting life on Mars. As Frank Drake and other pioneers of astrobiology sought to detect radio signals coming from distant alien civilizations — an ongoing effort called the search for extraterrestrial intelligence (SETI) — Lovelock reasoned that the presence of life on other planets could be deduced by looking for incompatible gases in their atmospheres. If two gases that react with each other can both be detected, then some lively biochemistry must be continually replenishing the planet’s atmospheric supplies.

In Earth’s case, though it readily reacts with hydrocarbons and minerals in the air and ground to produce water and carbon dioxide, diatomic oxygen (O2) comprises a steady 21 percent of the atmosphere. Oxygen persists because it is poured into the sky by Earth’s photosynthesizers — plants, algae and cyanobacteria. They enlist sunlight to strip hydrogen atoms off water molecules, building carbohydrates and releasing the oxygen byproduct as waste. If photosynthesis ceased, the existing oxygen in the sky would react with elements in the crust and drop to trace levels in 10 million years. Eventually, Earth would resemble Mars, with its carbon dioxide-filled air and rusty, oxidized surface — evidence, Lovelock argued, that the Red Planet does not currently harbor life.

But while oxygen is a trademark of life on Earth, why should that be true elsewhere? Meadows argues that photosynthesis offers such a clear evolutionary advantage that it is likely to become widespread in any biosphere. Photosynthesis puts the biggest source of energy on any planet, its sun, to work on the most commonplace of planetary raw materials: water and carbon dioxide. “If you want to have the uber-metabolism you will try and evolve something that will allow you to use sunlight, because that’s where it’s at,” Meadows said.

NASA/Chris Gunn

A full-scale model of the tennis court-size James Webb Space Telescope on display in Austin, Tex., in 2013.

