Solar Hegemony: M Dwarfs Are Unlikely to Host Observers Such As Ourselves

Abstract With no firm evidence for life beyond our solar system, inferences about the population of observers such as ourselves rest upon the Earth as a single input, at least for now. While the narrative of our home as a “humdrum” system has become ingrained in the public psyche via C. Sagan, there are at least two striking facts about our existence which we know are certainly unusual. First, the stelliferous period spans ∼10 Tyr—yet here we are living in the first 0.1% of that volume. Second, over three-quarters of all stars are low-mass M dwarfs, stars with no shortage of rocky habitable-zone planets—and yet, again, our existence defies this trend, previously dubbed the red sky paradox (RSP). Two plausible resolutions are that (a) stars below a certain mass, M crit , do not produce observers, and (b) planets have a truncated temporal window for observers, T win , negating the longevity advantage of M dwarfs. We develop a Bayesian model that encompasses both data points and jointly explores the two resolutions covariantly. Our analysis reveals that (1) the hypothesis that these observations are mere luck is disfavored with an overwhelming Bayes factor of ≃1600; (2) some truncation of low-mass stars is indispensable, lowering T win alone cannot well explain the observations; and (3) the most conservative limit on M crit occurs when fixing T win = 10 Gyr, yielding M crit > 0.34 M ⊙ [0.74 M ⊙ ] to 2 σ [1 σ ]. Our work challenges the tacit assumption of M dwarfs being viable seats for observers and, indirectly, even life.