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Rapid Water Loss can Extend the Lifetime of the Planetary Habitability

Two habitable planetary states are proposed: an aqua planet like the Earth and a land planet that has a small amount of water. Land planets keep liquid water under larger solar radiation compared to aqua planets. Water loss may change an aqua planet into a land planet, and the planet can remain habitable for a longer time than had it stayed an aqua planet. We calculate planetary evolution with hydrogen escape for different initial water inventories and different distances from the central star. We find that there are two conditions to evolve an aqua planet into a land planet: the critical amount of water on the surface M_ml consistent with a planet being a land planet, and the critical amount of water vapor in the atmosphere M_cv that defines the onset of the runaway greenhouse state. We find that Earth-size aqua planets with initial oceans < 10 % of the Earth's can evolve into land planets if M_cv = 3 m in precipitable water and M_ml = 5 % of the Earth's ocean mass. Such planets can keep liquid water on their surface for another 2 Gyrs. The initial amount of water and M_cv are shown to be important dividing parameters of the planetary evolution path. Our results indicate that massive hydrogen escape could give a fresh start as another kind of habitable planet rather than the end of its habitability.