Scientists revise famous Drake equation

Taking into account the uncertainties of the astronomical and biological parameters involved, the Drake equation typically predicts that there should be many exoplanets (< 100 to millions) in our Milky Way galaxy that host active, communicative civilizations. However, these optimistic calculations are not supported by evidence, which is often referred to as the Fermi Paradox. Professor Robert Stern of the University of Texas at Dallas and Professor Taras Gerya of ETH Zurich delve deeper into this long-standing puzzle by showing the importance of long-term plate tectonics, as well as oceans and continents, in the development of active, communicative civilizations.

Stern and Gerya suggest that the lack of evidence for active, communicative civilizations reflects the rarity of long-lived plate tectonics and/or continents and oceans on exoplanets with primitive life. Image credit: Sci.News.

In 1961, American astrophysicist and astrobiologist Dr. Frank Drake developed an equation that multiplies several factors to estimate the number of intelligent civilizations in our galaxy capable of making their presence known to humans:

N = R * f_P * N_t * F_m * F_I * F_C * M

N: the number of civilizations in the Milky Way whose electromagnetic emissions (radio waves, etc.) are detectable;

R: the number of stars born annually;

F_P: the proportion of stars with planetary systems;

N_t: the number of planets per solar system with an environment suitable for life;

F_m: the proportion of suitable planets on which life actually occurs;

F_I: the proportion of life-bearing planets on which intelligent life emerges;

F_C: the proportion of civilizations that develop a technology that produces detectable signs of their existence;

L: the average time (years) in which such civilizations produce such signs.

Assigning values ​​to the seven variables was a guessing game and led to the assumption that such civilizations must be widespread. But if that’s true, why is there no conclusive evidence of their existence?

This contradiction is known as the Fermi Paradox, named after the Italian and later naturalized American nuclear physicist and Nobel Prize winner Dr. Enrico Fermi, who informally posed this question to his colleagues.

“Life has existed on Earth for about 4 billion years, but complex organisms such as animals did not appear until about 600 million years ago, not long after the beginning of modern plate tectonics,” said Professor Stern.

“Plate tectonics really gets the evolutionary machinery going and we think we understand why.”

In their article, Professor Stern and Professor Gerya propose to refine one of the factors of the Drake equation – f_Ithe proportion of life-bearing planets on which intelligent life arises – to take into account the need for large oceans and continents and the existence of plate tectonics for more than 500 million years on these planets.

“The original formulation assumed that this factor was close to 1 or 100% – that is, evolution on all planets with life would proceed and, given enough time, lead to an intelligent civilisation. In our view, this is not true,” said Professor Stern.

The researchers propose a revision of the Drake equation, which defines f_I as a product of two terms:

F_oc: the proportion of habitable exoplanets with significant continents and oceans;

and f_pt: the proportion of planets on which there was long-lasting plate tectonics.

Based on the team’s analysis, the fraction of exoplanets with optimal water volume is likely very small.

The authors estimate the value of f_oc is between 0.0002 and 0.01.

They also conclude that plate tectonics lasting longer than 500 million years is also highly unusual, leading to an estimate of f._pt less than 0.17.

“If we multiply these factors together, we get a refined estimate of f_I “That is very small, between 0.003% and 0.2% instead of 100%,” said Professor Stern.

“This explains the extreme rarity of favorable planetary conditions for the development of intelligent life in our galaxy and solves the Fermi paradox.”

“Biogeochemistry assumes that the solid Earth, especially plate tectonics, accelerates the evolution of species,” he added.

“Studies like ours are useful because they stimulate thought about larger mysteries and provide an example of how we can apply our knowledge of Earth systems to interesting questions about our universe.”

The article appeared in the April 2024 issue of the magazine Scientific reports.

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RJ Stern & TV Gerya. 2024. The importance of continents, oceans, and plate tectonics for the evolution of complex life: implications for the discovery of extraterrestrial civilizations. Scientific Representative doi: 10.1038/s41598-024-54700-x

This article was adapted from an original publication from the University of Texas at Dallas.