Alien life capable of communication interstellar space it may not be able to evolve if its home planet doesn’t have plate tectonics, not to mention just the right amount of water and dry land.
Plate tectonics is 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. ACTIvE earthcomplex multicellular life appeared during a period known as the Cambrian explosion, 539 million years ago.
“We believe that the onset of modern-day plate tectonics greatly accelerated the evolution of complex life and was one of the main causes of Cambrian explosion“, Gerya told Space.com.
Plate tectonics describes the process of continental plates, which rise up in a molten mantle, slide over each other, leading to subduction zones and mountains, rift valleys and volcanoes, as well as earthquakes.
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The modern form of plate tectonics, say Stern and Gerya, began only a billion and a half billion years ago, in a geologic era known as the Neoproterozoic. Prior to this, the Earth had what is known as stagnant lid tectonics: the earth’s crustnamed lithosphere, it was a solid piece and had not broken into different plates. The change in modern plate tectonics occurred only after the lithosphere had cooled enough to become dense and strong enough to be able to subduct—that is, push under other parts of the lithosphere by a significant amount time before being recycled back to the surface where two tectonic plates are moving apart.
The environment states that modern plate tectonic sites in the biosphere may have spurred the evolution of complex life just over half a billion years ago, as life suddenly found itself living in an environment where it was forced to adapt or die, creating an evolutionary pressure that pushed the development of every kind of life that existed in the oceans and on the dry land connected to the continental plates. Given that beginning, life eventually—through no design or evolutionary imperative other than natural selection—ended up evolving into us, the idea goes.
“The long-term coexistence of oceans with dry land seems critical to achieve intelligent life and technological civilizations as a result of biological evolution,” said Gerya. “But having continents and oceans is not enough by itself, because the evolution of life is very slow. To accelerate it, plate tectonics is needed.”
However, there is a problem. Earth is the only planet in the solar system that has plate tectonics. Additionally, the models show that plate tectonics may be rare, especially in a class of exoplanets known as super-Earths, where the stagnant lid configuration may predominate.
Coupled with the need for plate tectonics is the need for oceans and continents. Models of planet formation show that planets covered entirely in oceans tens of miles deep may be common, as well as desert worlds with no water at all. earthwith a relatively thin veneer of ocean water and topography that allows the continents to rise above the oceans, it appears to occupy a sweet spot that is carefully balanced between the two extremes of deep ocean planets and dry desert worlds.
Having oceans is essential because it is strongly suspected that life on Earth began in the sea. Earth is also critical, not only for providing nutrients through weathering and facilitating the carbon cycle, but also for enabling combustion (in conjunction 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.
“What we have tried to explain is, Why haven’t we been contacted?Gerya said.
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To illustrate this, Gerya and Stern used the Drake equation. Created in 1961 by the late SETI pioneer Frank Drake, it was intended to provide an agenda for the first SETI (Search for Extraterrestrial Intelligence) science conference, held that year at the Green Bank Observatory in West Virginia, by summarized various factors necessary for the development of technological civilizations, resulting in an estimate of the number of extraterrestrial civilizations that may 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 than an absolute guide to the number of civilizations out there.
“Previous estimates for the lower limit of the number of civilizations in our galaxy were quite high,” Gerya said.
One of the terms in Drake’s equation is fi, the fraction of exoplanets that develop intelligent life (how we define “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 must be the product of two other terms, specifically the fraction of planets with continents and oceans (focus), and the fraction of planets with long-term plate tectonics (fpt).
However, given the apparent rarity of plate tectonics, and worlds that may have oceans and continents, Stern and Gerya find that fi is a very small number. They estimate that only 17% of exoplanets have plate tectonics, and the proportion with the right amount of water and land is likely even smaller – between 0.02% and 1%. Multiply these together and they give a fi value of between 0.003% and 0.2%.
Then, plugging this value into Drake’s equation, Stern and Gerya arrive at a value for the number of extraterrestrial civilizations somewhere between 0.0004 and 20,000. This is still quite a large range, the outcome of the other terms in the Drake equation is not well known, if at all. However, it is still orders of magnitude less than the value of a million civilizations that Drake predicted in the 1960s.
“A value of 0.0004 means that there could be only 4 civilizations per 10,000 GALAXIES“, Taras said.
There are some caveats to all of this. One is, as mentioned, that some of the other terms in 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 longevity of those civilizations are completely unknown. If their values turn out to be unusually high—for example, if civilizations typically survive for billions of years—then the chances that more of them are now around will increase.
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 sets foot on the ground can evolve. However, these would be specific, outlier cases that are the exception to the rule.
There is also a risk of jumping the gun when it is said that we have not yet been contacted. SETI astronomer Jill Tarter likes to say that if the galaxy were an ocean, we’d only be asking for a cup. While research has been accelerated recently thanks to the ambitious Progress Listen project, the issue still stands. We haven’t searched for every star yet, and those we have, we haven’t heard or seen for a long time. We could have easily missed an extraterrestrial signal.
A final point to consider is that of “Great 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 obstacle is provided by the lack of plate tectonics, oceans and continents. However, despite their estimate of the number of low civilizations, it is not zero, and there is a school of thought that plays their role in this field. Copernicus principle, which says that Earth should not be treated as special and is just another planet orbiting a noisy 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 in?
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Perhaps Stern and Gerya have jumped the gun, stating that planets with plate tectonics and the right amount of water and land are rare, before we have observational evidence to support this statement.
“Of course, it would be ideal to have observational data on how common continents, oceans and plate tectonics are on exoplanets,” Gerya said. “Unfortunately, this is well beyond our current observational capacities. On the other hand, the process of planet formation is understood to some extent, and models of planet formation are able to make predictions about what we can expect. These predictions can be used to estimate the probability of rocky exoplanets having continents, oceans, and plate tectonics.”
If Stern and Gerya are right, then we may effectively be on our own UNIVERSE. If so, we have a great responsibility on our shoulders. “We must take every possible precaution to preserve our civilization – very rare! -,” said Gerya. Otherwise, we could kill ourselves and wipe out the only technological life in our Milky Way galaxy.
Stern and Gerya’s analysis was published April 12 in the journal Scientific Reports.