What we now know of the infernos of Venus and the super-arctic wastes of Mars is a clear indication that being either too near or too far from the Sun may have perilous consequences. The geological record of the Earth, on the other hand, has shown that whatever the vicissitudes of the surface environment, perhaps most spectacularly manifested in terms of the episodes of major glaciations ('snowball' earths99) and super-torrid equatorial regions,100 the surface temperature has been remarkably constant. Whether or not this is a reflection of self-regulating homeostasis mediated by biotically driven feedback (as most influentially promoted by James Lovelock101) is less material to this chapter than the wider question that needs to be posed: how likely are planets to find themselves in a zone which ensures long-term habitability?
The question of how to define the Circumstellar Habitable Zone (CHZ), how wide (or narrow) it might be, and how it might evolve, was addressed some years ago by Michael Hart.102 The fate of our nearest planetary neighbours is a clear enough indication that this habitable zone must be relatively narrow (somewhere between the orbits of Mars and Venus), and most calculations suggest that it is substantially narrower. The principal problem in arriving at precise parameters is that the Sun's luminosity is increasing steadily, but various feedback processes on the planet, such as those linked to the amount of atmospheric carbon dioxide and water vapour (both greenhouse gases), mean that the calculations are not simple. Assuming that a planet's orbit is fixed, which may not always be a reliable supposition,103 the general conclusion has been that the inner boundary of the CHZ will change little, but the outer boundary will extend quite markedly.104 There are further complications in these estimates that need to be taken into account: the type of star, and hence its brightness; whether it produces a drenching of ultra-violet radiation; its lifespan; and its size. For example, if the star is two-and-a-half times as large as our Sun, then it will burn very quickly, probably too fast for life to get much beyond the first stage.105
Nor do the complexities end there, because planets like the Earth are not only dynamic systems but they also change through geological time. The net result of all these factors is a complex set of possibilities, in terms of stellar and terrestrial histories, that mean that both the definition and the evolution of the habitable zone are not easy to constrain and must depend on various assumptions. An accurate estimate, however, will have implications not only for the long-term habitability of our Earth, but also for the likelihood of finding near-equivalents elsewhere in the Galaxy. One recent analysis is of particular interest. Siegfried Franck and his colleagues suggest that not only has the habitable zone contracted significantly in the past billion years, but that in another billion years it will have moved beyond the Earth (and also as it happens will have narrowed significantly) (Fig. 5.8), so that higher life at least will have disappeared under the increasingly intolerable temperatures (Fig. 5.9).106 Although the long-term prognosis for the Earth is not good, indeed Franck and his colleagues simply remark, 'In about 500 Myr [500 Ma] - the biosphere ceases to exist',107 their calculations of both the evolution of the habitable zone and the number of Earth-like planets are used to suggest that in our Galaxy the total of such planets might be 500 000 (give or take). How many are suitable for the emergence of life, let alone civilizations, is much more of an imponderable. The strong hint, on the basis of what I have already said, is that of the half-million, 499 999 are not suitable.
A further indication that this assessment, which might be regarded as vaguely discouraging, might be realistic comes from a rather different line of evidence. This indicates that even if earth-like planets are relatively common, most of them will be significantly older than the Earth, on average by a rather remarkable 1.8 Ga.108 Plenty of time for civilizations to evolve and develop a taste for interstellar
Time (billion years)
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