N = A x B x C x D x E x F x G x H x I
How can we estimate how many advanced civilizations might exist in our Galaxy? Frank Drake (now President of the SETI Institute) conceived the Drake Equation. It comes in various forms, but its aim is to help us think clearly about the factors involved in detecting such civilizations.
The number of civilizations in the Milky Way Galaxy whose radio emissions are detectable.
Stars form and are destroyed.
The fraction of stars that are single (not members of double or multiple star systems), and yellow/white.
The value here is probably 1: all single, Sun-like stars should have planets, we think.
All stars have a habitable zone where a planet would be able to maintain a temperature that would allow liquid water. A planet in the habitable zone could have the basic conditions for life as we know it.
Although a planet orbits in the habitable zone around a star, other factors are necessary for life to arise, inclufing geological ones. Thus, only some fraction of suitable planets will actually develop life; but it might be close to 1.
Life on Earth began over 3.5 billion years ago. Complexity took a long time to evolve. On other life-bearing planets it may happen faster, it may take longer, or it may not develop at all.
January 8, 1997
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