Alien life capable of communication interstellar space it might not be able to evolve if its home planet doesn’t have plate tectonics, let alone the right amount of water and land.
Plate tectonics is absolutely necessary for complex life to evolve, say Robert Stern of the University of Texas at Dallas and Taras Geria of ETH Zurich in Switzerland. On The Earthcomplex multicellular life appeared during a period known as the Cambrian explosion, 539 million years ago.
“We believe that the advent of modern plate tectonics greatly accelerated the evolution of complex life and was one of the main causes of Cambrian explosion,” Gerja told Space.com.
Plate tectonics describes the process of continental plates rising on molten mantle sliding over each other, resulting in subduction zones and mountains, rift valleys and volcanoes, as well as earthquakes.
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The modern form of plate tectonics, Stern and Geria say, began only a billion and a half billion years ago, in a geologic era known as the Neoproterozoic. Previously, the Earth had what is known as stagnant lid tectonics: the earth’s crustcalled lithosphere, was one solid piece and was not broken into different plates. The change in modern plate tectonics occurs only after the lithosphere has cooled enough to become dense and strong enough to be subducted—that is, pushed beneath 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 environmental stresses that modern plate tectonics places on the biosphere could 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 all life existing in the oceans and on land associated with the continental plates. Given that push, life eventually—through no design or evolutionary imperative other than natural selection—eventually evolved into us, the idea goes.
“Long-term coexistence of oceans with land seems critical to obtain.” intelligent life and technological civilizations as a result of biological evolution,” said Gerja. “But the presence of 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. What’s more, the models show that plate tectonics may be rare, especially on a class of exoplanets known as super-Earths, where a stagnant lid configuration may dominate.
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 without any water. The Earthwith its relatively thin veneer of ocean water and topography that allows continents to rise above the oceans, it seems to occupy a sweet spot that is carefully balanced between the two extremes of deep ocean planets and dry desert worlds.
The presence of oceans is crucial 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 allowing combustion (along with oxygen) that can lead to technology when harnessed by intelligent life.
If planets with plate tectonics and the right amount of water and land are rare, then technological, communicative, extraterrestrial life may also be rare.
“What we tried to explain is, why didn’t they contact us?” said Gerya.
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To illustrate this, Geria 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, summarizing the various factors, necessary for the development of technological civilizations, leading to 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 bound on 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 (how to define “intelligence” in this context is still debated, but modern thinking includes all intelligent animals, such as chimpanzees and dolphins). Stern and Geria argue that fi must be the product of two more terms, specifically the fraction of planets with 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 have oceans and continents, Stern and Geria found that fi is a very small number. They estimate that only 17% of exoplanets have plate tectonics, and the proportion with the exact amount of water and land is probably even smaller, between 0.02% and 1%. Multiply these together and they give a value of fi between 0.003% and 0.2%.
Then, plugging this value into the Drake equation, Stern and Guerra arrive at a value for the number of extraterrestrial civilizations somewhere between 0.0004 and 20,000. This is still quite a large range, a result of the other terms in the Drake equation are not well known, if at all. However, it is still an order of magnitude less than the value of one million civilizations that Drake predicted in the 1960s.
“A value of 0.0004 means there could be only 4 civilizations in 10,000 galaxiesTaras said.
There are a few caveats to all of this. One is, as mentioned, that some of the other terms of the Drake equation, such as the fraction of planets that develop life in the first place, the fraction with intelligent life that develops 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 chances that more of them exist now will increase.
Another caveat is that while in general life as we know it needs plate tectonics, oceans and land to develop and thrive, it is possible to imagine scenarios where technological life inhabiting the ocean that never sets foot on the ground can evolve. However, these would be specific cases, exceptional cases that are an exception to the rule.
There’s also the risk of jumping the gun when we say they haven’t contacted us yet. SETI astronomer Jill Tarter likes to say that if the galaxy were an ocean, we’d search it in just one glass. While demand has accelerated recently thanks to ambitious Breakthrough Listen project, the point still stands. We haven’t searched for every star yet, and the ones we’ve searched for we haven’t heard or seen in a very long time. We could easily have missed an alien signal.
The final point to consider is that of “Great filter.” This is a concept first proposed by economist and futurist Robin Hanson, which suggests that there may be some universal bottleneck in the evolution of all life that prevents technological civilizations from existing. In Stern and Guerra’s model, this bottleneck place is provided by a lack of plate tectonics, oceans and continents However, although their estimate of the number of civilizations is low, it is non-zero and there is a school of thought that plays a role in Copernicus principle, which states that Earth should not be treated as special and is just another planet orbiting an ordinary star. Therefore, if life can develop on Earth, it should be able to develop on many planets, because Earth should not be special. The question then becomes, at what point does the Great Filter kick in?
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Perhaps Stern and Guerra jumped on the bandwagon by stating that planets with plate tectonics and the right amount of water and land were rare before we had observational evidence to support that claim.
“Of course, it would be ideal to have observational data on how common the continents, oceans and plate tectonics of exoplanets are,” Gerja said. “Unfortunately, this is well beyond our current observational capacity. On the other hand, the process of planet formation is somewhat understood, and models of planet formation are able to provide predictions of what we can expect. These predictions can be used to estimate the likelihood that rocky exoplanets have continents, oceans and plate tectonics.”
If Stern and Geria are right, then we could very well be alone The universe. If that is the case, we have a huge responsibility to shoulder. “We must take every possible care to preserve our own—very rare! — civilization,” Gerya said. Otherwise, we can kill ourselves and make the only technological life in our Milky Way galaxy extinct.
Stern and Geria’s analysis was published April 12 in the journal Scientific reports.
