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The Green Bank Formula Help (page 2)

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By McGraw-Hill Professional
Updated on Sep 17, 2011

Planets Suitable For Life: N E

Suppose that we look at a large number of star systems with planets. Some of these planets will have environments suitable for the evolution of life as we know it; others (probably most) will not. If we are able to look at a large enough sampling of star systems with planets, we will come up with a number n e , the average number of life-supporting planets per planetary system. The fact that a planet can support life does not necessarily mean that life exists but only that the environment is such that life could exist there. We have seen only one planetary system thus far in enough detail to get any idea of the value of n e , and statistically, it is nowhere near enough. We might guess that n e = 1 if our Solar System is an average one. However, the more we get to know our planet Earth, the more we realize what a special place it is. Again, let’s be conservative and suppose that there exists a planet suitable for life on only 1 out of every 2 star systems that have planets. Then n e = 0.5.

Development Of Life: F L

If a planet is ideal for the development of life, there is no guarantee that life arises and evolves. A large asteroid or comet impact would cut evolution short if it were violent enough. An unfavorable change in the behavior of the parent star also would snuff out life. A close call with a passing celestial object, such as a neutron star or a black hole, would disrupt the orderly nature of the planetary orbits of the star system. The big question is this: Was life created, and did it get going on its evolutionary way on Earth because of a series of flukes so rare as to have a “combined probability” of almost zero? Scientists have created complex molecules thought to be the precursors of living matter in a laboratory, but this is not the same thing as synthesizing life and demonstrating that its formation is a common thing.

The best we can do with respect to f 1 , the proportion of planets suitable for life on which life actually develops, is make a wild guess. Let’s call it 0.1, that is, only 1 out of every 10 planets with a good climate can support life.

Evolution Of Intelligence: F i

On Earth, life evolved to near perfection in the form of the dinosaurs. However, none of them had brain power approaching that of primates such as monkeys, let alone human beings. If it were not for a supposed small asteroid or comet splashdown around 65 million years ago, the dinosaurs would still be here, and the Earth would be a vastly different place. Many evolutionary scientists think that Homo sapiens wouldn’t exist. Evolution would never have produced our forebears. Dinosaurs would have eaten them!

Major cosmic collisions, once life has started to evolve on a planet, are not too likely. According to the model of Solar System evolution currently accepted, by the time life was underway on Earth, most of the debris from the primordial solar disk had been swept up into the planets and their moons. But minor collisions are common; we can expect that there will be more of these on Earth yet to come. Interestingly, these minor collisions can serve as a catalyst for evolution, not a fatal blow, as would be the case with a major collision.

What proportion f i of planets where life has gotten started undergo evolution to the point where intelligence arises? The answer to this question dictates the range of values we can realistically assign to f i . Evolution and natural selection seem to be relentless processes; we have seen adaptation of species all the way down from ourselves (if we consider Homo sapiens a species of animal) to bacteria that develop immunity to antibiotics and viruses that evolve new forms, evading extermination. Given the relentlessness of the evolutionary process and the relative likelihood of changes in the environment that spur the emergence of new evolutionary pathways, we might assign f i a value of about 0.1. This is a conservative estimate.

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