Only 4 in 10,000 galaxies may host intelligent aliens

Extraterrestrial life capable of communicating through space interstellar space might not be able to develop if the home planet doesn’t have plate tectonics, and 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. Soilcomplex multicellular life emerged during a period known as the Cambrian explosion, 539 million years ago.

“We believe that the onset of modern plate tectonics greatly accelerated the evolution of complex life and was one of the major drivers of the Cambrian explosion“, Gerya told

Plate tectonics describes the process by which continental plates, which are pushed up on a molten mantle, slide over each other. This creates subduction zones and mountains, rifts and volcanoes, but also earthquakes.

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The modern form of plate tectonics, Stern and Gerya say, began only between a billion and half a billion years ago, in a geologic period known as the Neoproterozoic. Before that, the Earth had what are known as stagnant-lid tectonics: Earth’s crustcalled the lithospherewas one solid piece and had not broken into separate plates. The change to modern plate tectonics did not occur until the lithosphere had cooled enough to become dense and strong enough to be subducted—that is, to be pushed under other parts of the lithosphere by a significant amount time before flowing back to the Earth’s surface, where two tectonic plates are moving apart.

The environmental stress that modern plate tectonics imposes on the biosphere may have fueled the evolution of complex life a little 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 fueled the development of all sorts of life existing in the oceans and on the dry land of the continental plates. Given that kickstart, life ultimately — by no design or evolutionary imperative other than natural selection — evolved into us, the idea goes.

“The long-term coexistence of oceans and dry land appears crucial for obtaining intelligent life and technological civilizations as a result of biological evolution,” Gerya said. “But continents and oceans are not enough on their own, because the evolution of life is very slow. To speed it up, plate tectonics is needed.”

There is a problem, however. Earth is the only planet in the solar system with plate tectonics. Moreover, models indicate that plate tectonics may be rare, especially on a class of exoplanets known as super-Earths, where the stagnant lid configuration may dominate.

Coupled with the need for plate tectonics is the need for oceans and continents. Models of planet formation indicate that planets completely covered by oceans tens of kilometers deep could be common, as could desert worlds without water. SoilWith its relatively thin layer of ocean water and topography that allows continents to protrude above the oceans, it appears to strike a balance between the two extremes of deep ocean planets and dry desert planets.

Oceans are crucial because it is strongly suspected that life on Earth originated in the sea. Land is also crucial, 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 and the right amount of water and land are rare, then technological, communication, and extraterrestrial life are likely rare as well.

“What we’ve been trying to explain is, why have we not been contacted“?” said Gerya.

Related: Fermi Paradox: Where Are the Aliens?

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 summarizing the various factors necessary for the development of technological civilizations, resulting in an estimate of the number of extraterrestrial civilizations that might exist. It should be noted, however, that the Drake Equation is more of a thought experiment to highlight what we do and do not know about the evolution of technological life, rather than an absolute guide to the number of civilizations 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 evolve 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 more terms, specifically the fraction of planets with both continents and oceans (foc), and the fraction of planets with long-term 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 only 17% of exoplanets have plate tectonics, and the fraction with just the right amount of water and land is likely even smaller — between 0.02% and 1%. Multiply these together and they give 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 alien civilizations somewhere between 0.0004 and 20,000. That’s still a pretty large range, the result of the fact that the other terms in the Drake equation are not well known, if they are known at all. However, it’s still orders of magnitude less than the million civilizations value that Drake predicted in the 1960s.

“A value of 0.0004 means that there can only be 4 civilizations per 10,000 galaxies“, said Taras.

There are several caveats to all of this. One, as noted, is that some of the other terms of the Drake equation, such as the percentage of planets that develop life in the first place, the percentage with intelligent life that develops technology, and the lifespans of those 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 likelihood that there are more of them now will increase.

Another caveat is that while life as we know it generally requires 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 land, could evolve. However, these would be specific cases, outliers that are the exception to the rule.

There is also a risk of getting ahead of ourselves when you say we haven’t been contacted yet. SETI astronomer Jill Tarter likes to say that if the galaxy were an ocean, we would only have searched a cup of it. Although the search has recently accelerated thanks to the ambitious Breakthrough Listen project, the point remains. We have not searched every star, and the stars we have searched, we have not listened to or looked at for long. We could easily have missed an alien signal.

One final point to consider is that of the “Great filter.” This is a concept first proposed by economist and futurist Robin Hanson, who suggests that there could be a universal bottleneck in the evolution of all life that prevents technological civilizations from existing. In Stern and Gerya’s model, that bottleneck is caused by the lack of plate tectonics, oceans, and continents. However, while their estimate for the number of civilizations is low, it is not zero, and there is a line of thought that plays a role in the Copernican principlewhich states that Earth should not be treated as special and is just a planet orbiting a boring star. So if life can develop on Earth, it should be able to develop on many planets, because Earth should not be special. The question then is: at what point does the Great Filter kick in?

Related: Why Haven’t Aliens Contacted Earth? New Fermi Paradox Analysis Suggests We’re Not That Interesting Yet


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Perhaps Stern and Gerya were premature in claiming that planets with plate tectonics and just the right amount of water and land are rare, before we have the observational evidence to support that claim.

“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 far beyond our current observational capabilities. On the other hand, the planetary formation process is understood to some extent, and planetary formation models are able to make predictions about what to expect. Those predictions can be used to evaluate the likelihood of rocky exoplanets having continents, oceans, and plate tectonics.”

If Stern and Gerya are right, then we could very well be the only ones the universe. If that is the case, we have a huge responsibility on our shoulders. “We must take every possible care to preserve our own — very rare! — civilization,” Gerya said. Otherwise, we could wipe ourselves out and wipe out the only technological life in our galaxy.

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

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