title: Habitability: A Review.
abstract: Habitability is a widely used word in the geoscience, planetary science, and astrobiology literature, but what does it mean? In this review on habitability, we define it as the ability of an environment to support the activity of at least one known organism. We adopt a binary definition of "habitability" and a "habitable environment." An environment either can or cannot sustain a given organism. However, environments such as entire planets might be capable of supporting more or less species diversity or biomass compared with that of Earth. A clarity in understanding habitability can be obtained by defining instantaneous habitability as the conditions at any given time in a given environment required to sustain the activity of at least one known organism, and continuous planetary habitability as the capacity of a planetary body to sustain habitable conditions on some areas of its surface or within its interior over geological timescales. We also distinguish between surface liquid water worlds (such as Earth) that can sustain liquid water on their surfaces and interior liquid water worlds, such as icy moons and terrestrial-type rocky planets with liquid water only in their interiors. This distinction is important since, while the former can potentially sustain habitable conditions for oxygenic photosynthesis that leads to the rise of atmospheric oxygen and potentially complex multicellularity and intelligence over geological timescales, the latter are unlikely to. Habitable environments do not need to contain life. Although the decoupling of habitability and the presence of life may be rare on Earth, it may be important for understanding the habitability of other planetary bodies.
authors.affiliation: 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK .
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authors.initials: CS
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authors.affiliation: 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK .
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authors.affiliation: 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK .
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authors.affiliation: 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK .
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authors.affiliation: 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK .
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authors.affiliation: 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK .
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authors.initials: JP
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authors.affiliation: 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK .
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authors.affiliation: 3 Division of Space Technology, Department of Computer Science, Electrical and Space Engineering, Luleå University of Technology , Kiruna, Sweden; and Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR), Armilla, Granada, Spain .
authors.forename: J
authors.initials: J
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authors.affiliation: 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK .
authors.forename: N
authors.initials: N
authors.lastname: Nicholson
authors.affiliation: 4 Department of Reference Systems and Planetology, Royal Observatory of Belgium , Brussels, Belgium .
authors.forename: L
authors.initials: L
authors.lastname: Noack
authors.affiliation: 5 School of Physics and Astronomy, University of St Andrews , St Andrews, UK; now at the Carl Sagan Institute, Cornell University, Ithaca, NY, USA .
authors.forename: J
authors.initials: J
authors.lastname: O'Malley-James
authors.affiliation: 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK .
authors.forename: S J
authors.initials: SJ
authors.lastname: Payler
authors.affiliation: 6 Centre for Ocean and Atmospheric Science (COAS), School of Environmental Sciences, University of East Anglia , Norwich, UK .
authors.forename: A
authors.initials: A
authors.lastname: Rushby
authors.affiliation: 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK .
authors.forename: T
authors.initials: T
authors.lastname: Samuels
authors.affiliation: 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK .
authors.forename: P
authors.initials: P
authors.lastname: Schwendner
authors.affiliation: 1 UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh , Edinburgh, UK .
authors.forename: J
authors.initials: J
authors.lastname: Wadsworth
authors.affiliation: 3 Division of Space Technology, Department of Computer Science, Electrical and Space Engineering, Luleå University of Technology , Kiruna, Sweden; and Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR), Armilla, Granada, Spain .|7 Centro de Astrobiología (CSIC-INTA) , Torrejón de Ardoz, Madrid, Spain .
authors.forename: M P
authors.initials: MP
authors.lastname: Zorzano
mesh_terms: D018559:Exobiology; D005118:Extraterrestrial Environment; D016083:Planets