Alien life capable of communicating across interstellar space it may not be able to evolve unless its home planet has plate tectonics, not to mention the right amount of water and dry land.
Plate tectonics is absolutely necessary for complex life to evolve, say Robert Stern of the University of Texas at Dallas and Taras Gerya of ETH Zurich in Switzerland. On Earthcomplex multicellular life emerged during a period known as the Cambrian explosion 539 million years ago.
“We believe that the advent of modern-style plate tectonics greatly accelerated the evolution of complex life and was one of the main causes Cambrian explosionGerya told Space.com.
Plate tectonics describes the process of continental plates, which are floating on molten mantle, sliding 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, Stern and Gerya say, began only a billion to half a billion years ago, in the geologic era known as the Neoproterozoic. Before that, Earth had what is known as stagnant lid tectonics: the earth’s crustcalled lithosphere, was one solid piece and not broken into different plates. The change to present-day plate tectonics only occurred after the lithosphere had cooled enough to be dense and thick enough to be subducted—that is, to be pushed beneath other parts of the lithosphere for significant amounts time before recycling back to the surface where the two tectonic plates are moving apart.
The environmental stress that current plate tectonics places on the biosphere may have spurred the evolution of complex life just over half a billion years ago, as life suddenly found itself in an environment where it was forced to adapt or die, creating an evolutionary pressure that pushed all species to develop life that existed in the oceans and on land associated with continental plates. Given this boot, life eventually—through no design or evolutionary imperative other than natural selection—eventually evolved into us, the thinking goes.
“The long-term coexistence of oceans with dry land appears to be critical for acquisition intelligent life and technological civilizations as a result of biological evolution,” Gerya said. “But having continents and oceans alone is not enough because the evolution of life is very slow. It takes plate tectonics to accelerate it.”
However, there is a problem. Earth is the only planet in the solar system that has plate tectonics. What’s more, the models suggest that plate tectonics could be rare, especially on a class of exoplanets known as super-Earths, where stagnant lid configurations could dominate.
The need for oceans and continents is also related to the need for plate tectonics. Models of planet formation suggest that planets covered entirely by oceans tens of miles deep could be common, as well as desert worlds without water. Earthwith its relatively thin layer of ocean water and topography that allows the continents to rise above the oceans, it seems to occupy a sweet spot carefully balanced between the two extremes of deep-ocean planets and dry desert worlds.
Having oceans is essential because there is a strong suspicion that life on Earth began in the sea. Soil is also critical, not only for providing nutrients through 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 be rare as well.
“What we tried to explain why haven’t we been contactedGerya said.
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To illustrate this, Gerya and Stern used the Drake equation. Designed in 1961 by the late SETI pioneer Frank Drake, it was intended to provide the agenda for the first ever SETI (Search for Extraterrestrial Intelligence) science conference, held that year at Green Bank Observatory in West Virginia, by summarizing the various factors necessary for the development of technological civilizations, leading to an estimate of the number of extraterrestrial civilizations 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 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 term in the Drake 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 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-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 the right amount of water and land is probably even smaller – between 0.02% and 1%. Multiply them together and you get a value of fi between 0.003% and 0.2%.
Then, plugging that value into the Drake equation, Stern and Gerya arrive at a value for the number of alien civilizations somewhere between 0.0004 and 20,000. That’s still quite a large range, a result of the other terms in the Drake equation not being well known, if at all . However, this is still an order of magnitude less than the value of a million civilizations predicted by Drake in the 1960s.
“A value of 0.0004 means there can only be 4 civilizations per 10,000 population. galaxiesTaras said.
There are a few caveats to all of this. One is, as already mentioned, that some of the other terms of the Drake equation, such as the fraction of planets that develop life, 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 chance that there are now more 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, ocean life which never set foot on land could evolve. However, these would be specific cases, outliers that are an exception to the rule.
There’s also the risk of jumping the gun if you say we haven’t been contacted yet. SETI astronomer Jill Tarter likes to say that if the galaxy were an ocean, we’d search it for just a cup. While the search has recently accelerated thanks to ambitious Breakthrough Listen project, the point still stands. We haven’t searched every star yet, and the ones we have, we haven’t listened to or watched for very long. We could easily have missed the alien signal.
A final point to consider is “Great filterThis is a concept first proposed by economist and futurist Robin Hanson, which suggests that there might be some universal bottleneck in the evolution of all life that prevents technological civilizations from existing. In Stern and Gery’s model, this bottleneck is provided by the lack of plate tectonics, oceans and continents However, despite their estimate of the number of civilizations being low, it is non-zero and there is a school of thought that plays a role. Copernicus principle, which says that Earth shouldn’t be considered special and is just another planet orbiting a boring star. So if life can evolve on Earth, it should be able to evolve on many planets, because Earth shouldn’t be special. The question then becomes: At what point does the Great Filter kick in?
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Perhaps Stern and Gerya jumped the gun and declared that planets with plate tectonics and the right amount of water and land are rare before we have observational evidence to support this claim.
“Of course, it would be ideal to have observational data on how continents, oceans and plate tectonics are common on exoplanets,” Gerya said. “Unfortunately, this is far beyond our current observational capabilities. On the other hand, the process of planet formation is somewhat understood, and models of planet formation are able to make predictions about what we can expect. These predictions can be used to evaluate the likelihood that rocky exoplanets have continents , oceans and plate tectonics.”
If Stern and Gerya are right, then we could be pretty much on our own universe. If that is the case, we have a huge responsibility on our shoulders. “We should take the utmost care to preserve our own—very rare!—civilization,” said Gerya. Otherwise, we could kill ourselves and destroy the only technological life in our Milky Way galaxy.
Stern and Gerya’s analysis was published April 12 in the journal Scientific reports.