The Number
The twin questions for extraterrestrial civilizations are how many are there and why have we no evidence them, either through communications or from a visit. Obviously if there is no such civilization, then we know why they have not turned up. On the other hand, their non-appearance does no confirm their non-existence. So the first question is how many extraterrestrial civilizations might exist. If we have a positive answer to this we can ask how far away they might be
Inevitably we approach the first question as members of a civilization on Earth and our estimate is based on our own experience. From this we conclude we are the outcome of biological evolution, based on carbon and making use of RNA and DNA as the means of reproducing copies of ourselves for each new generation. Our initial assumption, not having information to the contrary, is that extraterrestrial life leading to civilization could make use of the same chemistry on a different planet.
New observations have led to updates of the fraction of stars with planets in the habitable zone where liquid water can exist. This makes possible a re-estimate of the possible number of extraterrestrial civilizations in the Milky Way galaxy. Of interest are civilizations who seek evidence of other planetary civilizations, because they might communicate with us. However, the time required for such civilizations to evolve and the time they continue in existence set the limit to the number present. If they are randomly dispersed throughout the Galaxy, that number determines how far away they are and whether communications is possible. Unfortunately, these civilization times are so uncertain that it is only possible to show how assumptions of particular values affect the possibility of communication.
The estimates of the fraction of stars that have witnessed each of the different steps involved in evolution of a civilization are combined in the Drake equation to find the number civilizations that might be in existence. The equation was developed at the 1961 Conference at Green Bank Observatory on Intelligent Extraterrestrial Life. The probabilities of achievement for each step and an estimate of civilization lifetime are simply multiplied together to yield the total number of civilizations. The individual fractional probabilities are as follows (some of the probabilities in the original formulation of the Drake equation have been combined for simplicity).
The rate of star formation in the galaxy
This may be based on the number of stars we see now and the lifetime of the galaxy, or on the rates of formation and burnout of different types of stars. Typically an estimate assumes a current number of stars of 200 billion and the galaxy lifetime of10 billion years. This yields an average rate of star formation of 20 per year. Alternative values that have been proposed are 350 billion stars and 8.5 billion years. 200 billion is selected here to be representative of the single and multiple stars that may have planets.
The fraction of stars with planets
The preliminary results from the Kepler spacecraft mission suggested that the Galaxy contains 50 billion stars with planets, or a factor of .25 planets per star. An estimate from gravitational microlensing suggests that many stars in the galaxy have more than one planet, causing the average number of planets per star to be 1.3. Here, a factor of 1.0 is used as representative.
The fraction of planets favorable to life
This is the fraction of planets in a star’s habitable zone. the region where liquid water would be able to exist on a planet's surface. A recent estimate puts 0.15 planets between 0.5 and 1.4 Earth radii in size occurring in habitable zones of cool type M dwarf stars. Another estimate for sun-like stars with a temperature range of 5,000K to 6,500K, suggests habitable zone planets between 0.5 and 2.0 Earth radii in size occur at the rate of 0.34 per star. As there are many more red dwarfs than sun-like stars, an average figure of 0.2 earth-like planets per star is used here.
The fraction on which life emerges
Significant resources are being spent on space exploration to obtain a value for this fraction. Many spacecraft are being sent to Mars and other planets to find out if life has arisen more than once in the solar system. Stars are being investigated with ground and space-based telescopes to try to detect whether life has evolved outside the solar system. Studies of the resilience of life on Earth are leading to a consensus forming that the emergence of life in the conditions of a habitable zone is highly probable. This has come about as a result of discovery of living bacteria existing near the boiling point of water, in highly alkaline environments, inside nuclear reactors, in Antarctic ice, in sandstone, and in igneous rocks at depths of a kilometer or more, and at the boundary of the troposphere. So it is reasonable to put the probability of a simple form of life being present at 0.5.
