Only 4 out of 10,000 galaxies may host intelligent aliens

Alien life able to communicate across interstellar space he might not be able to evolve if his home planet doesn’t have plate tectonics, let alone the right amount of water and dry land.

Plate tectonics is essential if complex life is to evolve, argued Robert Stern of the University of Texas at Dallas and Taras Gerya of ETH Zurich in Switzerland. Ar Worldcomplex multicellular life appeared during a period known as the Cambrian explosion, 539 million years ago.

“We believe that the onset of modern-style plate tectonics contributed significantly to the evolution of complex life and was one of the main causes of the Cambrian explosion,” Gerya told

Plate tectonics describes the process of continental plates, mounted on molten mantle, sliding over each other, resulting in subduction zones and mountains, rift valleys and volcanoes, as well as earthquakes.

Related: The search for alien life (reference)

Today’s form of plate tectonics, Stern and Gerya say, began only between a billion and a half billion years ago, in a geological era known as the Neoproterozoic. Before that, the Earth had something called stagnant lid tectonics: the Earth’s crustcalled the lithosphere, it was one solid piece and was not broken into different plates. The transition to today’s plate tectonics only occurred when the lithosphere cooled enough to grow dense and strong enough to be subducted — that is, pushed under other parts of the lithosphere for a significant amount of it time before being recycled back to the surface where two tectonic plates are moving apart.

The environment emphasizes that the location of modern plate tectonics on the biosphere may have triggered the evolution of complex life a little over half a billion years ago, because life was suddenly living in an environment where it was forced he had to adapt or die, which created that change. an evolutionary pressure that pushed the development of all forms of life in the oceans and on the dry land associated with the continental plates. Since kickstart, life eventually – without any design or any evolutionary necessity other than natural selection – evolved into us, the thinking goes.

“The long-term coexistence of oceans with dry land appears to be key to its acquisition intelligent life and technological civilizations are the result of biological evolution,” said Gerya. “But the existence of continents and oceans alone is not enough, 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. In addition, models suggest that plate tectonics may be rare, especially on a class of exoplanets known as super-Earths, where a stagnant mantle configuration may dominate.

Along with the need for plate tectonics is the need for oceans and continents. Models of planetary formation show that planets completely covered in oceans thousands of miles deep could be common, as could desert worlds with no water at all. Worldwith a relatively thin veneer of ocean water and topography that allows continents to rise above the oceans, it seems to occupy a carefully balanced sweet spot between the two extremes of a deep ocean planet and a dry desert world.

Having the oceans is essential because there is a strong suspicion that life on earth began in the sea. Land is also vital, 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 tried to explain is, why were we not contacted?” said Gerya.

Related: Fermi Paradox: Where are the aliens?

To demonstrate this, Gerya and Stern used the Drake equation. Conceived in 1961 by the late SETI pioneer Frank Drake, it was intended to provide an agenda for the first SETI (search for extraterrestrial intelligence) scientific conference held that year at the Green Bank Observatory in West Virginia, by summarizing to do the different areas. factors needed to develop technological civilizations, leading to an estimate of the number of possible extraterrestrial civilizations. It should be noted, however, 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 a complete guide to the number of civilizations that exist.

“The previous estimates for the lower limit of the number of civilizations in our galaxy were quite high,” said Gerya.

One of the terms of 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 chimps and dolphins ). Stern and Gerya argue that fi should be the result of two other terms, specifically the fraction of planets with both continents and oceans (focus), and the fraction of planets with long-term plate tectonics (fpt ).

However, given its apparent lack of plate tectonics, and the possible existence of oceans and continents, Stern and Gerya see fi as a very small number. They estimate that only 17% of exoplanets have plate tectonics, and the proportion with the correct amount of water and land is probably even smaller – between 0.02% and 1%. Add these together and they give a fi value of between 0.003% and 0.2%.

Then, by inserting this value into the Drake equation, Stern and Gerya arrive at a value for the number of extraterrestrial civilizations as somewhere between 0.0004 and 20,000. That’s still quite a range, the result of the other terms in the Drake equation not being well known, if at all. However, it is still orders of magnitude less than the value of the million civilizations predicted by Drake in the 1960s.

“A value of 0.0004 means that there could be as few as 4 civilizations per 10,000 galaxies,” said Taras.

There are some caveats to all of this. One is that, as mentioned, some 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 life span of those civilizations completely unknown. If their values ​​happen to be extremely high – for example, if civilizations typically last for billions of years – the chances of more of them being around now will increase.

Another observation is that, in general, while life as we know it needs plate tectonics, the oceans and the land to evolve and thrive, it is possible to imagine situations where technological, ocean dwelling life that footsteps on earth could not emerge. However, these would be specific cases, outliers that are the exception to the rule.

There is also the risk of jumping the gun when it is said that we have not been contacted yet. SETI astronomer Jill Tarter likes to say that if the galaxy were an ocean, we’d only be searching for a cup’s worth. Although the search has recently accelerated thanks to the ambition Interrupted Listening project, the point remains. We haven’t searched for all the stars yet, and those that we have, we haven’t listened to or looked at for a long time. We could easily have missed an extraterrestrial signal.

A final point to consider is the “Great Filter“This is a concept first proposed by the economist and futurist Robin Hanson, which suggests that there may be a universal barrier in the evolution of all life that prevents technological civilizations from existing. lack 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 relates to life. Copernican principle, who says that Earth should not be treated as something special and is just another planet orbiting a humdrum 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 begin?

Related: Why haven’t aliens contacted Earth? A new analysis of the Fermi Paradox suggests we’re not so interesting yet


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Stern and Gerya may have jumped the gun, asserting that planets with plate tectonics are rare and have just the right amount of water and land, before we have the observational evidence to support that statement.

“Of course, it would be great 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 capabilities. On the other hand, the process of planetary formation is understood to some extent, and planetary formation models are able to deliver predictions about what we may be experiencing expected. Those predictions can be used to evaluate the likelihood of rocky exoplanets having continents, oceans and plate tectonics.”

If Stern and Gerya are right, we really could be on our own the universe. If that is the case, we have a huge responsibility. “We should take every care to preserve our own civilization – very rare! -,” said Gerya. Otherwise, we could kill ourselves and extinguish the only technological life in our Milky Way galaxy.

Stern and Gerya’s analysis was published on April 12 in the journal Scientific Reports.

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