'tis not the king's stamp can make the metal better or heavier.
William Wycherly, The Plaindealer
The biochemistry of terrestrial organisms — and the biochemistry of any extraterrestrial organisms we can plausibly imagine — depends crucially on six elements: sulfur (S), phosphorus (p), oxygen (O), nitrogen (n), carbon (C) and hydrogen (H). To an astronomer, any elements heavier than hydrogen and helium are known as metals. (The metallicity of a star, then, refers to the amount of heavier elements it possesses.) So in astronomical language, life depends upon the five metals SPONC. Soon after the Big Bang, the Universe contained essentially only hydrogen and helium (in the ratio 75% to 25%). The Big Bang would have produced small amounts of lithium, and even smaller traces of beryllium and boron. But that was it: none of the metals required by life were there at the beginning. One of the key findings of modern astronomy is that the heavier elements like SPONC were cooked in nuclear reactions inside stars, and became part of the interstellar medium only when stars reached the end of their energy-producing life. As time goes by, the concentration of metals in the Universe slowly increases.
One resolution of the paradox — often proposed and similar in spirit to Livio's suggestion — is that the heavier elements only recently became sufficiently concentrated in the interstellar medium to allow life to form. Planets around older stars, it is suggested, lack the metals SPONC. Only around quite young stars — stars like the Sun — can life arise. So mankind would inevitably be among the first civilizations, perhaps the first, to arise.
Like many of the proposed solutions we have discussed, the suggestion that the chemical enrichment of the Galaxy resolves the Fermi paradox by itself is too strong. It may be a factor in the final explanation, but for two reasons it is unlikely to stand alone as a resolution.
First, we do not know what metallicity is required of a star for it to possess viable planets. Would an abundance of heavy elements one third that of the Sun suffice? One quarter? One tenth? We simply do not know. So far, no planets have been found around any star that has a metallicity less than 40% that of the Sun, but these observations are in their infancy. If life can develop on planets possessing a much smaller abundance of heavy elements, then very old stars could be home to life.
Second, the metallicity of stars differs between the four stellar populations. Some types of star might be ancient and yet be metal-rich. The four stellar populations consist of the thin disk stars, the thick disk stars, the halo stars and the bulge stars. The halo stars, which form a spherical system around the center of the Galaxy, are old stars. They typically have a metallicity about 1% that of the Sun. Such stars are unlikely to possess planets. The bulge at the center of the Galaxy is old, and yet some of the stars are very rich in metals. However, bulge stars orbit within a few thousand light years of the Galactic center, which is a violently energetic environment. Whether complex life-forms can exist in such an environment is debatable, and too high a metallicity can itself be a problem, so it is safest to ignore bulge stars in these discussions. The thick disk consists of stars that stay reasonably close to the plane of the Galaxy. (But not too close; stars can move a few thousand light years above or below the plane — hence the term "thick" disk.) Such stars are old, and their metallicity is generally 25% that of the Sun. Finally, the thin disk stars, which stay within 1000 light years of the plane of the Galaxy, are the interesting ones from our point of view. Not only is the Sun a member of the thin disk population — so are 96% of its neighbors. These stars have a variety of ages, ranging from objects that are 10 billion years old to stars that have formed only recently. Similarly, the metallicities of the thin disk stars vary: some have less than 1% of the metallicity of the Sun and are poor candidates for life, but some have three times the Sun's metallicity. So the situation is more complicated than at first appears. It seems, though, that within all this variability there are many stars much older than the Sun but with the same abundance of heavy elements.
Consider, for example, 47 Ursae Majoris — a thin disk star only slightly more massive and only slightly hotter than our Sun. By coincidence, on the day I am writing this section astronomers have announced the discovery of a second Jupiter-sized planet orbiting the star.187 The discovery of 47 UMaj c (as the planet is tentatively called, and presumably will continue to be called until astronomers can decide upon a better nomenclature for extrasolar planets) is interesting for two reasons. First, 47 UMaj c is orbiting in a nearly circular orbit around the star, as is its companion 47 UMaj b. This planetary system is the first that might turn out to be like our own, in that the planetary orbits have low eccentricities and the Jupiter-sized planets are at a respectable distance from the star. (So arguing that the scarcity of "good Jupiters" resolves the Fermi paradox, as we do on page 160, may turn out to be wrong.) Second, 47 Ursae Majoris is 2.5 billion years older than the Sun and yet it has essentially the same chemical composition. Thus any Earth-like planet orbiting this star could in principle have given birth to life some 2.5 billion years ago; an ETC on that planet could be in advance of us by 2.5 billion years. This corresponds to almost 22 months in the Universal Year — much longer than the colonization time of the Galaxy.
(It must be stressed that astronomers do not know whether small, rocky planets exist in the inner planetary system of 47 Ursae Majoris. Our present techniques simply cannot detect such objects. Nevertheless, this planetary system is undoubtedly the most similar to our own. The ratio of the masses of 47 UMaj b to 47 UMaj c is 3.3 to 1, which is the same as the ratio of masses of Jupiter and Saturn. The ratio of their average distances from 47 Ursae Majoris is the same as the ratio of the average distances of Jupiter and Saturn from the Sun. Finally, since the observations suggest that there can be no further giant planets orbiting closer to the star than 47 UMaj b, there would seem to be "room" for Earth-like planets to exist. Unfortunately, numerical simulations suggest there are probably no Earths there: 47 UMaj b and 47 UMaj c orbit closer to their parent star than Jupiter and Saturn orbit the Sun, so their gravitational influence would disrupt the formation of terrestrial planets at the correct distance from the star. But one can dream.)
Regardless of whether 47 Ursae Majoris turns out to possess terrestrial planets, the fact remains it is a Sun-like star, it possesses the same chemical abundances as the Sun, and it has planets. The star is a neighbor — less than 50 light years away from us. Yet it is 2.5 billion years older than the Sun. If such stars are in our backyard, there must be many of them in the Galaxy. Perhaps the number of stars that are suitable for hosting life-bearing planets is much smaller than previously thought, but the suggestion that the Sun is among the earliest generation of stars that can give rise to life seems to be untenable.
There is, however, a further point to make. Although our Galaxy may possess millions of old stars with sufficient metals to sustain life, the same is not necessarily true of all galaxies. Elliptical galaxies, for example, generally contain metal-poor stars, and they are not the best place to look for life. Small irregular galaxies, too, are unlikely to be home to life as we know it. Furthermore, globular clusters (collections of millions of stars that orbit larger galaxies) are also metal-poor regions. Although Earth's first dedicated interstellar radio transmission was to the globular cluster M13 (see page 111), the signal is thus unlikely to cross an Earth-like planet there. The chemical enrichment of galaxies may help explain why we do not see K3 civilizations: there might be far fewer galaxies that are suitable for life than we at first expect.
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