How common are oxygenic photosynthesis and large coal deposits on exoplanets?

Abstract In the search for extraterrestrial intelligence (SETI), it is often assumed that intelligent life on an Earth-like exoplanet would inevitably develop the technological means for interstellar communication. This assumption ignores the critical role that fossil fuels played in driving the Industrial Revolution on Earth, which ultimately gave rise to our own advanced technological civilization (ATC) and the possibility of interstellar communication. We therefore propose that any habitable exoplanet that could potentially generate an ATC must contain sizable fossil fuel deposits, especially coal, which supplied most of the energy used in the Industrial Revolution during the 19 th century. Coal is critical because, based on an Earth-like geology, it is more accessible than the much deeper deposits of oil and gas. Without coal, it would have been impossible to tap into the vast underground deposits of oil and gas during the 20 th century. This raises the question of the inevitability of coal formation on an Earth-like exoplanet. Here we present arguments that coal formation may be unlikely, even on an Earth-like planet, because of the many contingent factors that have been recorded in the rock and biological record of our own planet, including the evolution of oxygenic photosynthesis itself, which generated the oxygen-rich atmosphere required for complex life to develop. Central to our argument is the host of highly contingent taphonomic factors, involving plate tectonics and climate, that were required to convert the tropical lycopsid swamp forests of the Pangean supercontinent to the massive coal deposits of the Carboniferous period. Finally, we discuss the need for synchronicity of the appearance of intelligent life forms and the maturation of vast deposits of coal. We conclude that the large number of contingencies involved in coal production justifies adding a term for coal to the Drake Equation for the number of ATCs in the galaxy.