New research suggests the closest technological civilization in the Milky Way could be about 33,000 light years from Earth. For such a society to coexist with humanity, it would need to have endured for at least 280,000 years. The findings, presented at a joint meeting in Helsinki, underscore the rarity of extraterrestrial intelligence.
Researchers Dr. Manuel Scherf and Professor Helmut Lammer from the Space Research Institute at the Austrian Academy of Sciences in Graz presented their study at the EPSC-DPS2025 Joint Meeting in Helsinki. Their work highlights the slim chances of finding Earth-like planets with plate tectonics and a nitrogen-oxygen atmosphere balanced for life, making complex intelligent life rare in the galaxy.
The study emphasizes the role of carbon dioxide in planetary habitability. Earth's atmosphere is 78 percent nitrogen, 21 percent oxygen, and just 0.042 percent carbon dioxide. Photosynthesis on Earth is expected to cease in 200 million to one billion years as carbon dioxide levels drop due to the carbon-silicate cycle trapping it in rocks.
Modeling other worlds, the researchers found that a planet with 10 percent carbon dioxide could sustain a biosphere for up to 4.2 billion years, while one with 1 percent would last about 3.1 billion years. Advanced life requires at least 18 percent oxygen; below this, fire and metalworking—key to technology—would be impossible.
Comparing these timelines to Earth's 4.5 billion-year path to technological life, Scherf and Lammer calculated that for one other civilization to exist simultaneously in the Milky Way, a species on a 10 percent carbon dioxide planet must persist for at least 280,000 years. "For ten civilizations 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 civilization."
This leads to the estimate that the nearest such civilization is roughly 33,000 light years away, potentially on the opposite side of the galaxy from our Sun, which is 27,000 light years from the center. "Extraterrestrial intelligences, ETIs, in our galaxy are probably pretty rare," Scherf notes. Despite the odds, he advocates continuing SETI efforts: "Although ETIs might be rare there is only one way to really find out and that is by searching for it." A detection would confirm we are not alone, while null results would bolster the theory of rarity.