Within the uncertainties of involved astronomical and biological parameters, the Drake equation typically predicts that there should be many exoplanets (< 100 to millions) in our Milky Way Galaxy hosting active, communicative civilizations. These optimistic calculations are however not supported by evidence, which is often referred to as the Fermi paradox. University of Texas at Dallas Professor Robert Stern and ETH-Zurich’s Professor Taras Gerya elaborate on this long-standing enigma by showing the importance of long-term plate tectonics as well as oceans and continents for the evolution of active, communicative civilizations.
Stern & Gerya propose that the lack of evidence for active, communicative civilizations reflects the scarcity of long-lived plate tectonics and/or continents and oceans on exoplanets with primitive life. Image credit: Sci.News.
In 1961, the American astrophysicist and astrobiologist Dr. Frank Drake devised an equation in which several factors are multiplied together to estimate the number of intelligent civilizations in our Galaxy capable of making their presence known to humans:
N = R * fp * ne * fl * fi * fc * L
N: the number of civilizations in the Milky Way Galaxy whose electromagnetic emissions (radio waves, etc.) are detectable;
R: the number of stars formed annually;
fp: the fraction of those stars with planetary systems;
ne: the number of planets per solar system with an environment suitable for life;
fl: the fraction of suitable planets on which life actually appears;
fi: the fraction of life-bearing planets on which intelligent life emerges;
fc: the fraction of civilizations that develop a technology that produces detectable signs of their existence;
L: the average length of time (years) such civilizations produce such signs.
Assigning values to the seven variables has been an educated guessing game, leading to predictions that such civilizations should be widespread. But if that is true, why is there no conclusive evidence of their existence?
This contradiction is known as the Fermi paradox, named for the Italian and later naturalized American nuclear physicist and Nobelist Dr. Enrico Fermi, who informally posed the question to colleagues.
“Life has been around on Earth for about 4 billion years, but complex organisms like animals didn’t appear until about 600 million years ago, which is not long after the modern episode of plate tectonics began,” Professor Stern said.
“Plate tectonics really jump-starts the evolution machine, and we think we understand why.”
In their paper, Professor Stern and Professor Gerya propose refining one of the Drake equation factors — fi, the fraction of life-bearing planets on which intelligent life emerges — to take into account the necessity of large oceans and continents and the existence of plate tectonics for more than 500 million years on those planets.
“In the original formulation, this factor was thought to be nearly 1, or 100% — that is, evolution on all planets with life would march forward and, with enough time, turn into an intelligent civilization. Our perspective is: That’s not true,” Professor Stern said.
The researchers propose a revision to the Drake equation that defines fi as the product of two terms:
foc: the fraction of habitable exoplanets with significant continents and oceans;
and fpt: the fraction of planets that have had long-lasting plate tectonics.
Based on the team’s analysis, the fraction of the exoplanets with optimal water volume is likely very small.
The authors estimate the value of foc ranges between 0.0002 and 0.01.
Similarly, they conclude that plate tectonics lasting more than 500 million years is also highly unusual, leading to an estimate of fpt at less than 0.17.
“When we multiply these factors together, we get a refined estimate of fi that is very small, between 0.003% and 0.2%, instead of 100%,” Professor Stern said.
“This explains the extreme rareness of favorable planetary conditions for the development of intelligent life in our Galaxy and resolves the Fermi paradox.”
“Biogeochemistry posits that the solid Earth, particularly plate tectonics, speeds up the evolution of species,” he added.
“Studies like ours are useful because they stimulate thinking broadly about larger mysteries and provide an example of how we can apply our knowledge of Earth systems to interesting questions about our Universe.”
The paper appeared in the April 2024 edition of the journal Scientific Reports.
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R.J. Stern & T.V. Gerya. 2024. The importance of continents, oceans and plate tectonics for the evolution of complex life: implications for finding extraterrestrial civilizations. Sci Rep 14, 8552; doi: 10.1038/s41598-024-54700-x
This article was adapted from an original release by the University of Texas at Dallas.
