Alien life on a planet with low levels of oxygen might never be able to develop technology because combustion would be impossible. This bottleneck for creating advanced civilisations may also help explain why we have yet to observe life elsewhere in a near-infinite universe.
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| Fires couldn’t be started on exoplanets that don’t have much oxygen Shutterstock/ppl |
The development of technology on Earth hinged on fire, also known as combustion, a chemical reaction that uses oxygen to generate large amounts of heat.
This is crucial for, among other things, extracting metals from ore, which almost every advanced human technology relies on. Laboratory experiments have found that combustion can’t fully occur in atmospheres with oxygen levels below about 18 per cent.
Now, Adam Frank at the University of Rochester in New York and Amedeo Balbi at the University of Rome in Italy have suggested that this might be a universal bottleneck for whether alien species can develop technology.
“You may have enough oxygen in the atmosphere of an exoplanet to have complex multicellular life, but you may not have enough oxygen to start combustion,” says Balbi.
We don’t know the exact balance of geological and biological processes that lead to an atmosphere with this level of oxygen, because we currently only have Earth as an example. “Oxygen in the atmosphere is also produced by photosynthetic activity, so that’s why it gets complicated, because it’s not just geology, it’s also biology,” says Balbi
There is also an upper limit to what is likely to be a useful amount of oxygen. At about 30 per cent and above, depending on the moisture content in the air, the chance of widespread fires killing lifeforms would become very high as combustion becomes so easy, says Frank.
Astronomers are looking for any type of life in the universe, but the search for intelligent extraterrestrial life might be guided by looking for planets with a level of oxygen in the atmosphere of between 18.5 and 21 per cent, the optimum for useful combustion.
For now, though, detecting atmospheric oxygen is beyond the current abilities of our telescopes for almost all known exoplanets. “We’re just getting started looking for the atmospheres of terrestrial planets,” says Frank. “This is the exciting thing – we’re at the frontier now.”
Combustion as a bottleneck for technology makes sense if life emerges the way it did on Earth, but might not apply if life evolved differently on another planet, says Ingo Waldmann at Imperial College London. “That was the evolutionary path we took, but I’m not necessarily sure whether that can be generalised across the universe.”
For instance, it is possible that an alien species in a low-oxygen environment may be able to harness heat from geothermal activity, says Waldmann, such as volcanoes, to do their smelting.
Another possibility could be concentrating sunlight using mirrors, says Ian Crawford at the University of Birkbeck, UK, although manufacturing mirrors in the first place may be tricky, he adds.
One downside for these alternatives is that they aren’t portable like wood and fossil fuels are, says Balbi. This might mean any civilisations would need to stay close to their heat source.
If the oxygen bottleneck does prove to be a limiting factor, then Frank and Balbi suggest it might help solve the Fermi paradox, which questions why there are no signs of advanced intelligent life if it is apparently likely, given the size of the universe.
“The Fermi paradox is telling us something about the nature of life in the universe that we don’t understand,” says Crawford. “In that context, all potential solutions to the Fermi Paradox are of interest.”
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