Eta Earth Revisited: How many Earth-like Habitats might there be in the Milky Way?

Without aerobic life, the simultaneous presence of N₂ and O₂ in the Earth's atmosphere, as on any other planet, would be chemically incompatible over geologic timescales. The existence of an N₂-O₂-dominated atmosphere on an exoplanet would, hence, not only constitute a strong biosignature of aerobic life. It would also have to meet certain astro- and geophysical conditions to originate, evolve and to sustain.

Our definition of Eta-Earth (η_Earth), therefore, builds on the concept of a so-called Earth-like Habitat (EH), i.e., a planet within the complex habitable zone for life, at which N₂ and O₂ are simultaneously present as the dominant species while CO₂ only comprises a minor constituent in its atmosphere. By our present scientific knowledge, certain criteria must be fulfilled to allow the existence of such an Earth-like atmosphere. These can be subsumed within a new probabilistic formula for estimating a maximum number of EHs which we will present within this talk. Some of these criteria, such as the initial mass function, the bolometric luminosity and XUV flux evolution of a star, or the distribution of rocky exoplanets within the habitable zones of different stellar spectral types, are already rather well studied and can be tested through further observations. Other important criteria, like the prevalence of working carbon-silicate and nitrogen cycles, or the origin of life are by now poorly, or entirely un-constrained. Further factors, like the presence of a large moon or the importance of an intrinsic magnetic field, are not only poorly constrained but its importance for the evolution and stability of an Earth-like Habitat are even debated. While our new formula for estimating the maximum number of EHs can in principle incorporate all these factors as well as unknowns, we by now must restrict ourselves to the ones that are either well understood or can at least be tested soon. Based on our current knowledge, this approach only allows us to probabilistically estimate a maximum number of exoplanets on which an Earth-like Habitat can in principle evolve. The real number of EHs might, therefore, be significantly lower than our current best estimate but additional criteria should be verifiable in near future by upcoming ground- and space-based instrumentation such as PLATO, the E-ELT, or by the kinds of the proposed space-based observatory LUVOIR.

By considering all the factors that are presently scientifically quantifiable to at least some extent, we will present our current best estimate for the maximum number of EHs that might exist within the galaxy. If we redefine η_Earth, the mean number per star of rocky planets within the habitable zone, to only account for the mean number of EHs per star, η_EH, we end up with a number much smaller than current best estimates for η_Earth. It is, therefore, scientifically not justified to presume the astrobiological Copernican assumption that all potential habitats inside a habitable zone for complex life will evolve similar to Earth.