The Allen Telescope Array in Northern California is dedicated to astronomical observations and a simultaneous search for extraterrestrial intelligence (SETI).
Alien life capable of communicating across interstellar space might not be able to evolve if its home planet doesn’t possess plate tectonics, not to mention just the right amount of water and dry land. Plate tectonics are absolutely essential if complex life is to evolve, argue Robert Stern of the University of Texas at Dallas and Taras Gerya of ETH Zurich in Switzerland. On Earth, complex multicellular life appeared during a period known as the Cambrian explosion, 539 million years ago.
“We believe that the onset of modern-day-style plate tectonics greatly accelerated the evolution of complex life and was one of the major causes of the Cambrian explosion,” Gerya told Space.com. Plate tectonics describe the process of continental plates, which are buoyed up on a molten mantle, sliding over one another, leading to subduction zones and mountains, rift valleys and volcanoes, as well as earthquakes.
The modern-day form of plate tectonics, say Stern and Gerya, only began between a billion and half a billion years ago, in a geological era known as the Neoproterozoic. Prior to that, Earth had what’s known as stagnant lid tectonics: Earth’s crust, called the lithosphere, was one solid piece and wasn’t broken into different plates. The change to modern-day plate tectonics only happened once the lithosphere had cooled enough to grow sufficiently dense and strong to be capable of being subducted.
“The long-lasting coexistence of oceans with dry land seems critical for obtaining intelligent life and technological civilizations as the result of biological evolution,” said Gerya. “But having continents and oceans is not sufficient on their own, because life’s evolution is very slow. In order to accelerate it, plate tectonics is needed.”
However, Earth is the only planet in the solar system to have plate tectonics. What’s more, models indicate that plate tectonics could be rare, especially on a class of exoplanets known as super-Earths, where the stagnant lid configuration could dominate.
Coupled with the need for plate tectonics is the need for oceans and continents. Earth, with its relatively thin veneer of ocean water and topography that allows continents to rise above the oceans, seems to occupy a sweet spot that is carefully balanced between the two extremes of deep ocean planets and dry desert worlds.
Having oceans is crucial because it is strongly suspected that life on Earth began in the sea. Land is also critical, not only for providing nutrients via weathering and facilitating the carbon cycle, but also for enabling combustion (in concert with oxygen) that can lead to technology when harnessed by intelligent life.
If planets with plate tectonics, as well as the right amount of water and land, are rare, then technological, communicative, alien life may also be rare.
“To illustrate this, Gerya and Stern used the Drake equation. Devised in 1961 by the late SETI pioneer Frank Drake, it was intended to provide an agenda for the first-ever SETI (search for extraterrestrial intelligence) scientific conference, held in that year at the Green Bank Observatory in West Virginia, by summarizing the various factors required for the development of technological civilizations, resulting in an estimate of the number of extraterrestrial civilizations that might exist.
One of the terms of the Drake equation is fi, the fraction of exoplanets that develop intelligent life. Stern and Gerya argue that fi should be the product of two more terms, specifically the fraction of planets with both continents and oceans (foc), and the fraction of planets with long-lasting plate tectonics (fpt).
However, given the apparent rarity of plate tectonics, and worlds that can have oceans and continents, Stern and Gerya find that fi is a very small number. They estimate that just 17% of exoplanets have plate tectonics, and the proportion with just the right amount of water and land is likely even smaller — between 0.02% and 1%. Multiply these together and they give a value of fi as between 0.003% and 0.2%.
Then, by plugging this value into the Drake equation, Stern and Gerya arrive at a value for the number of extraterrestrial civilizations as somewhere between 0.0004 and 20,000.
There are several caveats to consider. One is that some of the other terms of the Drake equation such as the fraction of planets that evolve life in the first place, the fraction with intelligent life that develops technology and the lifetime of those civilizations are completely unknown. Another caveat is that while, in general, life as we know it needs plate tectonics, oceans, and land to evolve and thrive, it is possible to imagine scenarios where technological, ocean-dwelling life that never steps foot on land could evolve.
There’s also a risk of jumping the gun when saying that we haven’t been contacted yet. We’ve not searched every star yet, and those that we have searched, we have not listened to or watched for very long. We could easily have missed an extraterrestrial signal.
If Stern and Gerya are correct, then we could very well be effectively alone in the universe. If that’s the case, we have an enormous responsibility to shoulder.
Stern and Gerya’s analysis was published on April 12 in the journal Scientific Reports.
