Astronomer Frank Drake formulated his influential equation in 1961 to estimate the number of civilizations in the Milky Way capable of communicating with us. Our understanding of planetary science has changed a lot since then, leading a team of scientists to propose a pair of important adjustments that produce an answer that could explain the Great Silence.
Despite its popularity and intuitiveness, the Drake Equation has faced criticism over the years for its broad assumptions and ambiguous parameters; it often results in an overly optimistic estimate for the value of N—the number of civilizations in our galaxy with which we might be able to communicate. This tends to feed a conundrum known as the Fermi Paradox: If intelligent life is common, why haven’t we found any evidence of it? New research published in Scientific Reports offers a potential fix via the addition of two new factors.
Planetary scientists Robert Stern from the University of Texas at Dallas and Taras Gerya from ETH-Zurich, the two co-authors on the study, suggest that the presence of both continents and oceans, along with long-term plate tectonics, is critical for the emergence of advanced civilizations. They consequently propose the addition of two factors into 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, significantly reduces the value of N in the Drake Equation.
“Our work suggests that both our planet Earth with continents, oceans, plate tectonics, and life and our active, communicative, technological human civilization are extremely rare and unique in the entire galaxy,” Gerya told Gizmodo.
The factors of life
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, the fraction of planets with life where intelligent life evolves, and so on. The proposed tweak to the equation refines the estimates of how many planets can develop life and how many civilizations have detectable technologies by including new environmental, biological, and technological factors.
The researchers argue that the presence of large oceans, plus Earth’s shift from single-lid tectonics (a stable surface layer) to modern plate tectonics about 1 billion years ago, were critical to the rapid development of complex life. This geological activity not only created the initial conditions necessary for life to emerge but also led to diverse environments with varying climates and ecosystems, which promoted the evolution of advanced life forms capable of developing technology and complex societies.
According to the new study, plate tectonics are crucial for developing complex life and advanced civilizations. Earth’s plate movements create diverse habitats, recycle nutrients, and regulate climate—all vital for life. It’s important for plate tectonics to last for 500 million years, Gerya explained, because biological evolution of complex multicellular life is extremely slow. “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 develops from everyday needs, such as making tools, farming, creating clothing, and making weapons, the authors argue, adding that fire and electricity are “essential” for the development of intelligent civilizations. Complex civilizations, they write, are unlikely to emerge in strictly ocean-based environments.
According to Stern and Gerya, it’s likely quite rare for planets to have both continents and oceans along with long-term plate tectonics, and this possibility needs to be factored into the Drake Equation.
Plugging in the numbers
To figure out how likely it is for a planet to have both continents and oceans, Stern and Gerya looked at how much water is needed on the planet’s surface. They found that an Earth-size planet needs to have between 0.007% and 0.027% of its mass in water for both continents and oceans to exist. Stern and Gerya then compared this to the overall possible range of water that planets can have, which is between 0% and 3.8% or even between 0% and 55%, depending on how they formed. For plate tectonics, the scientists used data showing that only about 33% of planets have the right chemicals to form sufficiently dense tectonic plates needed for plate tectonics. Of those, only about half are big enough and have enough gravity to support plate tectonics.
By including these new factors and estimates, the researchers estimate that the chance of a planet having both continents and oceans and long-term plate tectonics is very small—less than 0.2%. To put that into perspective, it’s like finding just two suitable planets out of every 1,000.
Plugging this value into the Drake Equation produces a rather discouraging result, at least as far as the presence of advanced aliens is concerned. The modified Drake Equation suggests that advanced civilizations are extremely rare, with the chance of planets having the right conditions being between 0.0034% and 0.17%. This means there could be anywhere from as few as 0.006 to as many as 100,000 active, communicative civilizations in our galaxy, with the actual number likely being on the lower end, considering the limited time these civilizations might communicate due to potential societal collapse or extinction.
“On the other hand, the chances of finding planets potentially suitable for civilizations—yet without any civilizations or with already extinct civilizations—are notably higher,” Gerya explained. “This could be done by remote sensing of exoplanets.”
Gerya explained that, while the upper bound value of 100,000 seems large, it’s the low number that’s more important. Because the low estimate is really close to zero, it means there’s a good chance there might not be any other civilizations in our galaxy. This would help explain why we haven’t detected any signals from other civilizations yet.
In the past, the Drake Equation gave a much higher low-end estimate, suggesting that it was almost certain we weren’t alone and that there should be at least 200 civilizations trying to communicate with us. Since we haven’t found any, this old estimate seems wrong, Gerya said. The new, much lower estimate (close to zero) makes it more understandable why we haven’t heard from anyone else: There might simply be no one else out there to hear from—a rather spooky possibility.
Fermi Paradox solved?
The Fermi Paradox refers to a frustrating situation: We haven’t found evidence of extraterrestrial civilizations, despite the high likelihood that they exist. Stern and Gerya’s study offers a possible solution by looking at how rare the right geological conditions are for advanced life. They found that Earth’s switch to modern plate tectonics sped up the evolution of complex species. They suggest that advanced civilizations are scarce because planets with both continents, oceans, and long-lasting plate tectonics are rare.
Stern and Gerya aren’t the first to propose the idea that suitable planets for advanced life are few and far between. This suggestion, known as the Rare Earth Hypothesis, was first articulated in the 2003 book Rare Earth: Why Complex Life is Uncommon in the Universe, written by scientists Peter Ward and Donald Brownlee. Interestingly, Ward and Brownlee were likewise fixated on plate tectonics as a factor.
The new study marks an important update to the debate, but the conversation surrounding the Fermi Paradox is far from over. The Rare Earth Hypothesis, while seductive, fails to account for the adaptability of life and the potential diversity of habitable environments. What’s more, the Drake Equation in its current form, or when updated with the new factors, still fails to account for an unassailable reality: The Milky Way is incredibly ancient and has likely been capable of fostering life for up to 10 billion years. Even with those slim odds calculated by the researchers, intelligent life has surely emerged at some earlier points in the galaxy’s history, giving it ample time to spread out throughout the galaxy. Yet we see no evidence for this. It’s very possible that other factors are at play—factors that still need to be sussed out for revising the Drake Equation even further, possibly incorporating temporal aspects and other unknown variables.
Another limitation of this study, and this is no fault of the researchers, is that we’re still far from knowing which values to plug into the equation. We lack an understanding of planetary formation rates and the types of planets that can support habitability elsewhere in the galaxy. Until then, we’re kind of stuck in the water with the Drake Equation, but future observations, such as those from the Webb telescope, should help.