Oxygen: the source of so much life on Earth. It's vital to every breath we take — and without it water couldn’t exist, and fires couldn’t burn.

Moreover, according to a new paper from astronomers Amedeo Balbi of the University of Roma Tor Vergato, and Adam Frank of the University of Rochester, if other worlds don’t have oxygen, and lots of it, then our chances of finding another technological society like our own are slim.

To understand why, let’s take a trip back in time, over 6,000 years to the final days of the Stone Age. By this stage, humans had been tool-bearers and tool-makers for some time, with stones hammered and chiseled into arrow heads, knives and spears. Yet metals such as copper and bronze offered greater options. These more malleable materials could be worked like stone, but the more they are hammered the more brittle they become. It’s a problem that is easily remedied by the process of annealing, which involved heating the metal.

To heat the metal, those early tool makers needed to make fire, which was no problem; humans had been using and controlling fire since the time of Home Erectus, one million years ago. So they took to their flint rocks and sparked a flame to anneal the copper, thus bringing about a new age: the Copper Age — to soon be followed by the Bronze Age. With access to metals new technology was invented, and refined, and civilization developed. 

Yet, like people, fire needs oxygen to breath. There’s plenty in Earth’s atmosphere now, an abundance of 21%. To facilitate open-air combustion, an abundance of at least 18% is required. Any lower and fires will sputter and go out. Lightning might strike a tree and cause it to explode, but the tree won’t burn. Sparks will fail to light up a campfire. The lack of wildfires would irrevocably alter our planet’s ecology. And without fire, generating the heat to anneal metal would not be possible. If Earth’s oxygen had been 17% some 6,000 years ago, then the Stone Age would never have ended, and our modern technological civilization would not have come to pass.

Luckily for us, Earth’s atmospheric abundance of oxygen has been higher than the critical value of 18% for at least the past 420 million years. Yet oxygen has not increased linearly throughout history; there have been fluctuations in the availability of atmospheric oxygen that has left traces in the geological and paleontological record. There’s no guarantee that Earth has to always have an oxygenated atmosphere and, by the same token, there’s no guarantee that any other potentially habitable planets around other stars will have high concentrations of oxygen either.

It’s this realization that has led to Balbi and Frank’s epiphany. In a new paper published in the journal Nature Astronomy, they point out that on worlds with oxygen abundances less than 18%, technology will not be possible for the reasons outlined above. As such, attempts by the Search for Extraterrestrial Intelligence (SETI) to find life that has developed technology that we can detect would be doomed to failure on such worlds.

“That’s what totally blew my mind,” Frank tells Supercluster. “They’d have no access to this source of energy that they would otherwise use for their tool-building and tool-using purposes.”

Without the ability to forge metal to build radio antennas, to provide the combustion necessary to launch rockets, or the means of generating energy from burning fossil fuels and developing technology to fire lasers into the sky, any aliens on an oxygen-poor planet would be largely undetectable, at least from the point of view of SETI. Frank and Balbi call it the ‘oxygen bottleneck’. There could be countless planets out there with life, even intelligent life, but lacking the oxygen to start fires.

Far from being bad news for the search for extraterrestrial intelligence, Frank thinks this could be a boon. Why waste time searching oxygen-poor worlds when we could more efficiently use our resources listening and watching planets with plenty of oxygen and therefore a greater likelihood of technological life? With billions of star systems in the galaxy to search, this would help cut the odds in SETI’s favor.

Which is great in theory, but the question is, how many potentially habitable planets in the galaxy have sufficient oxygen to support technology?

“We’re really at the hairy edge,” says Frank. “It’s conceivable that the James Webb Space Telescope could [find out], and we might get lucky.”

The JWST is probing the atmosphere of nearby exoplanets via a method known as transit spectroscopy. When a planet transits across the face of its star, producing a detectable dip in the star’s light, some of that starlight filters through the planet’s atmosphere. Molecules within the atmosphere absorb that light at specific wavelengths, casting dark lines in the star’s spectrum that betray the molecules’ presence. Already the JWST has discovered molecules such as carbon dioxide, carbon monoxide, methane and even water vapor in exoplanet atmospheres, and the Hubble Space Telescope has observed oxygen in the atmosphere of a gas giant planet (specifically the exoplanet HD 209458b, which is so close to its star that its atmosphere is evaporating) but an oxygen detection on a terrestrial world has thus far eluded us.

