2 The Calculation
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. 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. Typical probabilities for each step follow, with some steps in the original Drake equation combined for simplicity. You can insert your own values to produce your own forecast.
The rate of star formation in the galaxy
This rate can 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. Here the assumption is 200 billion stars and a galaxy age of 10 billion years. This yields an average rate of star formation of 20 per year. Other values proposed are 350 billion stars and 8.5 billion years. 200 billion is representative of the single and multiple stars that may have planets.
The fraction of stars with planets
The results from the Kepler spacecraft mission suggest that the Galaxy contains 50 billion stars with planets, or a factor of 0.25 planets per star. An estimate from gravitational microlensing suggests. 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 for simplicity. It can be easily changed.
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 solar planets to find out if life has arisen more than once in the solar system. Other stars are being investigated with ground and space-based telescopes to find out 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 required to evolve 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. There is also a theological argument demonstrating 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. The time for national civilizations to merge into a single planetary civilization is assumed to be short and not catastophic.