The search for extraterrestrial intelligence (SETI) has long focused on detecting signals from alien civilizations. But in recent years, scientists have expanded the search to look for potential “technosignatures” – signs of advanced alien technology that could be spotted with telescopes. In a paper published in Nature Astronomy, astronomers Amedeo Balbi and Adam Frank argue that oxygen may be the key ingredient needed for planets to develop technologically advanced civilizations that produce detectable technosignatures.
Oxygen Fuels Complex Life and Technology on Earth
On Earth, oxygen played a crucial role in enabling the evolution of complex multicellular life forms and, ultimately, an intelligent technological species. The authors explain that aerobic respiration, which uses oxygen to efficiently extract energy from organic compounds, provides a major advantage over anaerobic metabolism. Higher oxygen levels in Earth’s atmosphere were linked to increases in animal body size and complexity over evolutionary timescales.
Oxygen also facilitated the controlled use of fire by early humans. Fire provided access to much greater energy densities, allowing early technological innovations like metallurgy and eventually the industrial revolution. Many pivotal developments in metallurgy to produce copper, bronze, and iron tools involved combustion reactions using atmospheric oxygen. Thus, the presence of substantial free oxygen enabled rapid technological progress on Earth.
The Oxygen Bottleneck
While small amounts of oxygen may be enough to sustain microbial or simple animal life, the authors identify combustion and fire as the critical ingredients needed to transition toward advanced technology. Controlled combustion is only possible on Earth when atmospheric oxygen makes up at least 18% of the air. This 18% limit represents an “oxygen bottleneck” – a barrier that developing civilizations must cross by building up enough free oxygen.
Planets that lack sufficient oxygen may evolve intelligent creatures, but those creatures would lack the energy sources needed to develop high technologies using fire and metallurgy. Without technology like radio telescopes that produce detectable electromagnetic signals, intelligent life on oxygen-poor planets would be extremely difficult to find with SETI searches. Thus, oxygen abundance could be a key factor limiting which planets develop detectable technosignatures.
Implications for the Search for Alien Technosignatures
The oxygen bottleneck hypothesis suggests that SETI researchers may want to prioritize searching for technosignatures around planets with at least 18% free oxygen in their atmospheres. Detecting oxygen-rich atmospheres on exoplanets is within the capabilities of next-generation telescopes like the James Webb Space Telescope and extremely large telescopes now under construction.
The presence of high atmospheric oxygen content would provide an important clue that a planet may host an advanced technological civilization. Of course, life may find alternative pathways to technology that don’t require oxygen – but oxygen provides such an efficient energy source that most technological civilizations are likely to utilize it. By focusing on oxygen-rich worlds, our chances of detecting alien intelligence could be greatly improved.
The search for extraterrestrial intelligence has long focused on “listening” for radio signals from space. But by considering planetary environments and energy sources, we can better predict where advanced alien civilizations might arise. Using oxygen as a clue, the age-old question “are we alone?” may finally have an answer in the coming decades.
