The “oxygen bottleneck” may leave aliens stuck with primitive technology

What are the planetary prerequisites for the evolution of an intelligent, technological species? If humanity is going to search the galaxy for exoplanets with signatures of technological intelligence — and we’re starting to do just that — what kinds of planets should we focus on: Planets with a mix of oceans and land? With plate tectonics? Magnetic fields? In other words, what kinds of planets are conducive to the development of a world-spanning technological civilization?  

This was exactly the kind of question Italian astrobiologist Amedeo Balbi and I asked ourselves about a year ago. The research paper that resulted was recently published in Nature Astronomy, and today I want to unpack it a bit. If we are right, there could be some pretty big implications for where and when intelligent life in the Universe could form.

The “oxygen bottleneck”

The paper was called “The Oxygen Bottleneck for Technospheres” and its idea was simple: To make advanced technology, you need to be able to raise the temperature of the stuff you use to make that technology. Think about metallurgy. If you want to build something like a radio telescope, you’ll need to extract a bunch of iron, nickel, copper, and other raw materials from the ground and then you’ll need to heat them. The heat is required so that the metals melt and can be mixed to make alloys or be fashioned into the shapes you need (like struts or wires). Getting high temperatures may also be important for other things beyond metallurgy, as even the ability to cook food has been implicated in the development of human intelligence (nutrients are more available in cooked food).  

So, what does it take for a young intelligent species to get ready access to high temperatures? One answer from human history is burning stuff (i.e. combustion). If smart creatures who are starting to use tools have easy access to combustion, they can more easily climb the ladder of technological sophistication. 

But what, then, is combustion? This sounds like a simple question, but it took me and my relative ignorance of chemistry some effort to truly understand. Combustion is basically an exothermic chemical reaction that requires a fuel and an oxidizer. The “exo” here means that once a spark is applied, combustion reactions begin giving off heat. The reactions continue until either the fuel or the oxidizer are exhausted. A while back I wrote an entire post on my joy at discovering the details of combustion, so I won’t go through all that again. The main point of it all was that oxygen turns out to be the best all-around oxidizer. (Other elements like fluorine actually give off more energy in combustion, but they tend to be so reactive that they corrode everything they touch.)

It was this simple fact, drawn from the famous periodic table you slept through in high school, that led us to conclude that only planets with oxygen in their atmosphere could host technological civilizations. The next question is how much oxygen an atmosphere needs. Drawing on experiments carried out across disciplines as varied as combustion engineering to biogeochemistry, we found that an atmosphere with anything less than 18% oxygen would not allow open-air combustion. Remarkably, for most of our planet’s 4.5-billion-year history, Earth’s oxygen levels have been way, way below 18%. In fact, only over the past 500 million years or so has the atmosphere held enough oxygen for anything to freely burn in the open air.

Why does any of this matter? Imagine a young and intelligent species on an alien world with an atmosphere that’s just 1% oxygen. Those clever tool-using creatures would never get the chance to watch a tree burn after being hit by lightning and get the idea of using fire for their own purposes. They would never have the chance to learn how fire could be used to cook food, clear land, or, most importantly, melt metals. The poverty of oxygen in their air would likely box these creatures in forever, limiting their development. This is what Prof. Balbi and I meant by the “oxygen bottleneck.” Widespread technological development requires simple and easy access to high temperatures and open-air combustion is the simplest and easiest way for that to happen (would volcanic vents, for example, be so prevalent to allow industries to evolve?). That’s why oxygen-rich atmospheres matter. Planets with them may be the only ones that host intelligence and civilizations. 

But how many of these sorts of high-oxygen planets does the galaxy hold? If the numbers are really small, then we might end up being oxygen-rich but companion-poor.