Key Takeaways:
- A new study presented at the 2025 EPSC/DPS Joint Meeting proposes that the rarity of specific geological and atmospheric conditions necessary for technologically advanced life significantly limits the probability of encountering extraterrestrial civilizations within the Milky Way galaxy.
- The study highlights the crucial need for a finely balanced atmospheric composition of carbon dioxide and oxygen, maintained over billions of years by active plate tectonics, as a prerequisite for advanced life capable of interstellar communication.
- The researchers' model incorporates the requirement of sufficient atmospheric oxygen (at least 18%) to enable fire-based technologies, a critical step for the development of advanced tools and communication infrastructure necessary for detectable signals.
- Based on these constraints and Earth's timeline for technological development, the study suggests that even a single co-existing civilization in the Milky Way would need to be exceptionally old (over 280,000 years), while multiple such civilizations would require extraordinarily long lifespans.
For decades, scientists have grappled with a profound question known as the Fermi Paradox: if the galaxy is teeming with the potential for extraterrestrial life, why is the cosmos so quiet? A new study offers a potential answer, suggesting the specific geological and atmospheric conditions needed for a technological civilization to arise are so rare that we may be one of the very few in the Milky Way.
The study, presented Sept. 9 at the 2025 Europlanet Science Congress (EPSC) and Division for Planetary Sciences (DPS) Joint Meeting in Helsinki, lays out a series of hurdles a planet must clear to host advanced life. First, it must achieve a finely tuned atmospheric balance of carbon dioxide and oxygen. Second, it needs a geological mechanism in the form of active plate tectonics to maintain that balance over billions of years. Based on these and other constraints, the authors calculate that the nearest technological civilization could be 33,000 light-years away and would need to have survived for at least 280,000 years to be co-existing with us now.
“Extraterrestrial intelligences – ETIs – in our galaxy are probably pretty rare,” Manuel Scherf, a researcher at the Space Research Institute at the Austrian Academy of Sciences who presented the findings, said in a Sept. 12 press release.
An ear to the void
This sobering analysis provides a new framework for understanding the silence that has so far characterized the Search for Extraterrestrial Intelligence (SETI). As outlined by the SETI Institute, the modern search began in 1960 with astronomer Frank Drake, who used a radio telescope to listen for signals from nearby stars. For more than six decades, scientists have used massive antennas to eavesdrop on the cosmos, hoping to detect radio or light signals that would serve as proof of intelligent beings elsewhere. This new research, however, focuses on the immense challenges that must be overcome before an advanced civilization could ever send such a signal.
A delicate atmospheric balance
At the heart of the researchers’ analysis is the atmospheric challenge: maintaining a delicate balance of carbon dioxide for billions of years. According to the researchers’ framework, a planet’s atmosphere needs enough CO₂ to support a long-lived biosphere through photosynthesis, but not so much that it triggers a runaway greenhouse effect — a process where a planet gets progressively hotter until its oceans boil away.
The more CO₂ a planet has, the longer it can sustain life, as the gas is inevitably drawn out of the atmosphere and locked away in rocks over geological time. The researchers’ calculations explored this trade-off, finding that a world with a higher initial concentration of CO₂ (10 percent) could sustain its biosphere for up to 4.2 billion years. In contrast, a planet with a lower concentration (1 percent) would see its biosphere collapse after only 3.1 billion years, a potentially shorter window than it takes for complex life to evolve.
Planetary thermostat
To maintain the requisite CO₂ levels, the framework assumes any technological civilization would need a planet with active plate tectonics. On our world, this geological mechanism drives the carbon-silicate cycle, a process that acts as a planetary thermostat. Natural processes like the weathering of rocks constantly pull CO₂ out of the atmosphere, where it eventually gets locked away in sediments on the ocean floor. Plate tectonics complete the cycle by subducting these carbon-rich rocks deep into the Earth, where they melt, releasing the CO₂. This gas is then vented back into the atmosphere through volcanic activity, which is common at plate boundaries. Without this volcanic recycling system, CO₂ would be permanently locked away, and the planet’s atmosphere would more rapidly be depleted, leading to a global freeze.
The assumption about the importance of plate tectonics is bolstered by a 2024 study in Scientific Reports from Robert Stern, a professor of geosciences at the University of Texas at Dallas, and Taras V. Gerya, a professor of earth and planetary sciences at ETH-Zurich. Their research suggests that plate tectonics provides other essential benefits for life. They showed that Earth’s own evolution was stagnant for a billion years under a more static “single lid” tectonic regime. Life’s diversification only accelerated dramatically after the planet transitioned to modern plate tectonics, which supercharged the nutrient cycle, sped up the creation and destruction of habitats, and maintained both deep oceans and exposed continents — a critical combination, as primitive life likely needs water to evolve, while technological life needs land to discover fire and build tools.
Filtered by fire
Assuming a planet has the right atmospheric and geological conditions to develop primitive life, the researchers add another constraint to their calculations: fire. In order to have open-air combustion, an atmosphere needs at least 18 percent oxygen. Without this level of free oxygen, large-scale combustion is impossible. That means no fire to smelt metals, no forges to create complex tools, and ultimately, none of the industrial or technological leaps required to build the radio telescopes or powerful lasers that SETI experiments are designed to detect. This final filter narrows the search from merely habitable planets to those that can support a civilization capable of sending signals across the stars.
Sobering numbers
By combining these geological and atmospheric requirements with the 4.5 billion years it took for technological life to evolve on Earth, the researchers arrived at their stark conclusion. For even one other civilization to exist in the Milky Way at the same time as us, it would have to be, on average, more than 280,000 years old. For 10 to exist, their average lifetime would need to exceed 10 million years.
The researchers acknowledge that their analysis does not include every potential barrier to intelligent life. Other enormous hurdles — such as the initial origin of life, the evolution of photosynthesis, the leap to multicellular organisms, and the frequency with which intelligence leads to technology — were not factored into their calculations because their probabilities cannot currently be quantified. If these steps prove to be common throughout the galaxy, ETIs might not be so rare after all. But if they are also great filters, the outlook for a populated galaxy is even more pessimistic.
This suggests that if we ever do make contact, the civilization on the other end is likely to be vastly older than our own. Despite the bleak odds, Scherf believes the search must go on.
“Although ETIs might be rare there is only one way to really find out and that is by searching for it,” he stated. “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.”
