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Another Look at the Drake Equation
or
E.T. Phone Home, But Was Anyone There? (Part II)

by Billy Cooke

Last month I presented my reasons as to why I feel that the current SETI experiments imply that there are fewer than one million intelligent communicable civilizations in the galaxy. I also pointed out a major problem with the current (and planned) SETI experiments, namely that a civilization must transmit in the radio frequencies we consider "likely" channels for interstellar communication in order to be detectable. If the intelligences "out there" do not transmit in this set of frequency ranges, then we won't detect there and we are wasting time and money. This month I am going to discuss the Drake equation, that infamous equation which supposedly gives the number of intelligent communicable civilizations in the galaxy, and hopefully convince you that civilizations are a rarity in the galaxy, numbering far fewer than the one million figure quoted by many SETI advocates. Here goes...

Let me begin by writing down the Drake equation, after which I will explain what each symbol means ...

N = S x P x E xL x I x C where

N is the number of communicable civilizations in the galaxy,

S is the number of stars in the galaxy capable of sustaining life,

P is the probability that a star will have planets,

E is the probability that a planet is capable of sustaining life, i.e., the chances that a planet is habitable,

L is the probability that life will evolve on a habitable planet.

I is the probability that intelligent civilized life will evolve from less complicated organisms, and

C is the fraction of the star’s lifetime that the civilization lasts.

Before we begin, a word of caution -- When a scientist talks about life, he is referring to life as we know it. We are not postulating the existence of life forms based on other elements, like silicon. Further, we assume that the same principles that govern the evolution of life here on earth also apply to other worlds. With that in mind, let us now look at each of the terms of the Drake equation and see if we can come up with some numbers.

"The number of stars capable of sustaining life (S) -- We know there are about 400 billion stars in the galaxy. But are all of these capable of sustaining life as we know it? The answer is definitely not as intelligent(?) life on earth has taken 5 billion years to evolve, requiring the Sun to have been stable for this length of time. Stars much bigger than the Sun have a lifetime of only a few hundred million years, so we can eliminate them as candidates. Stars much smaller than the Sun do not produce much in the way of heat requiring any possible life-support planets to be very close to the star’s "surface" and vulnerable to flares. Further, such stars are often variable in the amount of radiative energy they produce, again making it tough to develop life. We can therefore conclude that a star must be solar to the Sun in order for it to be capable of sustaining life. Such stars account for about ten percent of the stars in the galaxy, so

S = 40,000,000,000 or 40 billion

The fraction of stars with planets (P) -- As of yet we have no conclusive evidence for planets in orbit about other stars. However, there are some tantalizing clues and hints (some of which hopefully will be resolved by Space Telescope) that most stars have planetary systems. Lacking any better figure, let's assume seventy-five percent of the stars in the galaxy have planets. Thus,

P = 0.75 or 3/4

The number of habitable planets around a star (E) -- If we use our Solar System as an example, we get a number of 1. However, the latest computer models indicate that life can develop on a planet only if it meets the following conditions

1. The planet must have a mass between 85% and 120% that of Earth

2. The planet must rotate fairly rapidly, i.e., have length of day similar to that of Earth.

3. There must be large amounts of liquid water (oceans) on the surface.

The scientists believe this models estimate that only about one out of every one hundred solar-type stars will possess such planets. Based on these results, let us assign B = 0.01 or 1/100

The probability that life will develop on an habitable planet (L) -- We really have no idea as to the value of this part of the equation. Let us therefore adopt a middle-of-the-road stance and set L equal to 50%, thus assuming that half of all habitable planets develop life. So

L = 0.5 or 1/2

The probability that life will evolve into a communicable civilization (I) -- To get an idea Of this number we turn to the work of evolutionary biologists, who are the experts in this sort of thing. They say that the odds of an intelligent species developing from single-celled organisms are about 1 in 100 million. In fairness, I will state that this is the majority opinion, and that there are notable dissenters, such as Stephen Gould, who argue that this number is much closer to one. But let us stick with the majority opinion and assume

I = 0.00000001 or 1/100,000,000

The fraction of a star's lifetime that a communicable civilization exists about it (C) - This is another one of those "Pick a number" terms. How long can a civilization survive? Will we blow ourselves to bits in the near future? If we do so, and all civilizations follow suit, destroying themselves after a few hundred or thousand years, then C is very small, on the order of one in ten million! But if this is not the case, then there should be no reason why a civilization could not last through most of a star’s life, perhaps even moving to another star as theirs dies. Again let us take a middle-of-the-road stance and say

C = 0.5 or 1/2

Now let us put everything together and see what results:

N = 40,000,000,000 x 0.75 x 0.01 x 0.5 x 0.00000001 x0.5
or N = 0.75

Hmm.... According to these numbers, it would seem there is only one civilization in the galaxy -- us! Certainly not a million. Then, why SETI? Because a SETI scientist would argue that I is much closer to one, say one in a hundred, giving

N = 750,000

which is much closer to the often-quoted figure of one million. It ll boil down as to which term is dominant -- the number of stars capable of supporting life (S), or the chances of a civilization evolving from single-celled organisms (I). If the SETI people are right then S is the dominant part and we have lots of civilizations. If the majority biology opinion is right then I dominates and we could very well be alone in the galaxy. Personally, I’ll listen to the biologists -- after all they're the experts when it comes to life. I don't necessarily think we're the only civilization in the galaxy, but I do believe there are very few of them out there (less than one hundred). As a result, I don't think we will ever detect any signals from alien civilizations, and so SETI is wasting much-needed money that could help revitalize a floundering space program. Even if I'm wrong, and there are many civilizations in the galaxy, we will eventually discover them as we move out into the galaxy.

That's it. If I've got you to think a little about SETI and extra-terrestrial intelligence, then I've accomplished my purpose, whether you agree with me or not. Thinking is a great thing for a person to do -- astronomers should do it more often.

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