Formulation and resolutions of the red sky paradox

Significance Red dwarf stars are the most numerous and long-lived stars in the cosmos, and recent exoplanet discoveries indicate an abundance of rocky, temperate planets around them. This presents an apparent paradox as to why we do not see a red dwarf in our sky. This “red sky paradox” could plausibly be random chance at the 1-in-100 level but would then come into tension with the Copernican principle. Three additional resolutions to the paradox are outlined, which broadly inhibit the opportunities for complex life to develop around such stars: attenuated emergence rates, truncated evolutionary windows, and/or a paucity of suitable habitats. All three appear viable given our present limited knowledge but the potential for future observational tests is explored. Most stars in the Universe are red dwarfs. They outnumber stars like our Sun by a factor of 5 and outlive them by another factor of 20 (population-weighted mean). When combined with recent observations uncovering an abundance of temperate, rocky planets around these diminutive stars, we are faced with an apparent logical contradiction—Why do we not see a red dwarf in our sky? To address this “red sky paradox,” we formulate a Bayesian probability function concerning the odds of finding oneself around an F/G/K-spectral type (Sun-like) star. If the development of intelligent life from prebiotic chemistry is a universally rapid and ensured process, the temporal advantage of red dwarfs dissolves, softening the red sky paradox, but exacerbating the classic Fermi paradox. Otherwise, we find that humanity appears to be a 1-in-100 outlier. While this could be random chance (resolution I), we outline three other nonmutually exclusive resolutions (II to IV) that broadly act as filters to attenuate the suitability of red dwarfs for complex life. Future observations may be able to provide support for some of these. Notably, if surveys reveal a paucity of temperate rocky planets around the smallest (and most numerous) red dwarfs, then this would support resolution II. As another example, if future characterization efforts were to find that red dwarf worlds have limited windows for complex life due to stellar evolution, this would support resolution III. Solving this paradox would reveal guidance for the targeting of future remote life sensing experiments and the limits of life in the cosmos.