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In 1961, astronomer Frank Drake developed a method for estimating of the number of extraterrestrial civilizations in the galaxy.  The Drake Equation simply begins with the number of stars, then multiplies that number by a series of probabilities (probability that a star has planets, that a planet can support life, etc.) to obtain the quantity desired (number of populated planets, number of civilizations whose existence is detectable, etc.).

In 1961, astronomer Frank Drake developed a method for estimating of the number of extraterrestrial civilizations in the galaxy.  The Drake Equation simply begins with the number of stars, then multiplies that number by a series of probabilities (probability that a star has planets, that a planet can support life, etc.) to obtain the quantity desired (number of populated planets, number of civilizations whose existence is detectable, etc.).

In principle, the Drake Equation can provide an upper and lower bound for the number of extraterrestrial civilizations by setting upper and lower bounds for each probability factor, which can then be improved upon as better information becomes available.  Because the number of stars in so huge, even the lower bound is often calculated to be a respectable number.  However, a serious problem is that some of the probabilities are completely unknown, or even unknowable.  For example, the probability of life emerging on a planet is a meaningless concept if life is created directly by God and not by some naturalistic process.  If any of the probabilities appearing in the equation is zero, then the number of extraterrestrial civilizations of course goes to zero.

In principle, the Drake Equation can provide an upper and lower bound for the number of extraterrestrial civilizations by setting upper and lower bounds for each probability factor, which can then be improved upon as better information becomes available.  Because the number of stars is so huge, even the lower bound is often calculated to be a respectable number.  However, a serious problem is that some of the probabilities are completely unknown, or even unknowable.  For example, the probability of life emerging on a planet is a meaningless concept if life is created directly by God and not by some naturalistic process.  If any of the probabilities appearing in the equation is zero, then the number of extraterrestrial civilizations of course goes to zero.

== Extraterrestrial Life in Science Fiction ==

== Extraterrestrial Life in Science Fiction ==