How Rare Is Life?
Until about 30 years ago, the ranges of conditions thought essential to life, such as those of temperatures and humidity, were thought to be narrow. Since then, scientists have found extremophiles (organisms that thrive in extreme conditions) living in adverse environments on Earth. Organisms may live deep in ice sheets or in boiling-hot water around vents in the ocean floor. Some exist in communities robbed from sunlight and live on energy from chemical sources. Bacteria are even found living 2 miles (3km) deep in the Earth's crust, living on hydrogen, which they convert to water. Extremophiles have encouraged the idea that life can exist in a wide range of conditions. Some scientists are still hopeful that extraterrestrial life will be found in the solar system, although exploration of the most likely location, Mars, has proved negative so far. Beyond the solar system, many scientists think that life must be widespread. At these remote distances, scientists are most interested in whether intelligent, contracting life exists. In the 1960s, American radio astronomer Frank Drake developed an equation for predicting the number of civilizations in the galaxy capable of interstellar communication. Because few of the factors in the equation can be estimated accurately, applying it can have an outcome less than one to millions, depending on the estimated values. Nevertheless, it is not unreasonable to suggest that at least a few such civilizations may exist in the Milky Way.
The Equation
Rate of start birth: A fair estimate would be 50 new stars per year in the Milky Way.
Stars With Planets Perhaps 50 percent of these stars develop planetary systems.
Habitable Planets: On average, maybe only 0.4 planets per system are habitable.
Planets With Life: Life may well develop on 90% of habitable planets.
Intelligent Life: Possibly about 10% of new instances of life develop intelligence.
Communicating Life: Possibly only 10% of such life develops interstellar communications.
Life Span of Civilization: These civilizations might, on average, last 10,000 years.
Conclusion: Using the estimates above, one might expect there to be about 50x0.5x0.4x0.9x0.1x0.1x10,000=900 alien civilizations in our galaxy that, in theory, we should be able to communicate with. However, some of the estimates may be wildly wrong.
Stars With Planets Perhaps 50 percent of these stars develop planetary systems.
Habitable Planets: On average, maybe only 0.4 planets per system are habitable.
Planets With Life: Life may well develop on 90% of habitable planets.
Intelligent Life: Possibly about 10% of new instances of life develop intelligence.
Communicating Life: Possibly only 10% of such life develops interstellar communications.
Life Span of Civilization: These civilizations might, on average, last 10,000 years.
Conclusion: Using the estimates above, one might expect there to be about 50x0.5x0.4x0.9x0.1x0.1x10,000=900 alien civilizations in our galaxy that, in theory, we should be able to communicate with. However, some of the estimates may be wildly wrong.