Purpose/2. Life/The Probability That Life Exists Elsewhere
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It is not difficult to estimate the probability that life has developed on other planets in the universe. All we need do is calculate the total number of stars, the number of these that may support habitable planets, and the likelihood that any of these planets would support life.[41]
First, the number of stars in our universe. It is estimated that our Milky Way galaxy contains between two and three hundred billion stars (2-3 x 1011), and that there are about one hundred billion galaxies (1011) in the visible universe. If the average number of stars in other galaxies is similar to ours, then there are 2-3 x 1022 stars all told. Using the smaller number we have 2 x 1022 stars to start with.
Second, there are many inhospitable zones within all galaxies (the planets of stars too close to the centre of a galaxy or to radiating black holes, for instance, are being sterilized by microwaves[42]) and stars in these regions are unlikely to support life-bearing planets, at least, not life as we think of it.[43] Let’s guess that only one tenth of each galaxy’s stars are clear of these areas, giving us 2 x 1021 stars in hospitable zones of the universe.
Third, using observations noted in section four of the previous chapter, we can guesstimate that some fifty percent of all stars possess planetary systems, so about 1 x 1021 (or 1021) stars are predicted to have orbiting planets.
Fourth, many exoplanets likely do not possess the conditions we consider necessary to support life (water, appropriate temperature ranges, appropriate elements and minerals, energy sources such as sunlight or planetary heat, etc.). A reasonable guess might be that of those possessing planetary systems, only one star in ten will hold a planet that is habitable. This gives us 1020 stars or 1020 habitable planets.
Fifth, we do not know if life will always arise on habitable planets.[44] If, as is turning out to be likely, the molecules from which life originates can form in space-ice, then probably all of the universe’s planets will have been inoculated by now. How much of this material then goes on to create life can only be a guess. Presumably, if the right conditions exist, eventually all will; but, to err on the conservative side, let us say that only one in a hundred habitable planets becomes a host to life.[45] Thus about 1018 (1,000,000,000,000,000,000 or one quintillion) life-bearing planets possibly exist in the visible universe. Of these, about 107, or ten million, could be in our own galaxy.
As we learn more about the nature of life and our universe, we will undoubtedly revise our estimates of the number of planets that could be home to living entities. The number may decrease or increase, even significantly, but it is very unlikely that the number will turn out to be one. Statistically, therefore, it is highly improbable that our planet is the only one to bear life; the universe contains an incredibly large number of stars, and the conditions and ingredients required to start and support life probably exist in many, many millions of places. Furthermore, these places may include intergalactic space, within gases where life’s precursors may first have formed, then evolved, to create living entities that waft through the heavens in forms vastly different from ones we might recognize.
