Estimated number of transits required to probe for Earth-like O₂ concentrations at 3σ significance on an Earth-sized habitable-zone planet orbiting M dwarfs as a function of telescope diameter. Shown are the models for hypothetical planets orbiting M2 V, M4 V, M6 V, and M8 V stars at distances of 5 (blue solid), 10 (magenta dotted), 15 (yellow dashed), and 20 pc (red dashed–dotted). These models show that a hypothetical Earth-sized habitable-zone planet orbiting an M4 V star would require the fewest number of transits to probe for Earth-like concentrations of O₂. In the M2 V, M6 V, and M8 V plots, we show the M4 V models in light gray for reference. Vertical dark gray lines denote the effective diameters (based on total collecting area) of the Keck (9.84 m, Mast & Nelson 1986), GMT (21.65 m, Jacoby et al. 2012), TMT (29.08 m, Skidmore et al. 2015), E-ELT (35.29 m, Liske et al. 2012), and theoretical 50, 75, and 100 m diameter equivalent telescopes.