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Sun Venus Earth figure 5.8 Evolution of the circumstellar habitable zone (CHZ; shaded) from 1 Ga (1 billion years) ago, through today to 1 Ga in the future, relative to the orbits (distance in Astronomical Units (AU)) of Earth, Venus, and Mars. Note that 1 Ga ago Mars was well within the HZ, but 1 Ga in the future the CHZ has shrunk dramatically and the Earth's oceans are boiling. (Reproduced with permission from Naturwissenschaften, from the article Planetary habitability: is Earth commonplace in the Milky Way?, by S. Franck, A. Block, W. van Bloh, C. Bounama, I. Garrido, and H.-J. Schellenhuber, vol. 88, pp. 416-426, fig. 5; 2001, copyright Springer-Verlag, and also with the permission of the authors.)

Sun Venus Earth figure 5.8 Evolution of the circumstellar habitable zone (CHZ; shaded) from 1 Ga (1 billion years) ago, through today to 1 Ga in the future, relative to the orbits (distance in Astronomical Units (AU)) of Earth, Venus, and Mars. Note that 1 Ga ago Mars was well within the HZ, but 1 Ga in the future the CHZ has shrunk dramatically and the Earth's oceans are boiling. (Reproduced with permission from Naturwissenschaften, from the article Planetary habitability: is Earth commonplace in the Milky Way?, by S. Franck, A. Block, W. van Bloh, C. Bounama, I. Garrido, and H.-J. Schellenhuber, vol. 88, pp. 416-426, fig. 5; 2001, copyright Springer-Verlag, and also with the permission of the authors.)

travel? The argument for this conclusion is important because it has an immediate bearing on the detection of the extra-solar planets and the so-called 'hot Jupiters'. The key observation is that the stars that are known to possess planetary systems are what astronomers call metal-rich.109 By this it is meant that they contain elements heavier than helium, which is the first product of the thermonuclear burning of hydrogen within the stellar interior. Metallicity, in an astronomical context, is, therefore, a wider concept than the more familiar notion of metals such as nickel and iron. The fact that metal-rich stars also possess planets is probably no coincidence, and such elements as silicon, oxygen, and aluminium are essential, not only for the rocky planets like the Earth, but also for the cores of the gas- and ice

figure 5.9 Surface temperatures of the Earth, from 2 Ga ago and looking forward 1.5 Ga; from a billion years on things are not looking so good. (Reprinted from Chemical Geology, vol. 159, S. Franck, K. Kossacki, and C. Bounama, 'Modelling the global carbon cycle for the past and future evolution of the earth system', pp. 305-317, fig. 9, copyright 1999, with permission of Elsevier Science.)

figure 5.9 Surface temperatures of the Earth, from 2 Ga ago and looking forward 1.5 Ga; from a billion years on things are not looking so good. (Reprinted from Chemical Geology, vol. 159, S. Franck, K. Kossacki, and C. Bounama, 'Modelling the global carbon cycle for the past and future evolution of the earth system', pp. 305-317, fig. 9, copyright 1999, with permission of Elsevier Science.)

giants. The converse is also true: stars of low metallicity will generally lack planets.110 It also seems likely, however, that stars with a proportionally higher metallicity are more likely to be accompanied by 'hot Jupiters'. But, as we have already seen, as such a giant migrates inwards from its original orbit to position very close to its star so it will either destroy or eject any earth-like planets. There is therefore a fine balance in a solar system between having a star with sufficient metallicity to ensure the existence of earth-like planets and having a star with too high a metallicity that will lead to the spawning of destructive 'hot Jupiters'. These observations can then be combined with calculations of the rate of star formation (and thus, in suitable circumstances, the formation of planets) and the assumption of a steady build-up of metallicity. This last assumption is based on the fact that the metals (in the astronomers' sense) are synthesized within the star, and may subsequently be blasted into interstellar space during a supernova (where more metals are made). Remnants of such supernovae eventually reaggregate as new stars, which will tend to be more metal-rich. Hence the overall conclusion is that, given these trends, the majority of planets similar to the Earth will have been formed at a substantially earlier stage. Plenty of time not only for life, but also for intelligence to evolve? So where are these planets?

