In our case, we will replay the tape of our planet's formation. We begin with a planetary nebula of exactly the mass and elemental composition that created our solar system. According to most theorists, this might create a star identical to our sun—but then it might not. For instance, the spin rate of the new star might be different from that of our sun, with unknown consequences. Well, then, 1000 such solar nebulae, perhaps 1000 clones of dear old Sol. Not so, however, with the planets coalescing out of this mix. If we rerun this particular tape, we will in all probability not get a repeat of our solar system with its nine planets, its one failed planet (now the asteroid belt), a Jupiter and three other gas giants orbiting outside four terrestrial planets; and a halo of comets surrounding the entire mix. Now we enter the realm of multiple contingencies. Of the 1000 newly formed planetary systems, none is likely to be identical to our solar system today—just as no two people are identical. In a coalescing planetary system many processes, including planetary formation, may be chaotic.
Planets form in what are known as "feeding zones," regions where various elements come together and eventually coalesce into planetesimals, which finally aggregate into a planet. Recent work by planetary scientists shows that the spacing of planets will probably be fairly regular. There might be as few as six planets or as many as ten or even more. James Kasting of Penn State University believes that planetary spacing is not accidental—that the positions of planets are highly regulated, and that if the solar system were to re-form many times, we would get the same number of planets each time. Yet the observational evidence to date does not back up the theory. The extrasolar planets that have been discovered exhibit an enormous diversity of spacing and orbits; their positions are not nearly so orderly as the theory suggests they should be. Ross Taylor, an astronomer who received the prestigious Leonard Award in 1998, disputes Kasting's views. "Clearly," he maintains, "the conditions that existed to make our system of planets are not easily reproduced. Although the processes of forming planets around stars are probably broadly similar, the devil is in the details."
No one knows whether a planet the size of Jupiter would always form or whether there would be a couple of planets like Mars instead. A planet would probably form in about the position of Earth, but it could be larger or smaller, somewhat closer to the sun or father away. Would the material (physical) quantities be essentially the same? Would plate tectonics develop? Would there be the same amount of water—and would that water end up on the surface of the planet, rather than locked up in its mantle or lost to space? Would there be few threats to life from Earth orbit-crossing asteroids? What is the chance that our Moon would form again, if, as we believe, it is important in making Earth a stable place conducive to animal diversification?
Even if all of these events occurred more or less the way they have, would life form again? And given life, would animal life appear once more? Can there be animal life without the utterly chance events that occurred in Earth's history, such as a Snowball Earth or an inertial interchange event, for instance?
Let us reorganize (and rephrase) this set of questions in the following way. We might ask: How many of all planets in the Universe are terrestrial planets (as opposed to the gas giants such as Jupiter, for instance)? What is the percentage for all planets of the Universe? (In our solar system there are five, but if we add the larger moons, that number more than triples.) Of the terrestrial planets in the Universe, how many have enough water to form an ocean (either as water or as ice)? Of those planets with oceans, how many have any land? Of those with land, how many have continents (rather than, say, scattered islands)? Yet these questions are only for the infinitesimally small slice of time we call the present. All of these conditions are subject to change.
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