Life is incredibly diverse on Earth, and wherever we find sufficient liquid water, we almost always find life. The only major caveat to this involves temperature. The highest known temperature at which metabolism and growth can still occur in water is 122 degrees Celsius (252 degrees Fahrenheit), for example at high-pressure hydrothermal vents. The lowest temperature seems to be about -18 degrees Celsius (about 0 degrees Fahrenheit).
Our astrobiology group at the Technical University in Berlin has used these known constraints on life to come up with a system of Planetary Environment Types, or PETs. The classification scheme identifies three main classes of water environment: PET 0, I, and II. It also includes subgroupings according to the type of water present: atmospheric water [A] such as clouds or rain; surface water [S] such as morning dew or oceans; and subsurface/ground water [G].
A planet designated PET 0 would have no surface water. But Mars would be classified PET 0-G, and Venus PET 0-A. Neither classification would necessarily exclude the possibility of life, since Mars could still host life in the subsurface and Venus in the lower atmosphere.
PET I and II planets have a surface water layer, ranging from a thin water film (for example morning dew) to massive Earth-like oceans. Water could be present either on or just beneath the surface, for example at a rock-water interface. Jupiter’s moon Europa would be an example of a PET I planetary body. A PET II planet is characterized by two liquid water layers, separated by an interstitial high-pressure water-ice layer.
Although some environments on Earth, such as the Atacama Desert, could be subclassified as PET 0-G, our planet as a whole is PET I-ASG, with all types of water—atmospheric, surface, and subsurface—being present and available.
This kind of classification scheme should be useful as we try to identify habitable worlds among the 3,500 (and counting) exoplanets now known to exist. The system does have one shortcoming, however: If an exoplanet’s biochemistry is markedly different from Earth’s—for example, if life uses a different type of liquid solvent or energy source—the assessment of its habitability would change. A solvent such as ammonia, or even an ammonia-water mixture, would lower the temperature at which life might be viable. And if life can use a different source of energy, such as magnetic fields or thermal gradients, this could open up new habitats as well. So the suggested PET system is just a first attempt to generalize some of the planetary habitats out there, and is unlikely to encompass all the possibilities for life in the Universe.