History
The concept of what is now widely known as the habitable zone originates in the 1950s. Two publications referring to the concept were written at about the same time. Hubertus Strughold wrote "The Green and the Red Planet: A Physiological Study of the possibility of Life on Mars" in which he used the term "ecosphere" and referred to "zones" in which life could exist. In the same year, Harlow Shapley wrote the "Liquid Water Belt" which described the same theory in further scientific detail. Both stressed the importance of liquid water to life. In 1955 Strughold wrote a follow-up called "Ecosphere of the Sun". Chinese-American astrophysicist Su-Shu Huang extended the debate in 1959 with "Life-Supporting Regions in the Vicinity of Binary Systems" proposing that life zones were rare due to the orbital instabilities of habitable zones in common multistar systems.
Habitable zone theory was further developed in 1964 by Stephen H. Dole in "Habitable Planets for Man" and then popularised by science fiction writer Isaac Asimov by capturing the imagination exploring possibilities of space colonization of other planetary systems. Dole estimated the number of habitable planets in the Milky Way to be about 600 million.
By the 1970s, Michael H. Hart's 1979 paper "Atmospheric Evolution, the Drake Equation and DNA: Sparse Life in an Infinite Universe" outlined the first evolutionary model for a habitable zone and his pessimistic conclusions on the distribution of extraterrestrial life fuelled the Rare Earth hypothesis.
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