Diatomic oxygen also boasts strong absorption bands in the visible and near-infrared — the exact sensitivity range of both the $8 billion James Webb telescope and the Wide Field Infrared Survey Telescope (WFIRST), a mission planned for the 2020s. With so many imminent hopes riding on oxygen, Meadows is determined to know “where the gotchas are likely to be.” So far, her team has identified three major nonbiological mechanisms that can flood an atmosphere with oxygen, producing false positives for life. On planets that formed around small, young M-dwarf stars, for instance, intense ultraviolet sunlight can in certain cases boil down the planet’s oceans, creating an atmosphere thick with water vapor. At high altitudes, as VPL scientists reported in the journal Astrobiology last year, intense UV radiation splinters off the lightweight hydrogen atoms. These atoms then escape to space, leaving behind a veil of oxygen thousands of times denser than Earth’s atmosphere. Because the smallness of M-dwarf stars makes it easier to detect much smaller, rocky planets passing in front of them, they are the intended targets for NASA’s Transiting Exoplanet Survey Satellite (TESS), a planet-finding mission scheduled to launch next year. The earthlike planets that will be studied by the James Webb telescope will be selected from among TESS’s finds. With these candidates on the way, astrobiologists must learn how to distinguish between alien photosynthesizers and runaway ocean boiling. In work that is now being prepared for publication, Meadows and her team show that a spectral absorption band from tetraoxygen (O4) loosely forms when O2 molecules collide. The denser the O2 in an atmosphere, the more molecular collisions occur and the stronger the tetraoxygen signal becomes. “We can look for the [O4] to give us the telltale sign that we’re not just looking at a 1-bar atmosphere with 20 percent oxygen” — an earthlike atmosphere suggestive of photosynthesis — Meadows explained, “we’re looking at something that just has massive amounts of oxygen in it.” A strong carbon monoxide signal will identify the false positive that Domagal-Goldman first encountered that drizzly morning in 2010. Now a research scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md., he says he isn’t worried about oxygen’s long-term prospects as a reliable biosignature gas. Oxygen false positives only happen in rare cases, he said, “and the planet that has those certain cases is also going to have observational properties that we should be able to detect, as long as we think about it in advance, which is what we’re doing right now.” He and other astrobiologists are also mindful, though, of oxygen false negatives — planets that harbor life but have no detectable oxygen in their atmospheres. Both the false positives and false negatives have helped convince Sara Seager of the need to think beyond oxygen and explore quirkier biosignatures. Encyclopedia of Gases Courtesy of Sara Seager Sara Seager, an astrophysicist and planetary scientist at the Massachusetts Institute of Technology. If the diverse exoplanet discoveries of the past decade have taught us anything, it’s that planetary sizes, compositions and chemistries vary dramatically. By treating oxygen as the be-all, end-all biosignature gas, Seager argues, we might miss something. And with a personal dream of discovering signs of alien life, the 44-year-old can’t abide by that. Even on Earth, Seager points out, photosynthesizers were pumping out oxygen for hundreds of millions of years before the process overwhelmed Earth’s oxygen sinks and oxygen started accumulating in the sky, 2.4 billion years ago. Until about 600 million years ago, judged from a distance by its oxygen levels alone, Earth might have appeared lifeless. Meadows and her collaborators have studied some alternatives to oxygenic photosynthesis. But Seager, along with William Bains and Janusz Petkowski, are championing what they call the “all-molecules” approach. They’re compiling an exhaustive database of molecules — 14,000 so far — that could plausibly exist in gas form. On Earth, many of these molecules are emitted in trace amounts by exotic creatures huddled in ocean vents and other extreme milieus; they don’t accumulate in the atmosphere. The gases might accrue in other planetary contexts, however. On methane-rich planets, as the researchers argued in 2014, photosynthesizers might harvest carbon from methane (CH4) rather than CO2 and spew hydrogen rather than oxygen, leading to an abundance of ammonia. “The ultimate, long-term goal is [to] look at another world and make some informed guesses as to what life might produce on that world,” said Bains, who splits his time between MIT and Rufus Scientific in the United Kingdom. Domagal-Goldman agrees that thinking both deeply about oxygen and broadly about all the other biochemical possibilities is important. “Because all these surprises have happened in our detections of the masses and radii and orbital properties of these other worlds,” he said, “[astronomers] are going to keep pushing on the people like me who come from an earth sciences background, saying, ‘Let’s think further outside the box.’ That is a healthy and necessary pressure.” Meadows, however, questions the practicality of the all-molecules approach. In a 3,000-word email critiquing Seager’s ideas, she wrote, “After you build this exhaustive database, how do you identify those molecules that are most likely to be produced by life? And how do you identify their false positives?” She concluded: “You will still have to be guided by life on Earth, and our understanding of planetary environments and how life interacts with those environments.” In contemplating what life might be like, it’s exasperatingly difficult to escape the only data point we have — for now. Uncertain Odds At a 2013 symposium, Seager presented a revised version of the Drake equation, Frank Drake’s famous 1961 formula for gauging the odds that SETI would succeed. Whereas the Drake equation multiplied a string of mostly unknown factors to estimate the number of radio-broadcasting civilizations in the galaxy, Seager’s equation estimates the number of planets with detectable biosignature gases. With the modern capacity to look for any life regardless of whether it’s intellectually capable of beaming messages into space, the calculation of our chances of success no longer depends on uncertainties like the rareness of intelligence as an evolutionary outcome or the galactic popularity of radio technology. However, one of the biggest unknowns remains: the probability that life will arise in the first place on a rocky, watery, atmospheric planet like ours. “Abiogenesis,” as the mystery event is called, seems to have occurred not long after Earth accumulated liquid water, leading some to speculate that life might start up readily, even inevitably, under favorable conditions. But if so, then shouldn’t abiogenesis have happened multiple times in Earth’s 4.5-billion-year history, spawning several biochemically distinct lineages rather than a monoculture of DNA-based life? John Baross, a microbiologist at the University of Washington who studies the origins of life, explained that abiogenesis might well have happened repeatedly, creating a menagerie of genetic codes, structures and metabolisms on early Earth. But gene-swapping and Darwinian selection would have merged these different upstarts into a single lineage, which has since colonized virtually every environment on Earth, preventing new upstarts from gaining ground. In short, it’s virtually impossible to tell whether abiogenesis was a fluke event, or a common occurrence — here, or elsewhere in the universe. Scheduled to speak last at the symposium, Seager set a light-hearted tone for the after party. “I put it all in our favor,” she said, positing that life has a 100 percent chance of arising on Earth-like planets, and that half of these biospheres will produce detectable biosignature gases — another uncertainty in her equation. Crunching these wildly optimistic numbers yielded the prediction that two signs of alien life would be found in the next decade. “You’re supposed to laugh,” Seager said. Meadows, Seager and their colleagues agree that the odds of such a detection this decade are slim. Though the prospects will improve with future missions, the James Webb telescope would have to get extremely lucky to pick a winner in its early attempts. And even if one of its targeted planets does harbor life, spectral measurements are easily foiled. In 2013, the Hubble Space Telescope monitored the starlight passing through the atmosphere of a midsized planet called GJ 1214b, but the spectrum was flat, with no chemical fingerprints at all. Seager and her collaborators reported in Nature that a high-altitude layer of clouds appeared to have obscured the planet’s sky from view. This article was reprinted on TheAtlantic.com and Wired.com View Reader Comments (13) Leave a Comment ## Reader CommentsLeave a Comment • Torbjörn Larsson says: If you are too pessimistic you won't get up in the morning. After all, you may have come down with one or more of a million diseases that won't allow you. 😉 I cam't find it now, but I recently saw an interesting seminar (AGU -15, I think), where the astronomer reviewed observational problems and found that biologically oxygenated atmospheres could be detected with little confusion if you used the entire available spectrum wisely. • Steve Baker says: It seems bizarre to me to assume that life requires a specific chemical process. All we really need is a molecule that can replicate itself – sometimes making mistakes when it does so. Once we have that, evolution can kick in and provide everything else. But evolution can easily get stuck in dumb, stupid pathways that make no logical sense – and which are far from optimal. Why don't cheetahs have wheels instead of legs so they can go faster than their prey? Why does the giraffe still have the recurrent laryngeal nerve? These things make no logical sense, but evolution isn't driven by an intelligent designer, and very often leaves organisms stuck in very inefficient solution. So why assume that oxygen-producing photosynthesis (as useful as it is) will necessarily evolve? We need to look for processes that fight entropy. That's not an easy thing criterion to look for when your only tool is the spectral analysis of atmosphere from a few hundred lightyears away. When the only tool you have is a hammer…every lifeform starts to look like something that generates gasses that we can see in the James Webb telescope. So we have to resign ourselves to searching only for the kinds of life that follow the general pattern taken by life here on Earth – and hope that we're not missing whole branches of nature in the process. I suspect that we're looking to find one of a tiny percentage of the life forms out there…but it's hard to imagine a way to do anything better. • Dennis Vaccaro says: One postulate that seems to never be considered is that alien life might have originally come from Earth. After all life has been on Earth for at least 3.5 billion years. There is a subreddit focused on whether Earth might be Planet Zero for life. (https://www.reddit.com/r/EarthPlanetZero/) • William Seeley says: I find the idea questionable that multiple instances of abiogenesis might have generated "several biochemically distinct lineages rather than a monoculture of DNA-based life" and then resulted in a "menagerie of genetic codes, structures and metabolisms on early Earth". One wonders how "gene-swapping and Darwinian selection" would have occurred across these different types of "upstart life" to "merge them into a single lineage". If these proto-organisms used different genetic codes, wouldn't any "genes" swapped from one form to another be expressed as different "proteins" in the recipient, if they could be expressed