Introduction

This chapter discusses general concepts of planetary habitability as well as major events in Earth's history that relate to habitability over its full past and future evolution. The Sun plays a determining factor for habitability in the solar system as other stars also are critical to habitability in other planetary systems. Stars provide well-known benefits to life, but they also cause life-ending processes such as the loss of oceans and the loss of planetary atmospheres.

Environmental limits for life as we know it

Because life has not been detected anywhere but on Earth, the nature of extraterrestrial life remains completely unknown. In light of this famous shortcoming we can still use environmental requirements for terrestrial organisms to estimate where organisms similar to life-as-we-know-it might plausibly exist elsewhere. While this can be criticized as being overly provincial, the Earth-biased approach provides a practical means to access the potential habitability of other worlds. For life based on complex interactions of compounds analogous to the biomolecules of life on Earth, it is relatively straightforward to set general constraints on environments that might support life similar to life-as-we-know-it.

Many of the environmental constraints are influenced by the central star in a planetary system, as the Sun does in our solar system. The Sun provides warmth and energy for photosynthesis, but it also influences many of Earth's fundamental atmospheric, oceanic, and biological processes.