Habitability

Habitability is the degree to which an environment can support the presence and persistence of living organisms. In astrobiology, habitability typically refers to environments that could sustain life as we know it, which requires liquid water, a source of energy, and chemical building blocks such as carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Planetary habitability often emphasizes long-term stability of surface conditions and a source of energy to drive metabolism.

Key factors include a source of liquid water within a temperature range that allows it to remain liquid; an atmosphere and pressure that keep surface temperatures within a range compatible with liquid water; a source of energy, such as starlight or geothermal heat; and a chemical inventory including essential elements. A protective magnetic field and moderate greenhouse effect can help maintain climate stability; plate tectonics and oceans may regulate cycles.

Context includes the circumstellar habitable zone, the region around a star where a planet with an appropriate atmosphere could maintain surface liquid water. However, habitability is a spectrum, and life may exist in subsurface oceans on icy moons or in environments far from stars. Temporal habitability concerns the duration over which conditions remain suitable, which can be affected by stellar evolution, orbital changes, and atmospheric evolution.

Researchers search for biosignatures and technosignatures, study exoplanet atmospheres, and model climate and geology to assess habitability. The term therefore denotes a potential rather than an assured state, varying with planetary, stellar, and cosmic context.