On which exoplanets will we most likely find alien civilizations with advanced technologies?

Alien hunters’ should look for life, especially technologically advanced life, on exoplanets with high oxygen levels, as oxygen is needed to burn fossil fuels, produce rocket fuel, and many other processes.

Amedeo Balbi from the University of Roma Tor Vergata in Italy and Adam Frank from the University of Rochester in the US argue that a planet's atmosphere must contain at least 18% oxygen for such a planet to develop a technological civilization. And the reason for this is simple: fire requires oxygen.

Back in 1996, British economist and futurist Robin D. Hanson hypothesized the so-called “Great Filter.” This hypothesis is one of the possible solutions to the Fermi paradox, according to which, over the billions of years of development of the Universe, multiple alien civilizations should have settled in it, but so far people have not found traces of the activity of any of them. According to Hanson, the lack of signs of extraterrestrial civilizations in the observable Universe means that the arguments of some scientists in favor of the relatively high probability of the emergence of intelligent life may be erroneous and factors currently unknown to science may interfere with the emergence and development of life forms to the point that traces of their activity could be detected from Earth.

According to scientists, the presence of at least 18% oxygen in the atmosphere may be one of the factors of the “Great Filter”. If there is less oxygen, it will not be enough for long-term combustion in the open air, and will create many restrictions for the development of metallurgy and many other areas. Limitations on the burning of fossil fuels will certainly help keep the atmosphere clean, but without it, as without metallurgy, it would be difficult to support any widespread industry or produce the materials needed to create radio transmitters capable of sending signals that could detect our devices.

Today on Earth the oxygen content in the atmosphere is 21%, but this was not always the case. In the first half of life, the planet's atmosphere was dominated by nitrogen and carbon dioxide, and the proportion of oxygen may have been only 0.001%. However, about 2.4 billion years ago, the content of molecular oxygen in our planet's atmosphere began to rise sharply. This increase is explained by the evolution of cyanobacteria, which produced oxygen as a waste product. Because of this, many anaerobic life forms were destroyed as a result.

For 400 million years after this event - approximately two billion years ago - oxygen levels in the atmosphere were at 10%, meaning they were still too low for intelligent and advanced life forms to emerge.

Considering how long it took for Earth's oxygen levels to rise above 18%, we can assume that the search for intelligent life should be on older planets that have had enough time to create their own oxygen reserves.

Balbi’s and Frank's study, published in the journal Nature Astronomy, is important for another reason: its data suggests that if Earth's oxygen levels drop below 18%, we will face serious problems. According to a 2021 study by Kazumi Ozaki of Toho University in Japan and Christopher Reinhardt of NASA and the Georgia Institute of Technology in Atlanta, as the Sun ages and becomes brighter, producing more heat, the Earth's atmosphere will become deprived of oxygen. And in about a billion years, oxygen levels will drop to 10%.

Of course, a billion years is quite a serious period, and future earthlings may have time to prepare for this problem and invent technologies that will help somehow mitigate the problem. Or they will have time to migrate to other planets.