The evolution of habitability: Characteristics of habitable planets

We review planetary habitability. We discuss the physical and biological constraints on the habitability of Earth and other rocky planets. The section on the “physics of habitability” examines the conceptual limitations of circumstellar habitable zones that are conventionally used to discuss habitability. Using the Earth as an example, we discuss how habitability can vary over spatial and temporal scales. In the section on the “biology of habitability,” we describe how life may be the dominant force in keeping a planet habitable over long durations. In the Gaian Bottleneck model, biotic regulation to provide the necessary level of negative feedback, to counter greenhouse and albedo positive feedbacks, is a prerequisite for sustaining habitability. We discuss how the conditions required to start life may be different from conditions required to sustain life and how abiogenesis habitable zones may be short-lived and highlight nuanced views on how planetary habitability evolves over time. We discuss problems associated with defining habitability based on anthropocentric or Earth-specific characterization of life. We suggest how studying the coevolution between life and the environment could offer better insights into the making of a habitable planet.