Extraterrestrial life capable of communicating through interstellar space might not be able to evolve if its home planet lacks plate tectonics, let alone the right amount of water and dry land.
Plate tectonics is absolutely essential to the evolution of complex life, say Robert Stern of the University of Texas at Dallas and Taras Gerya of ETH Zurich in Switzerland. EarthComplex multicellular life appeared during a period known as the Cambrian explosion, 539 million years ago.
“We believe that the emergence of modern-style plate tectonics dramatically accelerated the evolution of complex life and was a major cause of the Cambrian explosion“, Gerya told Space.com.
Plate tectonics describes the process by which continental plates, supported by molten mantle, slide past each other, creating subduction zones and mountains, rift valleys and volcanoes, and earthquakes.
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The current form of plate tectonics, according to Stern and Gerya, only began between a billion and half a billion years ago, in a geological epoch called the Neoproterozoic. Before that, the Earth had what is called stagnant tectonics: Earth’s crustcalled the lithospherewas a single solid piece and was not split into different plates. The change to modern plate tectonics only occurred once the lithosphere had cooled enough to become dense and solid enough to be able to be subducted – that is, to be pushed under other parts of the lithosphere for a significant amount of time. time before being recycled to the surface where two tectonic plates are moving apart.
The environmental stresses that modern plate tectonics place on the biosphere may have triggered the evolution of complex life just over half a billion years ago, when life suddenly found itself in an environment where it was forced to adapt or die, creating an evolutionary pressure that pushed all the life forms that existed in the oceans and arid lands associated with continental plates to evolve. Given this nudge, life eventually evolved toward us—without any evolutionary plan or imperative other than natural selection—the idea goes.
“The sustainable coexistence of the oceans with the land seems essential to achieve smart life “Modern, technological civilizations are the result of biological evolution,” Gerya said. “But the presence of continents and oceans alone is not enough, because the evolution of life is very slow. To accelerate it, you need plate tectonics.”
There is a problem, however. Earth is the only planet in the solar system that has plate tectonics. Moreover, models indicate that plate tectonics may be rare, particularly on a class of exoplanets called super-Earths, where the stagnant-lid configuration may dominate.
Plate tectonics is also essential for the formation of oceans and continents. Models of planet formation indicate that planets entirely covered by oceans tens of kilometers deep could be common, as could desert worlds devoid of water. Earthwith its relatively thin layer of ocean water and topography that allows the continents to rise above the oceans, seems to occupy a carefully balanced sweet spot between the two extremes of deep-ocean planets and dry desert worlds.
The presence of oceans is crucial, as it is strongly suspected that life on Earth began in the sea. Land is also essential, not only to provide nutrients through weathering and facilitate the carbon cycle, but also to enable combustion (in concert with oxygen) which can lead to technology when harnessed by intelligent life.
If planets with tectonic plates, as well as adequate amounts of water and land, are rare, then technological, communicative, and extraterrestrial life may also be rare.
“What we have tried to explain is, why haven’t we been contactedsaid Geria.
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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 that year at the Green Bank Observatory in West Virginia, by summarising the various factors necessary for the development of technological civilisations, which in turn provided an estimate of the number of extraterrestrial civilisations that might exist. However, it should be noted that the Drake equation is more of a thought experiment to highlight what we know and don’t know about the evolution of technological life, rather than an absolute guide to the number of civilisations that exist.
“Previous estimates of the lower limit of the number of civilizations in our galaxy were quite high,” Gerya said.
One of the terms in the Drake equation is fi, the fraction of exoplanets that develop intelligent life (the definition of “intelligence” in this context is still debated, but modern thinking includes all intelligent animals, such as chimpanzees and dolphins). Stern and Gerya argue that fi should be the product of two other terms, namely the fraction of planets with both continents and oceans (foc), and the fraction of planets with long-lived plate tectonics (fpt).
However, given the apparent rarity of plate tectonics and worlds that can contain oceans and continents, Stern and Gerya estimate that fi is a very small number. They estimate that only 17% of exoplanets have plate tectonics, and that the proportion of planets with just the right amount of water and land is probably even lower, between 0.02% and 1%. Multiply these values together and they get a value for fi between 0.003% and 0.2%.
Plugging this value into the Drake equation, Stern and Gerya arrive at a value for the number of extraterrestrial civilizations between 0.0004 and 20,000. This is still a fairly wide range, resulting from the fact that the other terms in the Drake equation are not well known, if at all. However, it is still orders of magnitude lower than the value of a million civilizations that Drake predicted in the 1960s.
“A value of 0.0004 means that there could be as few as 4 civilizations per 10,000 galaxies“, Taras said.
There are several caveats to this. One is that some of the other terms in the Drake equation, such as the proportion of planets that have given rise to life, the proportion of intelligent life that has developed technology, and the lifespan of these civilizations, are completely unknown. If their values turn out to be extremely high—for example, if civilizations typically survive for billions of years—then the odds that there are more of them today will increase.
Another caveat is that while, in general, life as we know it requires plate tectonics, oceans and land to evolve and thrive, it is possible to imagine scenarios where technological and oceanic life who never sets foot on the ground could evolve. However, these would be special cases, aberrant cases that would be the exception to the rule.
There is also a risk of jumping to the conclusion that we have not yet been contacted. SETI astronomer Jill Tarter likes to say that if the galaxy were an ocean, we would only be looking for a cup. Although the search has accelerated recently thanks to the ambitious project Listen to Breakthrough Still, the problem remains. We haven’t explored all the stars yet, and those we have explored, we haven’t listened to them or observed them for very long. We could easily have missed an alien signal.
A final point to consider is that of the “Excellent filterThis is a concept first proposed by economist and futurist Robin Hanson, who suggests that there may be a universal bottleneck in the evolution of all life that prevents technological civilizations from existing. In Stern and Gerya’s model, this bottleneck is provided by the absence of plate tectonics, oceans, and continents. However, while their estimate of the number of civilizations is low, it is not zero, and there is a school of thought that plays a role in this hypothesis. Copernican principlewhich states that Earth should not be considered special and is just another planet orbiting a mundane star. Therefore, if life can evolve on Earth, it should be able to evolve on many planets, because Earth should not be special. The question then becomes: at what point does the Great Filter come into play?
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Perhaps Stern and Gerya exaggerated when they stated that planets with plate tectonics and an ideal amount of water and land are rare, even before we have observational evidence to support this claim.
“Of course, it would be ideal to have observational data on the frequency of continents, oceans, and plate tectonics on exoplanets,” Gerya said. “Unfortunately, this is far beyond our current observational capabilities. On the other hand, the process of planet formation is to some extent understood, and planet formation models are able to provide predictions of what we can expect. These predictions can be used to assess the likelihood of rocky exoplanets having continents, oceans, and plate tectonics.”
If Stern and Gerya are right, then we may very well be alone in the universeIf so, we have a huge responsibility to shoulder. “We must take every precaution possible to preserve our own civilization – very rare!” Gerya said. Otherwise, we could kill ourselves and wipe out the only technological life in our galaxy, the Milky Way.
Stern and Gerya’s analysis was published April 12 in the journal Scientific reports.