The irony that the destructive cataclysm of a supernova is a necessary prerequisite for life, because only thus can the elements essential to biology be broadcast into space and ultimately gathered together in a new solar system, has often been remarked upon. Yet any biosphere unlucky enough to be relatively close to a neighbouring star that does explode will face a traumatic interval. Nor is this the only risk. Sudden and extremely energetic bursts of gamma-ray radiation are regularly detected by Earth-based instruments, although their origin is still rather mysterious. Perhaps they result from the collision of two neutron stars. Such events release some 100000 times more energy than a supernova, a colossal 1053 ergs,111 so placing biospheres thousands of light years away at risk. Now it is curious that calculations by Ray Norris112 suggest that such events should afflict the Earth roughly every 200 million years, yet here we are. Even if this risk factor (as well as that of more proximal supernovae) is decreased it does not easily explain the apparent immunity of our biosphere. One explanation (if that is the word) is that if 'we have already survived for some 20 times the mean interval between catastrophes ... [then we are] very lucky indeed.'113 If that is correct then one conclusion might be that 'we are alone in the galaxy.' James Annis,114 however, offers an interesting twist to this prospect, suggesting that while these gamma-ray bursts would reset the evolutionary clocks across much of any galaxy that experienced such an intense pulse of radiation, over time the frequency of these catastrophic events would decline, and ultimately the emergence of intelligence would become possible. On Earth the fossil record indicates that this process took about 108 years. What applies here should, on the Copernican principle of mediocrity (that is, there is nothing special about the Earth) apply elsewhere. Thus Annis suggests that as the Galaxy becomes relatively safe, so

'A previously forbidden configuration is now allowed. It is likely that intelligent life has recently sprouted up at many places in the Galaxy and that at least a few are busily engaged in spreading. In another 108 years, a new equilibrium state will emerge, where the galaxy is completely filled with intelligent life.'115

There is another turn to the story of habitable zones that might also narrow the likelihood of finding a real counterpart to Earth. This is not in the context of distance from a star, but within the galaxy as a whole. Guillermo Gonzalez and colleagues suggest that galaxies like our Milky Way also have relatively restricted zones of habitability.116 There are several reasons for this that range from the potentially violent nature of the galactic centre with its great density of stars, some of which will inevitably explode, not to mention the resident black hole, to possibly more frequent perturbations affecting the equivalents of the cometary Oort Cloud, so greatly enhancing the bombardment rates of the inner planets. In addition, as we have already seen, the relative metallicity of a star is correlated to the likelihood of its possessing both planets and destructive 'hot Jupiters'. Metallicity of stars, however, declines away from the galactic centre, and this also helps to define, at least approximately, the habitable zone of a galaxy.

That certainly still leaves millions of candidate stars, but, as Gonzalez and his colleagues argue, this leaves the Sun (and thereby the Earth) occupying 'an especially comfortable region of the Milky Way'. Recalling the paradox of the Fermi question ('Where are they?'), they continue, 'Any civilization seeking a new world would, no doubt, place our solar system on their home-shopping list.'117 Not only are we probably alone in our Galaxy, but looking further afield may suggest that any neighbours are very, very remote. As Gonzalez and his colleagues also remark, 'The broader universe looks even less inviting than our galaxy. About 80 per cent of stars in the local universe reside in galaxies that are less luminous than the Milky Way. Because the average metallicity of a galaxy correlates with its luminosity, entire galaxies could be deficient in Earth-size planets. Another effect concerns the dynamics of stars in a galaxy. Like bees flying around a hive, stars in elliptical galaxies have randomized orbits and are therefore more likely to frequent their more dangerous central regions. In many ways, the Milky Way is unusually hospitable: a disk galaxy with orderly orbits, comparatively little dangerous activity, and plenty of metals.'118

a cosmic fluke?

Earlier I remarked on the almost gleeful abasement of humans, not least to inform us that we are insignificant worms in the cosmic drama. One powerful ingredient of this dreary world picture is the Copernican triumph, of Earth the Insignificant. Perhaps so, but it could be that our planet and its Solar System are both very much odder than is realized. Life may be a universal principle, but we can still be alone. Suppose, at least for the sake of the argument, if not humility, we really are. Suppose also that the Earth is genuinely a cosmic accident, a chance fluke arising from spinning clouds of dust and gas. Such a view is now widely accepted, but so, too, are such principles applied with equal conviction to the history of life. From its starting point, however and wherever that strange event might have been, orthodoxy states that evolution can potentially explore a million different trajectories. Even if somewhere there is a planet like the Earth, so the argument continues, there may well be life but assuredly no biped writing lines similar to these. In the next few chapters I shall try to persuade you to take another view.

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