Facebook
Twitter
Pocket
Reddit
Email
The question of whether humanity is alone in the cosmos creates strange bedfellows. It attracts astronomers and abduction conspiracy theorists, pseudoarchaeology enthusiasts and physicists. And loads of science-fiction writers, of course, who have conjured extraterrestrials from Klaatu to Doctor Who. Douglas Adams imagined a galaxy so full of life that its interstellar travelers needed a hitchhiker’s guide.
Given the diversity of voices that have weighed in on the possibility that other civilizations may be out there, it is surprising that few geoscientists—people who study the one planet known to host life—have weighed in on the cosmic conundrum. Physicist Enrico Fermi’s famous question, “Where is everybody?” has long lacked a geological perspective.
Basic amenities we take for granted on Earth—continents, oceans, and plate tectonics—are cosmically rare.
That’s what Earth scientists Robert Stern and Taras Gerya offer in a recent paper published in Scientific Reports. Earlier speculations about extraterrestrial civilizations were based primarily on astronomical and technological considerations like the number of planetary systems in the galaxy and how long it might take an intelligent species to discover and begin using radio waves. That left little attention for the specific attributes of potential host planets—other than the presence or absence of water.
Stern is a geologist at the University of Texas at Dallas who studies the evolution of the continental crust, and Gerya is a geophysicist at the Swiss Federal Institute of Technology who models Earth’s internal processes. Their conclusion may disappoint extraterrestrial enthusiasts: The likelihood that other technologically sophisticated societies exist is smaller than previously thought, because basic amenities we take for granted on Earth—continents, oceans, and plate tectonics—are cosmically rare.
How can we estimate the number of alien civilizations that might exist? In the early 1960s, the radio astronomer Frank Drake conceived of an equation that many researchers still use to gauge how prevalent advanced extraterrestrial societies might be. Although the term “equation” suggests a certain level of precision, the Drake formulation is really no more than a crude, back-of-the-envelope guess at how many planets might conceivably host complex life—and have some possibility of communicating with us. Drake was optimistic about the possibility of interstellar communication: He later worked with Carl Sagan in developing the “Golden Record,” a gold-plated disc with information about Earth and human cultures that was launched in 1977 on a deep space journey aboard the Voyager I and II spacecrafts.
In the original equation, the estimated number of civilizations is a simple product of seven factors, or probabilities, multiplied together. Scientists have a fairly good handle on the first three of these for the Milky Way. There’s the rate of star formation in our galaxy; the fraction of those that have planets (probably most, given the burgeoning census of known exoplanets); and the average number of potentially habitable planets around such stars (based on how many would be sitting in the star’s “habitable zone,” the sweet spot where water can remain liquid).
The other four factors in the Drake equation grow successively more speculative. These are: the fraction of potentially habitable planets on which life likely has emerged (a variable that’s completely unconstrained, since only one case—ours—is known); the fraction of those planets on which intelligent life has developed (a criterion that often elicits dark humor about whether human life qualifies); the fraction of that fraction that have sent signals into deep space (again, just one known example, out-going calls only); and the length of time those civilizations have been sending such signals (to be determined).
Plate tectonics should be included as a criterion for planetary habitability.
The plus or minus values on the final product resulting from this formula are gigantic, and the “solution” to the Drake equation has been determined to be between 1,000 and 100,000,000 advanced civilizations in our galaxy (not 42, as Douglas Adams fans might have expected). While the size of this range is quite absurd, even the lower-end estimate of 1,000 suggests that we should have heard from someone by now. The fact that we haven’t encountered anyone is known as the “Fermi paradox.”
Bringing a geologic perspective to the problem, Stern and Gerya propose to resolve the paradox by adding two more factors to the already unwieldy Drake equation: the fraction of habitable planets with distinct continents and oceans; and the fraction of those planets with a plate tectonic system that has operated for at least 500 million years. The values of these terms are very small, they argue, because the development of distinct landmasses and water bodies, and the tectonic habit of crustal recycling—characteristics of Earth that we take for granted—are unlikely outcomes in the evolution of rocky planets.
With these new factors, the number of advanced civilizations in our galaxy that might communicate with us falls to … almost zero.
