Planetary habitability is the measure of a planet's or a natural satellite's potential to develop and sustain 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—in particular those 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.
It is possible to calculate absolute energy requirements for complex life and thus distinguish environments where no complex life can possibly evolve from those where complex life could evolve but did not in similar environments on Earth just because the equivalents on Earth were too small. For example, complex life must arguably be capable of some form of parasiting to fill their energy requirements, and that means using some form of oxidant. Oxygen is the most common oxidant in the Universe, but chlorine breathers could theoretically evolve on worlds extremely poor in alcalic metals, which binds the chlorine on Earth in salts.
An absolute requirement for life is an energy source, and the notion of planetary habitability implies that many other geophysical, geochemical, and astrophysical criteria must be met before an astronomical body can support life. In its astrobiology roadmap, NASA has defined the principal habitability criteria as "extended regions of liquid water, conditions favorable for the assembly of complex organic molecules, and energy sources to sustain metabolism."
It is howewer arguable that conditions that can support complex life is not the same as those where life can start in the first place. For instance, all known complex life requires oxygen, but spontaneous generation of life is impossible in the presence of oxygen.
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. The late 20th century saw two breakthroughs in the field. The observation and robotic spacecraft 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.