Planetary habitability

Planetary habitability is the measure of an astronomical body's (planets and natural satellites of planets) potential to develop and sustain life. The only absolute requirement for life is an energy source, but the notion of planetary habitability implies that many other geophysical, geochemical, and astrophysical criteria must be met before an astronomical body can support life.

As the existence of life beyond Earth is currently uncertain, planetary habitability is largely an extrapolation of conditions on Earth and the characteristics of the Sun and solar system which appear favorable to life's flourishing; of particular interest are factors that have sustained complex, multicellular organisms and not just simpler, unicellular creatures. Research and theory in this regard is a component of planetary science and the emerging discipline of astrobiology.

The idea that planets beyond Earth might host life is an ancient one, though historically it was framed by philosophy as much as physical science 1. The late 20th century saw two breakthroughs in the field. The observation and robotic exploration of other planets and moons within the solar system has provided critical information on defining habitability criteria and allowed for substantial geophysical comparisons between the Earth and other bodies. The discovery of extrasolar planets, beginning in the early 1990s and accelerating thereafter, has provided further information for the study of possible extraterrestrial life. Most importantly, it confirmed that the Sun is not unique among stars in hosting planets and expanded habitability research horizon beyond our own solar system.