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Semi-major axis (AU)

Semi-major axis (AU)

figure 5.5 Making terrestrial planets. Panel above shows a simulation with 154 planetary embryos with a bimodal distribution of masses: half the mass is housed in 14 larger embryos, and the other half in the remaining 140 miniature planets. In this simulation the basic structure of the solar system with four main planets has emerged by about 30 Ma, although about 25 smaller planetismals remain, of which many plunge into the star. The panel opposite shows the end-product of sixteen of the simulations, in comparison with our Solar System (top). Simulation 23 is the end result of the panel above. Relative sizes of planets are shown; also shown are their perihelion and aphelion distances (the horizontal line), i.e. the nearest and furthest distances to the star that thus define the eccentricity of the orbit. Spin axes of each planet are depicted with an arrow. Reprinted from Icarus, vol. 152, J. E. Chambers, Making more terrestrial planets, pp. 205-224, figs 3 and 10, copyright (2001), with permission from Elsevier Science, and also the author.

Such models give tantalizing glimpses of what might be the reality of other worlds, but at present they remain computed constructs, virtual possibilities that for one reason or another might never be manifest. On the other hand, there is, as we have already seen, a known reality of extra-solar systems (within the limits of how best to interpret minute fluctuations of distant starlight). What needs particular emphasis is that the configurations of these newly discovered systems

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