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Alan Alda in Scientific American Frontiers

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4 pages: | 1 | 2 | 3 | 4 |

planet imageIn "Alien Worlds," David Latham and his colleagues at the Harvard-Smithsonian Center for Astrophysics use cutting-edge science to locate planets beyond our own solar system. Until 1995, no one had ever found an "extra-solar" planet, and only a handful is known today. Yet, since the 1960's, scientists have been listening for alien signals nonetheless.

What are the chances intelligent life exists beyond Earth? In 1961, radio astronomer Frank Drake presented a means of estimating the number of technological civilizations that likely exist in the galaxy. The equation that bears his name is thought to be among the most important of the 20th Century, second only to Einstein's E=MC2. Drake's Equation factors together all of the assumed conditions required for advanced civilizations to evolve.
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The Mathematics of Life
Here's the equation: Drake Equation

N equals the number of civilizations in our galaxy with both the brains and the tools to communicate with us. N* is the total number of stars in the galaxy, estimated to be between 100 and 200 billion.

fp represents the fraction of those stars with planets. Before the 1995 discovery of the first extra-solar planet, astronomers didn't have any idea what this number might be. Subsequent surveys put fp at about one percent. In a galaxy with 100 billion stars, that's about one billion solar systems.

Photo of Frank Drake
  Frank Drake was the first to listen for extraterrestrial radio signals

But not every solar system is suitable for life. The term ne stands for the number of these planets that are roughly "Earth-like," rocky, temperate and relatively stable. Scientists are just beginning to collect the data needed to guess at a galaxy-wide value for ne.

The remaining variables are even harder to nail down. fl denotes the fraction of these Earth-like planets on which life has sprung out of the favorable conditions. Of those planets that harbor life, only some, represented by fI, will evolve an intelligent species. And only a fraction of those intelligent species, fc, will develop technology.

Drake's equation is thought to be among the most important of the 20th Century, second only to Einstein's E=MC2 .


Finally, L is the lifetime of a communicating civilization. How long can an advanced society exist before political, economic or environmental problems tear it apart? With such subjective variables, it's clear that no single solution to Drake's equation exists. Nonetheless, Drake and colleague Carl Sagan estimated there might be as many as one million communicative civilizations in the Milky Way. That figure, rough as it is, still compels researchers at leading institutes around the world to search for planetary needles in the galactic haystack.

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4 pages: | 1 | 2 | 3 | 4 |

Photos: NASA, Melita Wade Thorpe

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