Will ET Drink Water?

A team of astronomers announced this year that they had found no fewer than eight planets orbiting the so-called Goldilocks zone around their parent stars, inside which the temperatures are neither too hot nor too cold to support life as we know it on Earth. Two of these, called Kepler-438b and Kepler-442b, are the most Earth-like candidates yet seen among the 1,900 or so exoplanets (planets outside our solar system) now known to exist.

“Earth-like” means, in part, that a planet has a good chance of carrying liquid water—something more likely to be true inside the Goldilocks zone. “Follow the water” has become a mantra for astrobiologists seeking signs of life elsewhere in the cosmos. With the new ability to detect the characteristic fingerprint of water in the light reflected from the atmospheres of exoplanets, some astronomers hope to spot a world that hosts life soon.

But is liquid water really necessary to life?

There’s a long history to the belief that it is. In 1913, the Harvard biochemist Lawrence Henderson proposed a curious inversion of Darwinian evolution, in which organisms become “fit” for their environment by adaptation. Henderson’s book The Fitness of the Environment argued that the cosmic environment is itself peculiarly “fit” to host life.

This was deeply puzzling. How could an environment acquire fitness?

Henderson pointed out that water, in particular, seemed replete with “biocentric” attributes, as if it were uniquely designed as life’s solvent. The fact that it is a liquid at all on Earth is more unusual than it might seem. Other simple hydride molecules—methane, hydrogen sulfide, ammonia, hydrogen chloride— are all gases at room temperature and pressure—but not “oxygen hydride” H₂O. There seems to be some extra stickiness binding water molecules together.

And because water has a high heat capacity (it can absorb a lot of heat without much increase in temperature), ocean currents can redistribute huge amounts of solar heat and help make planetary conditions more uniform and stable. What’s more, while most liquids shrink and get denser when they freeze, ice expands and floats. As a result, ponds don’t freeze solid from the