Fate of the Earth

One issue of immediate importance is the fate of Earth's biosphere and, on even longer time scales, the fate of the planet itself. As the Sun grows older, it burns hydrogen into helium. Compared to hydrogen, helium has a smaller partial pressure for a given temperature, so the central stellar core must grow hotter as the Sun evolves. As a result, the Sun, like all stars, is destined to grow brighter as it ages. When the Sun becomes too bright, it will drive a runaway greenhouse effect through the Earth's atmosphere (Kasting et al., 1988). This effect is roughly analogous to that of global warming driven by greenhouse gases (see Chapter 13, this volume), a peril that our planet faces in the near future; however, this later-term greenhouse effect will be much more severe. Current estimates indicate that our biosphere will be essentially sterilized in about 3.5 billion years, so this future time marks the end of life on Earth. The end of complex life may come sooner, in 0.9-1.5 billion years owing to the runaway greenhouse effect (e.g., Caldeira and Kasting, 1992).

The biosphere represents a relatively small surface layer and the planet itself | lives comfortably through this time of destruction. Somewhat later in the Sun's evolution, when its age reaches 11-12 billion years, it eventually depletes its store of hydrogen in the core region and must readjust its structure (Rybicki and Denis, 2001; Sackmann et al., 1993). As it does so, the outer surface of | the star becomes somewhat cooler, its colour becomes a brilliant red, and its radius increases. The red giant Sun eventually grows large enough to engulf the radius of the orbit of Mercury, and that innermost planet is swallowed with barely a trace left. The Sun grows further, overtakes the orbit of Venus, and then accretes the second planet as well. As the red giant Sun expands, it loses mass so that surviving planets are held less tightly in their orbits. Earth is able to slip out to an orbit of larger radius and seemingly escape destruction. However, the mass loss from the Sun provides a fluid that the Earth must plough through as it makes its yearly orbit. Current calculations indicate that the frictional forces acting on Earth through its interaction with the solar outflow cause the planet to experience enough orbital decay that it is dragged back into the Sun. Earth is thus evaporated, with its legacy being a small addition to the heavy element supply of the solar photosphere. This point in future history, approximately 7 billion years from now, marks the end of our planet.

Given that the biosphere has at most only 3.5 billion years left on its schedule, and Earth itself has only 7 billion years, it is interesting to ask what types of 'planet-saving' events can take place on comparable time scales. Although the odds are not good, the Earth has some chance of being 'saved' by being scattered out of the solar system by a passing star system (most of which are binary stars). These types of scattering interactions pose an interesting problem in solar system dynamics, one that can be addressed with numerical scattering experiments. A large number of such experiments must be run because the systems are chaotic, and hence display sensitive dependence on their initial conditions, and because the available parameter space is large. Nonetheless, after approximately a half million scattering calculations, an answer can be found: the odds of Earth being ejected from the solar system before it is accreted by the red giant Sun is a few parts in 105 (Laughlin and Adams, 2000).

Although sending the Earth into exile would save the planet from eventual evaporation, the biosphere would still be destroyed. The oceans would freeze within a few million years and the only pockets of liquid water left would be those deep underground. The Earth contains an internal energy source - the power produced by the radioactive decay of unstable nuclei. This power is about 10,000 times smaller than the power that Earth intercepts from the present-day Sun, so it has little effect on the current operation of the surface biosphere. If Earth were scattered out of the solar system, then this internal power source would be the only one remaining. This power is sufficient to keep the interior of the planet hot enough for water to exist in liquid form, but only at depths 14 km below the surface. This finding, in turn, has implications for present-day astronomy: the most common liquid water environments may be those deep within frozen planets, that is, those that have frozen water on their surfaces and harbour oceans of liquid water below. Such planets may be more common than those that have water on their surface, like Earth, because they can be found in a much wider range of orbits about their central stars (Laughlin and Adams, 2000).

In addition to saving the Earth by scattering it out of the solar system, passing binaries can also capture the Earth and thereby allow it to orbit about a new star. Since most stars are smaller in mass than our Sun, they live longer and suffer less extreme red giant phases. (In fact, the smallest stars with less than one-fourth of the mass of the Sun will never become red giants -Laughlin et al., J"7.) As a result, a captured Earth would stand a better chance of long-term survival. The odds for this type of planet-saving event taking place while the biosphere remains intact are exceedingly slim - only about one in three million (Laughlin and Adams, 2000), roughly the odds of winning a big state lottery. For completeness, we note that in addition to the purely natural processes discussed here, human or other intentional intervention could potentially change the course of Earth's orbit given enough time and other resources. As a concrete example, one could steer an asteroid into the proper orbit so that gravitational scattering effectively transfers energy into the Earth's orbit, thereby allowing it to move outward as the Sun grows brighter (Korycansky et al., 2001). In this scenario, the orbit of the asteroid is chosen to encounter both Jupiter and Saturn, and thereby regain the energy and angular momentum that it transfers to Earth. Many other scenarios are possible, but the rest of this chapter will focus on physical phenomena not including intentional actions.

Continue reading here: Isolation of the local group

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