Perhaps the most important question we can ask about the universe is "Are we alone?" We already discussed the possibility of life in the solar system, but there we were really talking about the possibility of microbial life (or even evidence of past life). Just knowing that life can evolve elsewhere is tremendously important. But what about other intelligent life? We can be pretty sure that there are no non-human civilizations on other bodies of the solar system. So, what are the prospects for other intelligent life in the universe?
A useful place to start is with the Drake equation, created by Dr. Frank Drake. This equation expresses the probability of all of the different factors that have to be met in order for an intelligent civilization to exist in our galaxy, and be able to communicate with us. Here is the equation, which we will examine term-by-term:
N = R * • f p • n e • f l • f i • f c • L
The number N on the left is the number of civilizations that exist at any one time in our galaxy that are able to communicate with us. This is the number we want to estimate. See the Drake Equation calculator. See also this discussion of the Drake Equation. Here is an interesting debate on the subject.
The first term on the right, R*, is the rate of formation of stars in the galaxy that are suitable for intelligent life. How can we estimate this number? We could take the total number of stars in the galaxy (about 100 billion), and divide by the age of the galaxy (about 14 billion years), to get about 7/year. What is wrong with this estimate?
- Is the rate constant?
- Do all of the stars that have been born still exist today?
- Are all of the stars that are born suitable for intelligent life to evolve?
The best way to estimate this is to look at the ages of stars in our galaxy and in other galaxies. The number is very uncertain, but is probably about 1/year. Of course, it may have been higher in the past, which is what we care about if we are considering intelligent life (which should take billions of years to evolve).
The next term on the right, fp, is the fraction of such stars that have planets. The value is, by definition, somewhere between 0 and 1. At one time this fraction was very uncertain, since the only example solar system we knew about was our own! Now we know of more than 100 other solar systems, with more being discovered all of the time. As we learn new ways to discover them, undoubtedly more will be discovered. Therefore, this is probably a large fraction, perhaps 50%.
The next term on the right, ne, is the number of such planets in each solar system that could harbor life (are Earth-like?). What number should we choose here? It would be very helpful for us to know more about the chances of life in our solar system. Such planets do not HAVE to be Earth-like. For example, it may be that Europa, the second moon of Jupiter, harbors life in its sub-surface ocean. One could then make the claim that our solar system have at least three such places--Earth, Mars, and Europa. But there may be other solar systems with no Earth-like planets, so it is hard to guess based on only one example. We need to learn more about planets around other stars. We might take this number to be about 1.
The next term on the right, fl, is the fraction of such planets that actually have life. Again, the value is, by definition, somewhere between 0 and 1. And again, it would be very helpful for us to know more about actual occurrences of life in our solar system. The central question here is, how rare is the evolution of life? Was it a completely fluke occurrence, never to happen twice? If so, the value of fl would be zero, and there is no point in going further--we are alone in the universe. If we do discover independently evolved life in our solar system, then this would strongly suggest that fl would be about 1! What do you think? One thing to think about is how fast life appears to have evolved on Earth. The early solar system was under heavy bombardment from planetesimals for at least several hundred million years, yet there is evidence that life evolved very quickly after that--perhaps only 500 million years after the Earth was formed. That would seem to indicate that life evolves quickly whenever conditions are right. I will take a relatively conservative view and use 0.1.
The next term on the right, fi, is the fraction of life-bearing planets that have evolved intelligent life. Again, the value is somewhere between 0 and 1. As we look at the history of life on Earth, it is remarkable that it remained single-celled until only about 500 million years ago. Then there was an explosion of multi-celled life, which quickly left the sea (first as plants, then as animals). After a relatively short period of time, the land surface of Earth was teeming with life. Yet intelligent life (i.e. people) were rather slow in evolving. Only two million years ago, our ancestors first started to use tools. So perhaps intelligence is not very common. Still, 500 million years is not long in astronomical terms. Let's be relatively conservative again and use 0.1.
The last fraction term on the right is fc, the fraction of intelligent life that forms civilizations capable of communicating with us. That means that they use radio or other forms of long-distance communication. Intelligent whales on another planet probably will not be able to communicate with us, since they live in the sea and perhaps would never discover electricity. Still, any intelligent life-form that can use tools and live on land would eventually develop mathematics and science, and invent radio and other long-distance communication. This fraction is therefore probably at least 0.3 or so.
My estimates above result in a value of 0.0015. This brings us to the last term, L, which is the number of years that such a civilization will exist. During the 1960's, when the equation was developed, it was feared that we might wipe ourselves out with nuclear weapons. The speculation was that any civilization following our pattern of development might similarly survive for about 100 years and then blow themselves up. I think this is a very pessimistic view. More likely such civilizations will continue for thousands or millions of years. But here we are looking into the future, which of course is unknown, so it is very hard to imagine how we will change over the course of 1,000 -- let alone 1,000,000 -- years. If we take the lower number of 1,000 years, the number of other intelligent civilizations is only 1.5. If the number is 1,000,000 years, then the number is 1500!
