How common are biological ETIs in the Galaxy?

Earth-like Habitats are rocky exoplanets in the habitable zone that are able to host Earth-like N₂-O₂-dominated atmospheres with minor amounts of CO₂ on which complex life could, in principle, evolve (Lammer et al. 2024; Scherf et al. 2024). For animal-like, complex life to emerge, however, certain physical and toxicity limits of the surface partial pressures and mixing ratios of CO₂, O₂, and N₂ need be met (Lammer et al. 2024), although one has to keep in mind that complex life on other planets may evolve to withstand other toxicity limits than animals here on Earth. For an extraterrestrial intelligence (ETI) that develops technology to communicate over interstellar distances or that can even colonize other stellar systems, however, tighter physical limits on the atmospheric O₂ mixing ratio apply (e.g., Balbi & Frank 2024) based on, e.g., the minimum physical size of ETIs and the limits for combustion and flammability. In addition, subaerial land and some form of tectonics will be needed to allow for such ETIs to evolve (e.g., Stern & Gerya 2024; Scherf et al. 2024). The prevalence of planets that meet these specific conditions depends on certain astrophysical, geophysical, and biochemical criteria that must be met to allow for their evolution and environmental stability. One of the essential factors for their emergence and evolution is a planet's host star. Its radiation and plasma environment may affect atmospheric stability to such an extent that it can even render the existence of an Earth-like atmosphere unlikely. Here, the mixing ratio of CO₂ is of particular importance - together with N₂ and O₂ - as this parameter will not only provide tentative toxicity and physical limits for life as we know it to evolve, but also influence the thermal stability of an Earth-like atmosphere against escape into space. Recently, Lammer et al. (2024) and Scherf et al. (2024) developed a formalism to estimate the maximum number of Earth-like Habitats in the Galaxy, which suggests these specific kinds of habitats to be relatively rare (Scherf et al. 2024). In this presentation, we build on this work and derive a maximum number of ETIs that can, in principle, exist in the Milky Way by considering additional factors such as the lifetime of Earth-like Habitats and ETIs.

References:

• Balbi, A. Frank, A., The oxygen bottleneck for technospheres, 2024, Nature Astronomy, 8, 1, 39
• Lammer, H. Scherf, M., Sproß, L., Eta-Earth Revisited I: A Formula for Estimating the Maximum Number of Earth-Like Habitats, Astrobiology, 24, 897.
• Scherf, M., lammer, H., Sproß, L., Eta-Earth Revisited II: Deriving a Maximum Number of Earth-Like Habitats in the Galactic Disk, 2023, Astrobiology, 24, e916.
• Stern, R.J., Gerya, T.V., The importance of continents, oceans and plate tectonics for the evolution of complex life: implications for finding extraterrestrial civilizations, 2024, Scientific Reports, 14, 8552