In 1961, astronomer Frank Drake introduced his famous equation to estimate the number of civilizations in the Milky Way capable of communicating with us. However, our understanding of planetary science has evolved considerably since then. Now, a team of scientists has proposed crucial adjustments to the equation that might explain the Great Silence—the puzzling lack of contact with extraterrestrial civilizations.

The Drake Equation, while popular and intuitive, has faced criticism for its broad assumptions and ambiguous parameters. It often produces an overly optimistic estimate for the value of N—the number of civilizations in our galaxy with which we might communicate. This has fueled the Fermi Paradox: If intelligent life is common, why haven’t we found evidence of it? Recent research published in Scientific Reports offers a potential solution by introducing two new factors into the equation.

Planetary scientists Robert Stern from the University of Texas at Dallas and Taras Gerya from ETH-Zurich, co-authors of the study, argue that the presence of both continents and oceans, along with long-term plate tectonics, is critical for the emergence of advanced civilizations. They propose adding two new factors to the equation: the fraction of habitable planets with significant continents and oceans, and the fraction of those planets with plate tectonics operating for at least 500 million years. This adjustment, however, drastically reduces the estimated value of N.

“Our work suggests that Earth—with its continents, oceans, plate tectonics, and life, as well as our active, communicative, technological human civilization—is extremely rare and unique in the entire galaxy,” Gerya told Gizmodo.

The Crucial Role of Geological Factors

The traditional Drake Equation estimates the number of active extraterrestrial civilizations in the Milky Way by considering several factors, such as the rate of star formation, the fraction of stars with planets, the number of habitable planets, and the likelihood of intelligent life evolving. The proposed tweaks refine the equation by incorporating new environmental, biological, and technological factors.

The researchers argue that the presence of large oceans, coupled with Earth’s transition from single-lid tectonics to modern plate tectonics about 1 billion years ago, was essential for the rapid development of complex life. This geological activity not only created the initial conditions necessary for life but also led to diverse environments with varying climates and ecosystems, promoting the evolution of advanced life forms capable of developing technology and complex societies.

According to the study, plate tectonics are crucial for the development of complex life and advanced civilizations. Earth’s plate movements create diverse habitats, recycle nutrients, and regulate climate—all vital for sustaining life. Gerya emphasizes the importance of plate tectonics lasting for at least 500 million years, given the slow pace of biological evolution. “On Earth, it took more than 500 million years to develop humans from the first animals, which appeared around 800 million years ago,” he said.

Technology, the authors argue, develops from everyday needs such as tool-making, farming, clothing creation, and weapon production. They note that fire and electricity are “essential” for the development of intelligent civilizations, and that complex civilizations are unlikely to emerge in strictly ocean-based environments.

Stern and Gerya suggest that it is quite rare for planets to possess both continents and oceans along with long-term plate tectonics, and that this rarity needs to be accounted for in the Drake Equation.

Quantifying the Rarity

To estimate how likely it is for a planet to have both continents and oceans, Stern and Gerya examined how much water is needed on a planet’s surface. They found that an Earth-sized planet needs to have between 0.007% and 0.027% of its mass in water for both continents and oceans to exist. They compared this to the possible range of water that planets can have, which varies between 0% and 3.8% or even between 0% and 55%, depending on how they formed.

For plate tectonics, the researchers used data indicating that only about 33% of planets have the necessary chemical composition to form dense tectonic plates. Of those, only about half are large enough and have sufficient gravity to support plate tectonics.

By including these new factors, the researchers estimate that the chance of a planet having both continents and oceans, along with long-term plate tectonics, is less than 0.2%. This means that, statistically, only about two suitable planets might be found out of every 1,000.

Plugging this value into the Drake Equation results in a sobering conclusion: Advanced civilizations are extremely rare. The chances of planets having the right conditions range between 0.0034% and 0.17%, translating to anywhere from as few as 0.006 to as many as 100,000 active, communicative civilizations in our galaxy. However, the actual number is likely on the lower end, considering the limited time these civilizations might communicate due to potential societal collapse or extinction.

By Impact Lab