If technologically advanced civilizations exist elsewhere in the Milky Way, they are likely to be both extremely rare and much older than our own, according to new research.
Presented at the EPSC–DPS 2025 Joint Meeting in Helsinki, the findings reveal the specific planetary conditions needed for such civilizations to arise, which may be so limited that the nearest example could be located 33,000 light-years away—potentially on the far side of our galaxy.
The new research builds on the idea that specific planetary factors are essential for life and technological development. Led by Dr. Manuel Scherf and Professor Helmut Lammer of the Austrian Academy of Sciences, the study examines how features such as plate tectonics and atmospheric composition might determine whether a planet can host advanced societies with technological capabilities.
The Role of Plate Tectonics
The analysis places particular emphasis on the role played by plate tectonics. As Earth’s plates move over time, they trigger the carbon–silicate cycle, which gradually alters the amount of carbon dioxide present in the atmosphere. The geological changes that result from this movement help maintain temperatures within a range that supports life, stabilize the atmospheric mix of gases, and enable photosynthesis to occur. Over extended geological timeframes, tectonic activity locks away carbon dioxide within rock formations.
“At some point, enough carbon dioxide will be drawn from the atmosphere so that photosynthesis will stop working,” said Scherf. For Earth, that point is expected to come in roughly 200 million to one billion years.
The team ran simulations using various atmospheric scenarios to determine how long biospheres could last under different conditions. They found that if a planet’s atmosphere contains 10% carbon dioxide, photosynthesis could be sustained for up to 4.2 billion years. However, if the carbon dioxide level drops to just 1%, the period during which photosynthesis remains possible shrinks to about 3.1 billion years. The results suggest that the conditions required for long-term habitability may be narrower than earlier models suggested.
Oxygen and the Spark of Technology
In addition to carbon dioxide, the amount of oxygen present in the atmosphere is another key factor that determines whether a planet is capable of technological advancement. While Earth’s atmosphere contains about 21% oxygen, the study finds that a minimum of 18% is necessary for fire to burn in open air.
Without fire, the refinement of metal would not be possible. “Not only is more oxygen needed by larger, complex animals,” said Scherf, “but without fire the smelting of metal would be unfeasible and a technological civilisation would be impossible.”
This challenge highlights how, even with a simple life form present, the leap to advanced technology may be restricted by a planet’s atmospheric environment, further illustrating how rare such conditions might be.
The Odds of Overlap
The team compared the estimated lifespans of habitable planets with the time required for technological life to evolve on Earth, which is approximately 4.5 billion years, and also considered the potential longevity of such civilizations once they emerge.
Their analysis implies that for even one other technological species to exist concurrently with humanity, it would need to have existed for at least 280,000 years. For ten civilizations to overlap, each would have to survive for more than 10 million years.
This implies that if extraterrestrial intelligence is ever detected, it is likely to represent a civilization much older than our own. As Scherf notes, the number of technological societies strongly depends on how long such civilizations can endure.
A Civilization Across the Galaxy
Based on their estimates, Scherf and Lammer conclude that the nearest technological civilization could be around 33,000 light-years away and would likely be located on the opposite side of the Milky Way from our solar system. With these estimates in mind, the researchers emphasize that significant uncertainties remain, especially in the steps leading from the origin of life to the rise of technology. These uncertainties directly impact SETI strategies as well as expectations.
Searching Anyway
Despite the odds, Scherf argues that the Search for Extraterrestrial Intelligence (SETI) remains a worthwhile endeavor. “Although ETIs might be rare, there is only one way to really find out, and that is by searching for it,” he said.
Continued null results from SETI would strengthen the case that the existence of other technological civilizations is extremely rare. On the other hand, detecting a signal would mark a major scientific milestone, showing that humanity is not alone and that at least one other society has endured much longer than ours.
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds a Master of Business Administration and a Bachelor of Science in Business Administration, along with a certification in Data Analytics. His work combines analytical training with a focus on emerging science, aerospace, and astronomical research.
