The water that life knows and needs, the water that makes a world habitable, the water that acts as the universal solvent for all the myriad and fantastically complicated chemical reactions that make us different than the dirt and rocks, can only come in one form: liquid.
The vast, vast majority of the water in our universe is unsuitable for life. Some of it is frozen, locked in solid ice on the surface of a world too distant from its parent star or bound up in a lonely, wayward comet. The rest is vaporized, existing as a state of matter where molecules lose their electron companions, boundless and adrift through the great nebular seas that dot the galaxies, or ejected completely into the great voids between them. Either way, that water exists only one molecule at a time, at a temperature of over a million degrees yet its density so low that you could pass through it and mistake it for the cold, hard vacuum of space itself.
No, for water to be liquid it must exist in special place around a star, not too cold for it turn to ice, not too hot for it to turn to gas. It must lay within what astronomers call the habitable zone, or, if they’re feeling playful, the Goldilocks zone.
The habitable zone is different for every star throughout the galaxy, because no two stars are alike. The smallest red dwarfs are barely a tenth the mass of the Sun, with luminosities a thousand times weaker. The largest are great beasts, a hundred solar masses or more, so bright they can be seen from thousands of light-years away by the unaided eye. Around each star a simple iron law holds, the fact that the intensity of light, and all the warmth and comfort that light brings with it, diminishes with the square of the distance from the source. An object twice as far away will experience a quarter of the brightness; at a distance of four times that drops to a sixteenth, and so on. That is why Pluto, despite only sitting about 30 times further away from the Sun than the Earth, is forced to experience never-ending dim twilight, even at the height of its day.
Too far from a star and the radiant temperatures are too cold, and any water freezes. Too close, and the water slips its bonds, free to roam as a gas. In between, in a special band determined by the star’s mass, age, and brightness, sits the habitable zone, where a planet is capable – yes, merely capable – of hosting water in its liquid state on its surface.
For our own Sun, the habitable zone stretches from just within the orbit of Venus to just beyond the orbit of Mars. Three planets perfectly situated within the warm embrace of our Sun, and yet only one has life. What happened? What made our planet so special, or so lucky? It’s impossible to say for sure, because the potential of habitability is not a promise.
There is, however, one other place where we know liquid water can exist. Ironically, it’s in the frozen moons of the outer solar system. There, under surfaces of frozen ice a hundred kilometers thick sit globe-spanning liquid water oceans, with more liquid water than exists on the surface of the Earth. There the habitability isn’t given by the rays of the Sun, but from their molten cores emanating heat, driven by the gravitational warping of the giant planets they orbit. Life could certainly find a purchase there, in places of darkness that the Sun never can touch, even though there worlds are not, according to the traditional definition, habitable.