This seems plausible, given the grand sweep of Earth history: Although life had emerged and diversified by at least 3.5 billion years ago, it remained mainly unicellular until about 560 million years ago, when macroscopic marine organisms first appeared. It took another 100 million years before plants and animals began to move onto land, and a further 450 million for tool-making humans to show up. And we’ve only been transmitting signals for about 50 years.
Stern and Gerya assert that while “life must evolve in the sea, advanced communicative civilizations must evolve on dry land.” This is because landscapes are more varied than seascapes, and therefore foster more evolutionary innovation, giving rise to creatures with more sophisticated sensory organs. This generalization may be true, but like so many other astrobiological hypotheses, it is limited by the fact that we have only one planetary example.
There could be other paths to technologically advanced life. The enormous diversity of marine organisms here on Earth, both in the fossil record and the modern oceans, is a reminder that there has been plenty of evolutionary experimentation in the seas over time. Plus, some recent origin-of-life theories suggest that the first living cells popped up in land-based hot springs, not marine environments. Still, we are faced with our own facts: that the existence of distinct continents and oceans on Earth has engendered great biodiversity and that the one species to develop advanced technology is a landlubber.
The number of advanced civilizations in our galaxy falls to … almost zero.
Most geologists will agree with Stern’s and Gerya’s argument that plate tectonics should be included as a criterion for long-term planetary habitability. Earth’s tectonic system allows the planet’s atmosphere and hydrosphere to remain in communication with its interior, in a remarkable, self-perpetuating cycle. Subducted ocean crust—seafloor that slips down into Earth’s interior—carries water back into the mantle, and at shallow depths, this water lowers the melting temperature of mantle rock, giving rise to unusual magmas that create the continental crust—what we surface dwellers live on—which is rich in rare elements, like phosphorus, that are critical to life.
At greater depths, subducted water acts to decrease the viscosity of the mantle, allowing it to churn, or convect, more vigorously—which in turn drives plate motion. When the Earth’s mantle exports heat via convection, it encourages the liquid iron outer core to convect as well, and this generates Earth’s protective magnetic field, which shields the surface environment from harmful cosmic radiation. Without plate tectonics, continents would quickly be eroded to sea level. But tectonic collisions continuously rejuvenate Earth’s topography, providing rivers with more energy to transport nutrient-rich sediments to shallow marine environments. In other words, plate tectonics is entangled with all the phenomena that support life on Earth.
A plate tectonic system like Earth’s requires a very specific thermal combination of a cool, crisp outer shell, broken into movable pieces, and a warm, gooey underlying mantle capable of flowing in the solid state. Exactly when Earth achieved this balance is unknown. Since Venus and Mars show no evidence of subduction, it is unlikely that the early Earth had a full-fledged plate tectonic system from the start.
Stern and Gerya are well known in the geological community for arguing for a comparatively late start to Earth’s plate tectonics—at the end of the Proterozoic Eon about 550 million years ago. In their view, diagnostic “plate tectonic indicators,” such as evidence for seafloor spreading and subduction, do not appear in the geologic record before that time, and they posit that until then Earth had a Mars-like “single-lid” tectonic system. They use their new paper to further advance this less common viewpoint, arguing that the emergence of multicellular life at that time was triggered by the beginning of modern-style tectonics, which would have accelerated delivery of critical nutrients like phosphorus to the oceans, leading to phytoplankton blooms that sequestered carbon and increased atmospheric oxygen levels.
But the mainstream consensus among geologists is that Earth had adopted its modern plate tectonic habits by at least 2.5 billion years ago—much earlier than Stern and Gerya contend. Starting around this time, ancient mountain belts (which survive as eroded remnants) began to have the same internal “architecture” as more recent ranges like the Rockies and Alps, and some of these belts preserve high-pressure metamorphic rocks diagnostic of subduction.
Regardless of when our planet’s plates fractured and began moving, Stern’s and Gerya’s paper makes a compelling case that planets anything like Earth are exceedingly scarce in the cosmos. That message should give pause to anyone who imagines that we might “terraform” another planet in a matter of a few human generations.
Does it really matter that the probability of receiving a communiqué from deep space has just become a little slimmer? It might turn out to be a good thing if it causes us to look down with new reverence at the beautiful, bountiful, mysterious planet right under our feet. 
Lead image: Triff / Shutterstock
Get the Nautilus newsletter
Cutting-edge science, unraveled by the very brightest living thinkers.
