## You Just Love the Drake Equation

With a few hundred billion stars in the Galaxy, the likelihood of life somewhere in the Milky Way other than on the earth seems great. But is there any way to get a handle on just how great? Not every star has a fair shake, however. Some are so massive that they don't last long enough, and others are not hot enough to warm any planets that might be there. Perhaps 15 percent of the stars in the Galaxy have the proper mass to be luminous enough (but not too luminous) to support habitable planets. In the 1960s, one astronomer attempted to roughly quantify the odds that intelligent life capable of communication exists elsewhere in the Galaxy.

In 1961, at a conference on the Search for ExtraTerrestrial Intelligence (SETI), held at the National Radio Astronomy Observatory in Green Bank, West Virginia, Professor Frank Drake proposed an equation to estimate the possible number of civilizations in the Milky Way. The Drake Equation contains a number of highly uncertain terms—variables that must simply be guessed at—however, it is a very useful way of breaking down a complex question into more readily digestible portions. In addition, some of the variables that were highly uncertain in 1961—the number of stars with planets, for example—are becoming more certain as further research is conducted.

Here's what the Drake Equation, in its basic form, looks like:

N = R * x fp x Np x fe x f x f x f x L And here's what the terms of the Drake Equation mean:

> N (the left-hand side of the equation) is the number of civilizations in our Galaxy with which we should be able to communicate via radio signals.

> R* is the rate at which our Galaxy produces stars—its productivity in solar masses per year.

> fp is the fraction of stars with planetary systems.

> Np is the average number of planets per star.

> f is the fraction of Earth-like planets (planets suitable for life).

> f is the fraction of f planets on which life actually develops.

> f is the fraction of f planets on which intelligent civilizations arise.

> fC is the fraction of f planets on which technological civilizations arise.

> L is the lifetime of a civilization in years.

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