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5 - Number of planets, per solar system, with an environment suitable for life, ne, pre-1961

Published online by Cambridge University Press:  05 July 2015

Douglas A. Vakoch
Affiliation:
SETI Institute, California
Matthew F. Dowd
Affiliation:
University of Notre Dame, Indiana
Frank Drake
Affiliation:
SETI Institute, California
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Summary

Abstract

The Drake Equation factor ne, the number of planets per planetary system with an environment suitable for life, has recently received increased attention as astronomers discover many planetary systems within the Milky Way. This factor is closely associated with the notion of planetary habitability, which is not new. During the second part of the nineteenth century, astronomers such as Richard Proctor (1837–1888) and Camille Flammarion (1842–1925) studied planetary habitability in our solar system a century before this concept was updated by the systematic exploration of the solar system and the detection of exoplanets. The question was tackled in scientific terms while studies of the Martian surface were intensifying. The term “habitability” was used mainly for Mars, which was regarded as a sister planet of Earth and looked like a possible abode for life. More generally, studies during this period attempted to compare Earth with other planets in the solar system, in spite of the lack of accurate data on their environments. Analogy soon became the basic way in which to calculate the level of habitability of each planet. In the meantime, spectroscopy was developing quickly and began to provide more data about planetary atmospheres.

At the dawn of the space age, Hubertus Strughold (1898–1987) suggested that we examine the planets of the solar system using the concept of “planetary ecology.” This notion was very similar to the concept of habitability employed earlier by nineteenth-century pioneers. Strughold also coined the term “ecosphere” to describe the region around a star that has conditions suitable for life-bearing planets, a concept equivalent to the habitable zone. A decade later, Stephen Dole redefined the concept of habitability as the planetary conditions suitable for human life. He used probability methods to estimate the number of planets in the galaxy where human life might be possible. This chapter reviews the main historical contributions up to the 1960s regarding the concept of planetary habitability and its different meanings over time.

Type
Chapter
Information
The Drake Equation
Estimating the Prevalence of Extraterrestrial Life through the Ages
, pp. 90 - 113
Publisher: Cambridge University Press
Print publication year: 2015

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