The fraction with an advanced civilization
This is a much more difficult estimate because of the complicated sequence of evolutionary steps that lead to intelligent social organisms able to form a civilization capable of interplanetary communication. It is a process that has required 3.8 billion years on Earth, whereas the initial creation of life took a mere 2-300 million years. Problems include accounting for catastrophes of various sorts that regularly destroy most of the species present and the unusual stability of the Earth's environment within benign limits. These and other challenges to evolution of complex animals have led James Trefil in Are We Alone? (a book he wrote with Robert Rood) to argue that such evolution is so extremely rare that civilization in the Milky Way may have emerged only once. A similar conclusion was reached by Peter Ward and Donald Brownlee in Rare Earth, making use of additional information that later became available. There is also a theological argument that demonstrates that there can be only one civilization in the universe.
Lifetime of the Civilization
The remaining factor is the lifetime of the civilization in years -- the length of time it exists after becoming able to of investigate interplanetary communication. In the example of the calculation in Table 1, the fraction of planets where civilization evolves out of the earliest forms of life has been put at 0.0001 (one in ten thousand) and the lifetime of such a civilization is put at 5,000 years
Table 1: Sample Drake Equation Calculation
Stars per Year | 20 |
Planets per Star | 1 |
Fraction in Habitable Zone | 0.2 |
Fraction with Life | 0.5 |
Fraction with civilization | 0.0001 |
Civilization Lifetime, years | 5,000 |
Present Number of Civilizations | 1 |
Present Number of Civilizations = 20 x 1 x 0,2 x 0.5 x 0.0001 x 5000 = 1
On the basis of these arbitrary numbers (Assumptions 1), we are the only civilization present at this time (but how many civilizations have existed in the past has not been calculated.). If the civilization lifetime were 500 years, the number of civilizations would be 0.1, which can be interpreted as the probability that one civilization exists now, or that one civilization would appear in the course of a decade. If civilization lifetimes averaged five million years, there would be 1000 civilizations. Changing the fraction of civilizations emerging from primitive life affects the estimate in a similar way.
It is a very useful feature of the Drake Equation that civilization lifetime combines so easily with the other factors to give civilization numbers. It works because the multiplication by years converts the per-year figures to a pure number. However, the per-year calculations that have been carried out in previous years are still valid and suggest that many civilizations have flourished in the past but are no longer in existence. The implications can be seen by examining the calculation in more detail.
The previous calculation can be understood better by adding an extra term that shows what happens over the course of time, and extending the average civilization lifetime to 10,000 years to allow the Existence of Earth 2.0. Table 2 shows that the combination of factors in the equation before civilization lifetime is inserted can be associated with the period within which one civilization can be expected to appear.
Table 2: Explicit Drake Equation, Assumptions 2
Stars per Year | 20 |
Planets per Star | 1 |
Planets per Year | 20 |
Fraction in Habitable Zone | 0.2 |
Habitable Zone Planets per Year | 4 |
Fraction with Life | 0.5 |
Planets with Life per year | 2 |
Fraction with civilization | 0.0001 |
Planets with civilization, per year | 0.0002 |
Civilization Lifetime, years | 10,000 |
Number of Civilizations | 2 |
The assumptions have been modified by doubling the civilization lifetime to allow for the emergence of Earth 2.0.
In this second example, the civilization lifetime has been chosen deliberately to show the assumptions that lead to the conclusion that there are only two civilization present in the Galaxy at this time at this time. Given a population of 200 billion galaxies in the universe, this still implies many billions of active civilizations. However, as far as our own galaxy is concerned, the assumptions allow for many other civilizations in the past and in the future.
The estimate for concurrent civilizations will increase significantly if we are more optimistic about the ability of civilizations to evolve from primitive life. We could put the probability at one success in a hundred attempts at evolution (0.01). And if we are more optimistic about the average length of their survival, we might put this at 500,000 years. We then estimate that a much larger number of civilizations are present in the Galaxy today. The number are shown in Table 3.