More probably, it will require the next generation of space telescope to be able to measure the oxygen abundance of exoplanets. “They’ll be designed with this problem in mind,” says Frank. “Certainly in the next 20 or 30 years we’ll have the telescope technologies that will be able to make these observations.”

A next-generation space telescope, which is already on the drawing board for a launch in the 2040s, after being highlighted as a priority by the National Science Foundation’s most recent astrophysics decadal survey, won’t have to rely on transit spectroscopy. It will instead blot out the light of a star using a coronagraph, reducing the star’s glare substantially so that the telescope, which will be in the eight-meter class at the very least, can directly image exoplanets instead.

Until then, in lieu of any actual exoplanetary data, perhaps we can use our own planet as a guide. For the first half of its life, Earth had no oxygen in its atmosphere. There was life though – anaerobic microbes that gained their sustenance by converting sunlight into energy. Then, around 2.4 billion years ago, cyanobacteria evolved – microbes that form the basis of photosynthesis in plants today, consuming carbon dioxide and exhaling oxygen as a waste product. The sky filled with oxygen instigating the ‘Great Oxygenation Event’, and 400 million years later the oxygen abundance had reached 10%. Alas, the oxygen was toxic to many of the older species of microbes that existed at the time, leading to a mass extinction that some scientists refer to as the ‘oxygen catastrophe’.

Since then, it’s been a story of fluctuating oxygen levels rather than a steady rise. There were two more big rises in oxygen, known as the Neoproterozoic Oxygenation Event spanning a period of time between 850 million and 540 million years ago, and the Paleozoic Oxygenation Event some 420 million years ago, but even these events had their bumps in the road.

Take the Neoproterozoic Oxygenation Event, for example. About 750 million years ago the atmospheric oxygen abundance was 12%, but within a few tens of millions of years it had plummeted to just 0.3%. The cause? An ice age where almost the entire planet froze over, quite possibly triggered by life itself, as vast mats of macroalgae removed enough carbon dioxide – an important greenhouse gas – from the atmosphere to send Earth into a snowball state. As ice covered the land it cut off the supply of nutrients that ordinarily ran-off into the ocean to feed the macroalgae, slowly starving them. As they died off, so too did their production of oxygen. However, without the macroalgae to draw it out of the atmosphere, carbon dioxide belched out by volcanoes was able to steadily accumulate once more, warming the planet and bringing the snowball state to an end. 

Following the end of global winter, oxygen levels began to reassert themselves and, for the last half-billion years, Earth’s atmosphere has been oxygen rich. “My modeling suggests that oxygen levels in the atmosphere reached 18% roughly 420–423 million years ago,” says Alex Krause, who is a biogeochemical modeler from the University of Leeds who has been studying the development of ancient Earth’s atmosphere. “There is some data that hints that 18% might have been reached around 480 million years ago, but I am not entirely convinced it would have been that high by this point in time.”

Krause says that there’s even evidence that by the Permian period, which lasted between 250 and 299 million years ago, Earth’s oxygen abundance could have been as great as 35%. So, if potentially habitable exoplanets develop in a similar fashion to Earth, we can expect to find them across a huge spectrum of oxygen abundances, from barely nothing to perhaps a third of their atmosphere being composed of oxygen.

As the huge fluctuations during the Neoproterozoic Oxygenation Event illustrate, life can have a dramatic impact on a planet’s oxygen abundance. Even something as simple as the evolution of angiosperms — flowering plants — during the Cretaceous period when large dinosaurs walked the Earth had a marked effect. Wildfires are one way that nature can regulate oxygen levels, burning huge swathes of oxygen-emitting plants, but research conducted by Claire Belcher of the University of Exeter has shown how angiosperms altered the behavior of forest fires, and therefore how those fires regulated atmospheric oxygen.

What’s evident in all of this is how life is inextricably linked to the ebb and flow of atmospheric oxygen, and therefore how life itself can be the arbiter of whether the oxygen abundance reaches 18%. Certainly large, complex life — particularly life with fast metabolisms — require more oxygen, but how much oxygen large animals need is unclear,

“Where exactly is the dividing line?” asks Frank rhetorically. “Do you need 18% for something as big as a brontosaur? I don’t think so, but you probably need more than 1%, and in between that we just don’t know. That 18% is a physical limit, not a biological limit.”

There are some caveats to this physical limit. One is that we could easily fall foul of false positives. A 2021 study led by Joshua Krissansen-Totton of the University of California, San Diego found multiple ways in which an exoplanet can develop an oxygen-rich atmosphere without the presence of life, often by being too hot or too wet for ordinary weathering, which can draw oxygen down from an atmosphere, to take place. So we might find planets with greater than 18% oxygen levels, which in Frank’s paradigm would make them high-priority targets for SETI, but they would prove to be red herrings because the oxygen would have accumulated geochemically, rather than biologically.

