A habitable zone is an area where liquid water can exist on a planet’s surface. The habitable zone is also sometimes called the “Goldilocks Zone” because it is not too hot and not too cold for water to remain a liquid. Earth is located in the habitable zone around our sun. If Earth were any closer to the sun, our oceans would boil away. If we were any farther from the sun, our oceans would freeze solid.
The distance from a star that determines the inner edge of the habitable zone depends on the star’s luminosity. The more luminous a star, the hotter its planets will be and thus the further away from the star they can be and still have liquid water. For example, Proxima Centauri b, which orbits Proxima Centauri (a low-luminosity red dwarf), has an estimated inner edge of just 0.4 AU (astronomical units) from its star—closer than Mercury is to our Sun—whereas Gliese 667C c, orbiting a bright orange dwarf star, has an inner edge of about 1.7 AU—farther out than Neptune’s orbit in our Solar System. The outer edge of the habitable zone is determined by how much infrared radiation a planet emits back into space as it cools down after formation; if it radiates too much heat then it will lose all its water and become uninhabitable.
The concept of a circumstellar habitable zone was first proposed by Joseph Harold Schwab in 1961, although he called it the “ecosphere”. In 1964, Carl Sagan defined two regions which he called “the conservative ecosphere” and “the maximum ecosphere”. These corresponded roughly to what are now known as the inner and outer edges of habitability.
The conservative ecosphere was defined as “the region surrounding a star within which an earth-like planet could maintain long-term stable temperatures conducive to life”. It extended from just inside Venus’ orbit out to just inside Earth’s orbit; beyond this point conditions became too cold for photosynthesis or other energy production processes essential for life as we know it. This limit moved inward as stars became more Luminous over time due to stellar evolution; by 3 billion years ago even G-type main sequence stars like our Sun had reached their present day brightnesses,:33–34 so their corresponding conservative ecospheres had contracted around them well before that time period.:377 The maximum ecosphere was defined as “the greatest radial distance from a given central body at which intelligent beings could exist.” It corresponded approximately to modern estimates for Venus’ maximum greenhouse effect radius: between about 0.75–2 times Venus’ current orbital distance (0.72–1.08 AU).:309 However unlike today’s scientific understanding of habitability,:129 Sagan assumed that advanced civilizations would have technology capable of artificially maintaining livable conditions on otherwise hostile worlds outside this region such as Mars;:183 he later wrote that his choice of name (“maximum”) may have been unfortunate in this regard since many people interpreted it literally rather than recognizing it referred only potential future colonization efforts by technologically advanced societies..