Table 3: Drake Calculation, Assumptions 3
Stars per Year | 20 |
Planets per Star | 1 |
Fraction in Habitable Zone | 0.2 |
Fraction with Life | 0.5 |
Fraction with Civilization | 0.01 |
Civilization Lifetime, years | 500,000 |
Present Number of Civilizations | 10,000 |
Variations in Civilization Lifetimes
In actual fact, civilizations will not all have the same life span, as assumed in the Drake equation. And if variations in lifespan are be taken into account, there will be variations in the type of numbers that the equation throws up. A number less than one, for example, will indicate the probability of a civilization appearing. If the number is low, there will be no civilization present for long periods of time.
The Drake Equation gives the number of civilizations in existence simply by inserting the average lifetime of a Galactic civilization as the final factor. But it does not indicate the variation in current lifetimes, the number of civilizations that have occurred in the past and gone extinct, or the effect of multiple lifetimes and the effect on distances. These variables are implicit in the initial assumptions but rarely addressed explicitly.
Having observed the different fortunes of civilizations on Earth, we can assume that the lifetimes of civilizations are unlikely to be equal to each other. To use one lifetime in the calculation is unrealistic. The effect of different distributions of lifetimes is examined in Multiple Lifetimes.
Cumulative Numbers of Civilizations
To find the number of civilizations that have appeared in the past, we need to know the span of time over which civilizations may appear. The basic assumptions that can be used are as follows. The Solar System, including the Earth, is 4.5 billion years old. On the assumption that the Earth is a typical living planet, I assume that it takes 4.5 billion years for a civilization to evolve. The Galaxy is 10 billion years old. When the first Galactic stars condensed, planets started to form around them; so the first civilization emerged after 4.5 billion years. That is, there are 5.5 billion years for the subsequent population of the Galaxy with civilizations. We arrived after 10 billion years.
The assumptions have been modified by doubling the civilization lifetime to allow examination of the relation between concurrent civilizations and cumulative numbers. (It also allows for the possibility of Earth 2.0.). The total number of civilizations emerging in the Galaxy is shown in Table 3. While it is unrealistic to expect civilizations to emerge strictly on after another, it is important to get some sort of idea of how many might exist at the same time on the average. This is done here by dividing the total time available by the number of civilizations to find out the average time slot available to a civilization. Dividing the civilization lifetime by the length of the time slot gives the average number of civilizations in that time slot. Under the assumptions of Table 2, there are two concurrent civilizations on the average.
This calculation, with its low civilization lifetime, suggests that over a million civilizations have passed out of existence, while only two currently exist. Us and Earth 2.0.
Table 3: Cumulative Number of Civilizations, Assumptions 2
Civilization Time Available, years | 5,500,000,000 |
Planet Civilizations Emerging, per year | 0.0002 |
Total Number of Civilizations | 1,100,000 |
Available Sequential Time-slots | 5,000 |
Assumed Civilization Lifetime, years | 10,000 |
Concurrent Civilizations | 2 |
Table 5: Cumulative Civilizations, Assumptions 3
Civilization Time Available, years | 5,500,000,000 |
Planet Civilizations Emerging, per year | 0.02 |
Total Number of Civilizations | 110,000,000 |
Available Sequential Time-slots | 5000 |
Assumed Civilization Lifetime, years | 500,000 |
Concurrent Civilizations | 10,000 |
Theological Implications
A theologian's examination of the implications of extraterrestrial civilizations has indicated that there can only be one planet in the universe that has a civilization.. If Earth is the only planet in 5.5 billion years on which a civilization has evolved, then the fraction of planets in the Galaxy on which primitive life gives rise to a civilization is 0.0000000001. Whether the probability of a civilization arising in 5.5 billion years is one in a billion is a matter of opinion.
The theological argument leading to the conclusion that there is only one civilization in the Galaxy made use, perhaps unwisely, of the reductio ad absurdum argument, based on two premises. The argument examines the conclusion reached if both premises are true. If an absurd conclusion is reached, then at least one of the premises is false. An absurd conclusion was indeed reached by the argument, so the premise that extraterrestrial civilizations exist was determined to be false. If, in fact, extraterrestrial civilizations are found to exist, then the other premise is false, namely, the Christian dogma of salvation.
Multiple Lifetimes Spacing between Civilizations
Likelihood of Communications