Are we alone in the Universe? This question has been puzzling humans for several centuries since we realized that the Sun is a star at the center of a planetary system and that many of the bright spots we see in the night sky are also stars possibly accompanied by several planets. Some of these planets might be similar to Earth and host civilizations; do they? This question is very difficult to answer confidently but recent advances in astronomy,  especially the detection of remote extrasolar planets or exoplanets in our galaxy – the Milky Way - and better understanding of our own planet – Earth – create new exciting opportunities to address this question on a quantitative scientific basis.  The Drake equation proposed by Frank Drake in 1961 provides a framework for constraining the number of active, communicative civilizations (ACCs) in our galaxy, based on a set of 7 astronomical and biological parameters, which can be estimated with various degree of uncertainties.  Drake and his colleagues acknowledged these uncertainties and inferred that there should be probably between 1000 and 100,000,000 ACCs in our galaxy. The Fermi Paradox (Where is everybody?, spelled out by Enrico Fermi) points out that all these estimates are much too high as we are not communicating with any ACC at all.  This implies that some important variables are missing from the Drake Equation or their magnitudes are incorrectly estimated.

In our new paper in Scientific Reports (https://doi.org/10.1038/s41598-024-54700-x), we  - Robert Stern from U Texas Dallas, USA and Taras Gerya  from ETH Zurich, Switzerland - elaborate on the paradox by focusing on one parameter of the Drake equation: f_i – probability of planet with any life to develop intelligent life. This probability was originally estimated to be very high (100%), implying that any planet with primitive life will sooner or later host intelligent life leading to an ACC. This assumption may be overly optimistic due to the extreme slowness of biological evolution, which on Earth required four billion years to make the myriad evolutionary steps in a continued chain of subsequent complex species leading to the appearance of intelligent ACC-forming humans. The study identifies two key environmental features needed to enable and accelerate evolution leading to ACC: (1) the long term existence of plate tectonics and (2) the presence of large oceans and continents (i.e., dry land areas), and provides the first estimates for the likelihood of each.

In order to quantify the effects of these environmental factors for the Drake equation term, we reviewed the tectonic styles of rocky bodies in the Solar System and noted that actively convecting planets generally do not have plate tectonics, rather they have a tectonic style called «single lid», in which global mosaic of tectonic plates present on Earth does not develop (as for example seeing on Mars and Venus). Reviewing the last 1.6 billion years of Earth’s tectonic history, we noted that a prolonged transition from Mesoproterozoic active single lid tectonics (1.6 to 1.0 Ga) to modern plate tectonics occurred in the Neoproterozoic Era (1.0 to 0.541 Ga), which dramatically accelerated emergence and evolution of complex species. This is because plate tectonics is much more effective than single lid tectonics for increasing nutrient supply, ameliorating climate, creating isolated habitats, and applying moderately strong environmental pressure on life.  For these reasons, we concluded that ~500 million years of plate tectonics is needed for an ACC to evolve on a planet with primitive life.  We also evaluated the importance of the long-lasting (>500 million years) presence of large land areas (continents) and oceans for evolution leading to an intelligent ACC-forming species. Both continents and oceans are required for ACCs because evolution of simple to complex multicellular life must happen in water but further evolution leading to exploration of fire and electricity, creation of technologies and finally to the emergence of ACC must happen on land. We therefore revised the Drake equation by defining f_i as the product of two terms, f_oc and f_pt, where f_oc is the fraction of habitable exoplanets with significant continents and oceans and f_pt is the fraction of exoplanets that have had long-lasting plate tectonics.  We redefined f_i=f_ocf_ptand resolved the Fermi Paradox by showing that the f_i values computed from f_oc and f_pt estimates are very small (<0.003-0.2 %), explaining the extreme rareness of favorable planetary conditions for the development of intelligent life in our galaxy. 

Indeed, we are likely alone in the galaxy and have to preserve our very rare and unusual planet Earth – we will not be able to easily find another one!