Geosciences 2018 8 x FOR PEER REVIEW 19 of 48 be followed up with observations

Geosciences 2018 8 x for peer review 19 of 48 be

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Geosciences 2018 , 8 , x FOR PEER REVIEW 19 of 48 be followed up with observations. In contrast, glaciated surfaces may characterize planets near the outer edge of colder stars [2]. 7.3. Methane Daisyworld for Hotter Stars If methane productivity increases with surface temperature, a stabilizing feedback loop suggested for the Archean Earth ala “Daisyworld” [172,173] can be proposed for habitable planets near the outer edges of hotter stars [2]. In this formulation, increases in atmospheric CH 4 /CO 2 ratio yield a parabolic response in surface temperature, whereas CH 4 /CO 2 exhibits linear responses to increases in surface temperature (Figure 9). A positive (and therefore unstable) feedback ensues for points on the left half of the curve (e.g., P 1 ), with surface temperatures rising as CH 4 is increased. Such a positive feedback may operate if methanogens could evolve on these planets, assuming methane productivity increases with temperature [173]. The resultant methanogenesis yields the following, assuming atmospheric H 2 is available: 2 2 4 2 4H CO CH 2HO (8) However, hazes form once CH 4 /CO 2 ratios exceed ~0.1 [158] and the surface cools until a stable equilibrium point (e.g., P 2 ) is reached, which intersects a line with a negative (and therefore stable) slope. Further increases in surface temperature increase the CH 4 /CO 2 ratio, thickening the haze and countering the warming. However, if more complex models confirm that high FUV fluxes inhibit haze formation on blue stars (A and F spectral class) and active M-dwarfs [160], the system could potentially achieve even higher temperatures until runaway conditions are triggered or life extinguishes itself [2]. Figure 9. Proposed “Daisyworld” scenario for planets at the outer edge of the methane HZ with CH 4 in their atmospheres orbiting hotter (~A–G class) stars. The parabolic lines illustrate the effect that CH 4 /CO 2 has on surface temperature, whereas the straight lines depict the effect of temperature on the CH 4 /CO 2 ratio (figure adapted from Ramirez and Kaltenegger [2] and Domagal-Goldman et al.[173]). 8. Planetary Habitability: Extensions in Time Although the search for potentially habitable exoplanets has focused on worlds orbiting main- sequence stars, recent work shows that understanding the temporal evolution of the habitable zone is essential to determining a planet’s habitability (e.g., [51,130,131]). Hart [36] was the first to consider the temporal evolution of the habitable zone as a star ages, which he termed the “continuous Figure 9. Proposed “Daisyworld” scenario for planets at the outer edge of the methane HZ with CH 4 in their atmospheres orbiting hotter (~A–G class) stars. The parabolic lines illustrate the effect that CH 4 /CO 2 has on surface temperature, whereas the straight lines depict the effect of temperature on the CH 4 /CO 2 ratio (figure adapted from Ramirez and Kaltenegger [ 2 ] and Domagal-Goldman et al.[ 173 ]).
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Geosciences 2018 , 8 , 280 20 of 48 8. Planetary Habitability: Extensions in Time Although the search for potentially habitable exoplanets has focused on worlds orbiting
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