How Common Are Biological ETIs in the Galaxy?
The closest technological species to us in the Milky Way galaxy could be 33,000 light years away and their civilisation would have to be at least 280,000 years, and possibly millions of years, old if they are to exist at the same time that we do, according to new research presented at the EPSC–DPS2025 Joint Meeting in Helsinki this week.
These numbers are reflective of the strong odds against finding Earth-like worlds with plate tectonics and a nitrogen-oxygen dominated atmosphere with just the right amount of oxygen and carbon dioxide.
By considering these factors, the possibility for the success of SETI (Search for Extraterrestrial Intelligence) seems bleak, according to Dr Manuel Scherf and Professor Helmut Lammer of the Space Research Institute at the Austrian Academy of Sciences in Graz.
“Extraterrestrial intelligences – ETIs – in our galaxy are probably pretty rare,” says Scherf.
The more carbon dioxide a planet has in its atmosphere, the longer it can sustain a biosphere and photosynthesis for, and prevent the atmosphere from escaping into space, but it’s a careful balance: too much carbon dioxide and it can lead to a runaway greenhouse effect, or render the atmosphere too toxic for life. Plate tectonics regulate the amount of carbon dioxide in the atmosphere as part of the carbon-silicate cycle, and so a habitable planet requires plate tectonics. Gradually, though, the carbon dioxide that is drawn out of the atmosphere gets locked away in rocks rather than recycled.
“At some point enough carbon dioxide will be drawn from the atmosphere so that photosynthesis will stop working,” says Scherf. “For the Earth, that’s expected to happen in about 200 million to roughly one billion years.”
Earth’s atmosphere is dominated by nitrogen (78 per cent) and oxygen (21 per cent), but it also contains trace gases including carbon dioxide (0.042 per cent). Scherf and Lammer consider what would happen on a planet with ten per cent carbon dioxide (such a planet could avoid a runaway greenhouse if it is further from its sun, or its sun is younger and less luminous) and find that its biosphere could be maintained for 4.2 billion years. Alternatively, an atmosphere with one-per-cent carbon dioxide would maintain a biosphere for a maximum of 3.1 billion years.
This graph shows the maximum number of ETIs presently existing in the Milky Way. The solid orange line describes the scenario of planets with nitrogen–oxygen atmospheres with 10 per cent carbon dioxide. In this case the average lifetime of a civilization must be at least 280,000 years for a second civilization to exist in the Milky Way. Changing the amount of atmospheric carbon dioxide produces different results. Image credit: Manuel Scherf and Helmut Lammer.
These worlds would also need no less than 18 per cent oxygen. Not only is more oxygen needed by larger, complex animals, but previous studies have shown that if oxygen levels fall below this then there is not enough free oxygen to enable open-air combustion. Without fire the smelting of metal would be unfeasible and a technological civilisation would be impossible.
Scherf and Lammer then contrasted these biosphere lifespans with the amount of time it takes for technological life to evolve, which on Earth was 4.5 billion years, and the possible lifetime of a technological species. This is important because the longer their species survives, the greater the chance that they will exist at the same time that we do.
Combining all these factors is what led Scherf and Lammer to the conclusion that technological species living on a planet with 10 per cent carbon dioxide would have to survive for at least 280,000 years for there to even be one other civilisation in the galaxy at the same time we are.
“For ten civilisations to exist at the same time as ours, the average lifetime must be above 10 million years,” says Scherf. “The numbers of ETIs are pretty low and depend strongly upon the lifetime of a civilisation.”
This means that if we do detect an ETI, it is almost certainly going to be much older than humanity.
It’s these numbers that also lead to the estimate that the next closest technological civilisation is about 33,000 light years away. Our Sun is about 27,000 light years from the galactic centre, which means that the next closest technological civilisation to our own could be on the other side of the Milky Way.
These numbers are not absolutes – Scherf points out that there are other factors that should be included, such as the origin of life, the origin of photosynthesis, the origin of multi-cellular life and the frequency with which intelligent life develops technology, but they cannot be quantified at present. If each of these factors has a high probability, then ETIs might not be as rare. If each of these factors has a low probability, then a more pessimistic outlook is required.
Nevertheless, Scherf strongly believes that SETI should continue the search.
“Although ETIs might be rare there is only one way to really find out and that is by searching for it,” says Scherf. “If these searches find nothing, it makes our theory more likely, and if SETI does find something, then it will be one of the biggest scientific breakthroughs ever achieved as we would know that we are not alone in the Universe.”
Further information
EPSC-DPS2025 1512 How Common Are Biological ETIs in the Galaxy?
Manuel Scherf and Helmut Lammer,
Astrobiology, SETI, Technosignatures, Habitability,
