The Changing Concept of Habitability on Earth, the Solar System, and Beyond

Our concept of where life can thrive on Earth has advanced over the past 70 years to include extreme ionizing radiation, high temperatures, the deep subsurface, hydrothermal vents on the deep ocean floor, extreme arid deserts, and the ice-covered lakes and high mountain valleys of Antarctica. This expanding understanding of the biosphere has coincided with the development of space exploration programs, and it has informed those programs with regard to the search for life on other water worlds in our Solar System—especially Mars, Europa, and Enceladus. Titan presents a reverse of this approach. The interesting organic solids and fluids on that world have no analog in Earth habitability but have inspired suggestions of possible biological systems unlike any on Earth. If realized, the discovery of life on Titan would stretch the concept of habitability just as it stretches the concept of life as we know it. Habitability studies on exoplanets may follow both of these paths: we will look for habitability on exoplanets based on observed habitats on Earth, and we will also use observations of exoplanets as grist for contemplation of lifestyles different from anything we know on Earth.