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.

Introduction

Habitable planets are those bodies that provide environments, materials and processes that are advantageous for the formation and long-term evolution of life. Understanding the processes that lead to the formation of such planets is a central issue in astrobiology. We are of course handicapped in this quest since Earth is the only example of a planet with proven habitability – the only one known to have provided thermal, chemical, and other physical conditions that allowed life to form and survive for ~3.5 Gyr.

This chapter emphasizes the formation of Earth-like planets, those with environments capable of supporting complex life comparable to Earth's plants and animals. The focus on life comparable to Earth's multicellular organisms is partly due to the practical consideration that we better understand the environmental constraints of such life. Despite this restricted focus, note that most astrobiologists consider that the dominant form of life in the Universe, as it has been on Earth over most of its history, is probably far simpler and more rugged, analogous to Earth's bacteria and archaea (Section 3.2).

This interpretation of habitability is highly Earth-centric and assumes that life elsewhere is similar to terrestrial life and requires environments similar to those of terrestrial organisms. The actual cosmic limits of life are of course unknown, but the Earth-centric view is a reasonable, albeit conservative, place to start. Until there are detailed data on other inhabited planets, discussions of extraterrestrial life would be prudently biased by what is known from our Earth experience.