NASA identifies 17 exoplanets with possible subsurface oceans

A large, partially shadowed earth-like planet hangs in space. a yellow star is seen in the distance.

As far as we know, life needs water. 

Due to this simple truth, astronomers and astrobiologists have naturally focused their efforts on identifying exoplanets that might harbor liquid oceans. Water in its liquid form can exist on a planet's surface, where direct heat from its host star can keep the substance from freezing — but it can also exist  beneath a planet's surface, where internal sources of heat can sustain flowing, subsurface oceans.