Liquid water is the primary ingredient for life as far as we can determine. The search for habitable exoplanets focuses on this fact. Exoplanet scientists sift through data trying to determine which worlds might be in their stars' habitable zones, a zone with just the right amount of star energy to maintain liquid surface water. But exoplanets and their water isn't a binary issue. Determining habitability isn't as simple as determining if an exoplanet has water or doesn't have water. An exoplanet with a scant amount of water may not be hospitable whether it's in the habitable zone or not. Earth, the only habitable planet we know of, depends on its carbon cycle to maintain habitability. The carbon cycle depends on water, and exoplanets without enough water aren't likely to sustain their carbon cycles, making the prospects for long-term habitability bleak. But how much water is enough?…