Imagine that it is possible to make multiple copies of the Earth and then place them at random in circular orbits around the stars of one's choice. Presuming further that we wish to have liquid water available on the surface of each new Earth, the question is, given the parent stars luminosity, what range of orbital radii will satisfy the surface water existence condition? This question has, in fact, long ago been addressed by James Kasting (Pennsylvania State University) and co-workers, and the term ''habitable zone'' has been coined to describe the region in which liquid water might exist on the surface of an Earth-like planet.
The width of the habitable zone is bounded according to the distances at which water boils (the inner boundary) and freezes (the outer boundary), and these distances will change according to the star's luminosity.3 The lower the luminosity, the closer the habitability zone resides to the parent star; the higher the luminosity, the further it is away.
Kasting and co-workers have refined the determination of the habitability zone by studying detailed climate models. The inner edge of the habitability zone in such detailed models is set according to the rapid loss of water vapor (actually, its constituent hydrogen atoms) by photodissociation in the upper atmosphere. The outer edge is set according to the formation of CO2 clouds that, being highly reflective, dramatically increases the albedo and the planet is thereby cooled off. The variation of the width and radial location of the habitable zone, according to the calculations of Kasting and co-workers, is illustrated in Figure 5.9.
As one would expect for our Solar System (Figure 5.9), the Earth is situated within the habitability zone for a solar-mass star of age 4.5 billion years. Similar such diagrams for different-mass stars of other ages can also be constructed to gauge the location of the habitability zones for exoplanet-supporting stars (a topic we return to in Chapter 8). For the present, however, we note that terraform-ing might, in some sense, be described as the vertical shifting of a planet within the habitability zone diagram. The present orbit occupied by Venus, for example, would fall within the habitability
zone if it orbited a 0.75 M© star. Likewise, the current orbit of Mars would place it in the habitability zone if it orbited a 1.5 M© star.
Although not necessarily beyond the realms of possibility, we are not advocating changing the Sun's mass in order to make the planets Venus or Mars habitable (there are easier ways of achieving the same ends), but as the Sun ages there are, in fact, sound reasons for attempting to reduce its mass, as briefly described at the end of Chapter 4.
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