Alien civilizations may be far rarer than hoped study suggests

Alien civilizations may be far rarer than hoped study suggests
by Robert Schreiber
Berlin, Germany (SPX) Sep 17, 2025
New research presented at the EPSC - DPS2025 Joint Meeting in Helsinki indicates that the nearest technological civilization in the Milky Way could be about 33,000 light years from Earth, and that such a society would need to have survived at least 280,000 years - potentially millions - to coexist with us in time.

Dr Manuel Scherf and Professor Helmut Lammer of the Austrian Academy of Sciences argue that the odds of finding advanced life are severely constrained by planetary conditions. Specifically, worlds require active plate tectonics to regulate carbon dioxide and maintain long-term biospheres. Without this mechanism, atmospheres can become either depleted or toxic.

Scherf notes that eventually, even Earth will lose sufficient atmospheric carbon dioxide for photosynthesis to function, a process expected within 200 million to one billion years. On Earth, nitrogen (78 percent) and oxygen (21 percent) dominate the atmosphere, with trace carbon dioxide at 0.042 percent. Their modeling shows that planets with 10 percent carbon dioxide could support biospheres for 4.2 billion years, while 1 percent carbon dioxide would allow up to 3.1 billion years of habitability.

However, such planets would still need at least 18 percent oxygen to support complex animals and enable fire. Fire, in turn, is essential for metalworking, a prerequisite for a technological civilization.

By comparing biosphere longevity with the time needed for technological species to arise - 4.5 billion years on Earth - the researchers concluded that civilizations on high-carbon dioxide planets must endure at least 280,000 years to overlap with humanity. To have ten such civilizations alive simultaneously, average lifespans must exceed 10 million years.

This statistical framework suggests that if extraterrestrial intelligences exist, they are likely to be far older than humans. It also places the closest advanced civilization on the far side of the galaxy.

Scherf cautions that these results depend on poorly understood variables such as the likelihood of life's origin, photosynthesis, multicellularity, and technological development. If those probabilities are high, alien civilizations may be less rare than estimated.

"Although ETIs might be rare there is only one way to really find out and that is by searching for it," Scherf said. "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."