Alien hunters should search for technological life on planets that possess a high oxygen abundance in their atmospheres, according to new research that aims to hone the search for technosignatures from extraterrestrial civilizations.
Amedeo Balbi, who is an associate professor of astronomy and astrophysics at University of Roma Tor Vergata in Italy, and Adam Frank, who is a professor of physics and astronomy at the University of Rochester in the U.S., argue that a planet's atmosphere needs to contain at least 18% oxygen to facilitate a technological civilization. The reason for this, they say, is a simple one: oxygen is needed for fire.
"You might be able to get biology — you might even be able to get intelligent creatures — in a world that doesn't have oxygen," said Frank in a statement. "But without a ready source of fire, you're never going to develop higher technology because higher technology requires fuel and melting."
Balbi and Frank describe the 18% level as an "oxygen bottleneck" that potentially hinders intelligent life from developing sophisticated technology that we could detect via radio signals. This includes any alien city lights, infrared emissions of Dyson swarms or perhaps megastructures transiting alien stars. This concept joins other such bottlenecks scientists discuss during the search for extraterrestrial life, including a bottleneck that dictates the evolution of multicellular life, one that deals with the development of tool-using intelligence, and even one that regulates whether an alien society can avoid blowing itself up. Together, they form the "Great Filter," a concept introduced by economist and futurist Robin Hanson while describing a series of barriers to the development of technological life in an effort to explain why it seems scarce in our galaxy.
Exoplanets with an oxygen abundance below 18% would not be sufficient for prolonged open-air combustion, the team says. This would limit metallurgy, for example, and constrain the burning of fossil fuels. While a lack of the latter might lead to a cleaner environment, without the former it would be difficult to sustain any kind of widespread industry or fabricate materials necessary to build radio transmitters that can send signals detectable by our devices.
On Earth, the abundance of oxygen in our atmosphere is 21%, but this has not always been the case. For the first half of its life, Earth's atmosphere was dominated by nitrogen and carbon dioxide, and oxygen may have made up just 0.001%. Then, around 2.4 billion years ago, the molecular oxygen abundance in our planet's atmosphere began to dramatically rise. This increase is attributed to the evolution of cyanobacteria that produced oxygen as a waste product, but the rising oxygen levels are also known to have been harmful to the anaerobic life forms of the time, reducing the abundance of hydrogen sulfide and methane necessary for their life.
Hence, the arrival of extra oxygen in Earth's atmosphere is sometimes called the "oxygen catastrophe" because many microbial species were wiped out.
Within 400 million years of this event — two billion years ago — atmospheric oxygen levels were at 10%, meaning they would have still been too low for Balbi and Frank's bottleneck.
Consequently, the duo's research points to the kinds of planetary systems that the search for extraterrestrial intelligence (SETI) should focus on during the hunt for technosignatures.
"Targeting planets with high oxygen levels should be prioritized because the presence of high oxygen levels in exoplanet atmospheres could be a major clue in finding potential technosignatures," said Frank.
Furthermore, given how long it took for Earth's oxygen levels to rise above 18%, it might make sense to target older planets that would have had enough time to develop their own stores of oxygen.
The research can even be used to rule out false alarms, says Balbi, who suggests "we should be skeptical of potential technosignatures from a planet with insufficient atmospheric oxygen."
The findings also suggest that if technological life on Earth can continue to flourish for the next few billion years, we might find an additional challenge on our hands. In a 2021 study by Kazumi Ozaki of Toho University in Japan and Christopher Reinhard of NASA and the Georgia Institute of Technology in Atlanta, it was shown that, as the sun ages and brightens to produce more heat that warms our planet in turn, Earth's atmosphere will become deoxygenated, with oxygen levels dropping below 10%. Of course, a billion years is a long time to be able to prepare; by then, our descendants may have technology to mitigate this, or might have left Earth entirely.
The research was published on Dec. 28, 2023 in the journal Nature Astronomy.
