Formulation and Resolutions of the Red Sky Paradox

Abstract

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're faced with an apparent logical contradiction - why don't we 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 a 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. Whilst this could be random chance (resolution I), we outline three other non-mutually exclusive resolutions (II-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 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.

arXiv commentPublished in PNAS

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