at at all. And if they could be transcribed to proteins, wouldn't they result in completely different "structures and metabolisms" in the target form, with a high likelihood of lethality. This sounds more like science fantasy than science. • Greg says: There are, what, over 180 worlds in our own solar system, counting up all the moons and such. If life exists on some of these moons in the outer solar system, or even on subterranean Mars, a survey of their atmospheres would not yield many obvious clues. If/when we find other life in the solar system, hindsight will be 20/20, of course. Meanwhile, searching the stars for earth-like planets in a habitable zone with oxygen atmospheres is well and good; just as listening to the heavens for the bleep-bleep signal of some distant civilization is well and good. It's humility or hubris (depending on one's perspective) that we're essentially looking for the universe to reflect some version of ourselves — our "best guess" based on a current sample size of n=1. C'est la vie? Das ist leben? We certainly hope so. • Marc Troyer says: First off we have no idea how life might evolve on other planets. Life started on Earth way before we had this atmosphere and in fact life created this atmosphere ! We don't know how life will evolve to utilize the energy sources available elsewhere. At the bottom of our ocean there are many species that have never received any kind of energy from direct sunlight and yet they thrive on gasses lethal to almost everything else. If life can start in such a place then why would anyone have the ill conceived notion that life couldn't thrive in what we consider inhospitable environments ? What we have learned on Earth is this , energy = life and I don't think you will find many places in the universe with no energy 😉 • Wayne King says: Loved the article. Excellently focused. The one omission was earlier clarity (as background in the field) that before we ever look at an exo's atmosphere we know a host of very specific data about the planet and its relationship with its sun. And while we aren't guaranteed anything about what an exo-planet's atmosphere will be like by looking at the orbital mechanics, that planet/sun relationship will define what the atmosphere cannot be. • Wayne King says: @ Seeley I think its possible to have something similar to our RNA/DNA evolution and one form say a thermal/chemical development existing side be side with nothing more than limited competition or maybe some for of parasitism/co-operative environment. Wont say anythings possible but life does seem to emerge when ever certain basic requirements are met. • jgaa says: Coming from the Tor network, it took me always 5 minutes to get trough the captcha (CloudFlare). In the process, I lost the motivation to read the article. Bye. • bill wesley says: So far no one has been able to start up life from scratch in the lab, but we do know that some extremophiles can survive in hibernation on particles in space, such particles could ride the solar winds and seed wide areas. It may be possible for life to have arisen spontaneously on earth but to prove it one must replicate the results in an experiment in the lab otherwise its speculation. If planets are seeded by natural processes its not necessary for life to arise on each planet, evolution would adapt any alien colonist to earthly conditions. Time will tell if life can be created from basic compounds but so far this has not happened. Life seems to be one of natures entropy reducing processes seemingly in violation of the 2nd law of thermodynamics. The book "Ecophysics" by the late Physicist J P Wesley goes into some detail about the physics of living systems that might be quite pertinent to the search for extraterrestrial life. • Woody Stanford says: I love the algorythm here. Exactly how long does a civilization transmit, even passively their communications? Do they switch over to fiber optics after a point? Do they get really adventerous and hyper-forcused beam to near by systems (for power management) or do they mine moons for super-thin sheets of alumium foil that they continously orbit around their sun, transmitting mathematically-proven artificial matematical constants by their placement? But this is a great idea, scanning for life gasses. Let's say an intelligent culture can exist for million of years after the advent of science, it makes a lot more sense to look for the window of 100's of millions of years that life can exist on a planet instead. This is the kind of hard core science that makes me happy to read. The scientists involved have obviously thought it through, looked at the probablilities, thought of a few approaches and launched a great one. Kudos to you. It is really thought out. And you never know, we could be flying to some of the closer ones at some point. • SODOS says: OK – 8 billion US$ later and we detect what we think to be biochemical life on a planet 1.400 light years away which we will never really see or visit. Isn't there a better way to spend the 8 billion, ie on trying to save this planet for instance, our home to be for thousands of years to come, which will soon need to go onto a ventilation machine? Just a thought.

• SODOS says:

Applying the laws of probability alone, it is unquestionable among intelligent people not prone to religious prejudices that there must be life on other planets, abundant life, and intelligent life even, both within our own galaxy system with over 100.000.000.000 stars and throughout the entire universe as a whole. So why do we need to find it? Why go on spending billions upon billions every year to prove the inevitable. I know why, so that we can point our finger at the night sky and say ‘’you see there, 1.400 light years away, we think there is live bacteria clinging on a rock which might one day evolve into an intelligent life form and send us a message. It will take a couple of billion years but we’re not really going anywhere’’