Just the right size

In a fascinating discussion of alternative scenarios Jack Lissauer73 invites us to consider Earth-like planets, but somewhat smaller and larger than our familiar home (Fig. 5.7). In neither case is life necessarily excluded, but if it were present the biosphere would probably be impoverished as compared with what we enjoy, or at least should enjoy. On a smaller planet gravity is weaker, the consequences of which are much more precipitous mountains, a thinner atmosphere, and substantially lower surface temperatures. The internal heat budget of this smaller planet would be reduced, and its more rigid crust might rule out plate tectonics. There would probably not be large oceans, and possibly only shallow seas. On this rather undynamic planet, life might be constrained, not least because levels of atmospheric oxygen would be substantially lower than on Earth.

What of a planet substantially larger than the Earth? It would have stronger gravity to be sure, leading to a much more subdued topography and probably a globe mostly, if not entirely, covered in oceans. As we saw earlier, such a possibility was also raised for an excessively water-rich moon that might orbit an extra-solar giant planet. While aquatic life might develop in either situation it would be a problematic habitat because without continents there would be a greatly reduced influx of nutrients and minerals derived from weathering, although volcanic and hydrothermal activity might provide some compensation. Even if intelligent life, perhaps dolphin-like, emerged, the transition to a technology based on metallurgy and controlled combustion would probably be impossible. So, if we wish to meet conceptualizing beings we might concentrate on a planet pretty much the size of the Earth with a large Moon. It would be a further curious coincidence if the Moon were of a size and at a distance to eclipse exactly the nearby sun during the time a sentient species was becoming

figure 5.7 A comparison of Earth-like planets, with our Earth in the middle flanked by smaller (left) and larger (right) versions. Smaller means drier and precipitous landscape, while bigger means an ocean-world with little, if any, emergent land. (Reproduced from fig. 13.9 of Planetary sciences, by I. de Pater and J. J. Lissauer (Cambridge University Press, Cambridge, 2001) with the permission of the authors and Cambridge University Press.)

figure 5.7 A comparison of Earth-like planets, with our Earth in the middle flanked by smaller (left) and larger (right) versions. Smaller means drier and precipitous landscape, while bigger means an ocean-world with little, if any, emergent land. (Reproduced from fig. 13.9 of Planetary sciences, by I. de Pater and J. J. Lissauer (Cambridge University Press, Cambridge, 2001) with the permission of the authors and Cambridge University Press.)

interested in astronomy.74 Perhaps that is not asking too much - until, that is, we look at the Solar System's goalkeeper, mighty Jupiter.

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