Indeed, the only truly serious questions are ones that even a child can formulate. Only the most naive of questions are truly serious.
—Milan Kundera, The Unbearable Lightness of Being
"Do you feel lucky? Well do ya?" —Clint Eastwood, Dirty Harry
How rare is Earth? We have arrived at the end of a long grocery list of ingredients seemingly necessary to make a planet teeming with complex life. It involves material, time, and chance events. In this chapter we will try to assess these various factors and their relative importance; all can be thought of as probabilities. In some cases we understand these probabilities, but in others almost no research has been done, and our questions, like those referred to in the quotation above, are the simple questions of children—questions as yet with no answers. Some of these questions can thus be tackled only with our imagination. Others will be answered by the space voyages and instrumented investigations we discussed in the previous chapter.
Let us begin by imagining we have the power to observe 100 solar nebulae coalesce into stars and the planets that will encircle them. How many of these events will yield an Earth-like planet with animal life?
As we have seen, the first step in preparing the way for a habitable environment is the formation of a suitable star: one that will burn long enough to let evolution work its wonders, one that does not pulse or rapidly change its energy output, one without too much ultraviolet radiation, and most important, perhaps, one that is large enough. Of the 100 applicants, perhaps only two to five will yield a star as large as our sun. The vast majority of stars in the Universe are smaller than our sun, and although smaller stars could have planets with life, most would be so dim that Earth-like planets would have to orbit very close to their star to receive energy sufficient to melt water. But being close enough to get adequate energy from a small star leads to another problem: tidal lock, the condition where the same side of the planet always faces the sun. A tidally locked planet is probably unsuitable for animal life.
What if we increased the number to 1000 planetary systems, so that we might expect 20 stars of our sun's size or greater to be born? Even these numbers are too small to yield a high probability that we will find a truly Earthlike planet. Perhaps a better way to envision the various odds is to re-create the scenario that led to the formation of our solar system and then run through the process once again in a thought experiment. Stephen Jay Gould used this type of mental reconstruction in his interpretation of the Cambrian Explosion. In his 1989 book Wonderful Life, Gould described the exercise as follows:
I call this experiment "replaying the tape." You press the rewind button and, making sure you thoroughly erase everything that actually happened, go back to any time and place in the past—say, to the seas of the Burgess Shale. Then let the tape run again and see if the repetition looks at all like the original.
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