There’s also the problem of atmospheric pressure; the lower the pressure, the more rarefied the oxygen will seem. “It’s possible that life might not advance to a technological civilization without at least 18% oxygen in the atmosphere,” acknowledges Krause. “However, that also depends on atmospheric pressure, which might not be the same on other planets and has possibly changed through time on Earth.”

In the cases where there is life but the oxygen abundance is below 18%, we ought to be careful not to completely rule out technological life. It would be naive to assume that life couldn’t find a way.

We see surprising ways in which life is able to communicate here on Earth, and when it comes to SETI, communication of some form, whether deliberate or not, is the key. For example, some bacteria are able to communicate with each other using signaling molecules, and are able to sense the density of those molecules released into the environment to determine the number of bacteria in their colony. This is known as quorum sensing and some bioluminescent microbial colonies apply quorum sensing to regulate how they fluoresce; one could imagine an entire planet covered in microbial mats, all working together as one singular organism through quorum sensing to emit light on a global scale that could be detected light years away by astronomical observations.

“Communities of bacteria could find ways to leech metals out of the ground and build things with them in weird and interesting ways,” he proposes. “Maybe you could get natural radio telescopes or something. We have to be really open to not being so Earth-biased. I think when it comes to life in the Universe, we need to be prepared to be surprised.”

Frank and Balbi’s paper is also specifically about a certain level of civilization. When lightning struck on Earth, setting trees and grasslands ablaze, our intrepid ancestors saw the flames and realized they could make use of them, and even spark fires of their own. On a planet without enough oxygen for spontaneous open-air combustion, however, that knowledge about how fire can be tamed and wielded would never be accrued. The native lifeforms wouldn’t see any fire to inspire them to begin a journey that might eventually progress beyond the Stone Age, to one day reach for the stars.

“We’re really focused here on young species,” Frank explains. “How could a young tool-using species advance to a level where it can begin building the kinds of technologies that will leave technosignatures?”

Maybe, the lack of enough oxygen to fuel fires is one of the so-called great filters, a phrase first coined by Robin Hanson to describe the many barriers that might prevent life evolving to technological intelligence and filling the galaxy with their presence. These filters could contribute to the Fermi Paradox — the mystery of why we see no evidence of extraterrestrial intelligence. The nature and timing of these great filters is unclear. Some may lie early in a species’ history; others may await them in their future. It’s a foreboding concern for our future. If there is still something ahead of us that will curtail our development, be it nuclear armageddon, climate collapse, a war with artificial intelligence, an asteroid strike, or something else, our future existence could be threatened. On the other hand, if most of the great filters are found early in life’s evolution, then maybe we’ve got a good chance of surviving. The lack of oxygen preventing a technological civilization would therefore be one of these filters that we’ve already managed to slip past.

And once at our fingertips, technology could protect us from any future drops in oxygen. As they have in the past, oxygen levels will surely fluctuate in the future over timescales of millions of years. Should the oxygen abundance drop to below 18%, it wouldn’t mean that our technology would suddenly stop working; once a technological civilization becomes established, it can use its technology to mitigate the consequences of an oxygen drop. In the far future, a billion years or more ahead of us, the Sun will warm and the Earth will begin to sizzle. Plant life will wither away and oceans will evaporate, and atmospheric oxygen will bottom out. And yet, if we’re still around in that far future, it need not spell the end for us.

“In another billion years, if humanity can get through what we’re dealing with now, we might terraform Mars or put up sunshields,” says Frank. “With technology, a species can become functionally immortal, it can free itself from the constraints of stellar and planetary evolution. So in that sense, all bets are off, which is what makes it so interesting.”

Technology, combined with a dose of enlightened self-interest and an awareness sufficient enough to avoid wiping themselves out, can give a civilization the tools to hold the Universe in the palm of its hand. We can’t even begin to imagine what the upper bounds for such a civilization could ultimately be. But we do know that there are lower bounds, and to squeeze through the bottleneck requires a happy coincidence of timing, of oxygen levels, and the evolution of technological intelligence aligning in order to push past this particular great filter. If a planet just has one without the other, then technological life will never happen, and our modern human society of mod-cons, shining satellites and microchips would stand out among the stars like a lonely spark in the Universe.