  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.
