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The Green Bank Formula Help

By — McGraw-Hill Professional
Updated on Sep 17, 2011

Introduction to the Green Bank Formula

Suppose that the evolution of intelligent life on our planet Earth is not a miracle. Then extraterrestrial life exists! This part of the problem being solved by faith, let’s play a mind game that involves an attempt to calculate the number of other intelligent civilizations we would find if we could travel freely among the stars, galaxies, and clusters of galaxies throughout the known Universe. This mind game involves cheating on the “probability fallacy.”

We are going to talk about the likelihood , as a proportion, that certain things have happened, are happening, or will happen in our Cosmos. When we say that the chances of some event taking place on a planet are 1 in 100, we really mean that if we could visit a large number of planets, say, 1 million of them, then that event would be discovered on 1/100, or 10,000, of them.

The Conference

In 1961, a conference was held at the Green Bank radio telescope observatory, the same place where Project Ozma was conducted. The object of the meeting was to make an estimate of the number of technologically advanced civilizations (defined as capable of communicating by electromagnetic means such as radio) that exist in the Milky Way galaxy. We know this number is at least equal to 1 because we are here and we have radio. Is the number greater than 1? If so, how much greater?

At this conference, the astronomers, led by Frank Drake and Carl Sagan, developed a formula to determine the number of technologically advanced civilizations in our galaxy. It has been called the Green Bank formula , the Drake formula , or the Drake-Sagan formula . Several factors are involved in this mathematical equation; some of them are probabilities . The complete formula consists of the product of all these factors. Let’s look at the factors one at a time, and then we’ll evaluate the entire formula by “plugging in” some educated guesses.

Star Formation: R

Let R be the average number of new stars that are born in our galaxy each year. There are about 200 billion (2 × 10 11 ) stars in the Milky Way, and the galaxy is thought to be about 10 billion (10 10 ) years old. This might lead one to suppose that an average of 20 new stars are born every year, that is, that R = 20. Let’s be a little bit conservative because we can’t be certain that stars have always formed at the same rate during the lifetime of the galaxy. For our calculations here, let’s use R = 10. This value has been suggested as reasonable by many scientists.

Planetary Systems: F P

Let f p be the fraction, or proportion, of stars in our galaxy that have planets orbiting around them. Until recently, astronomers had almost no idea of what f p might be. However, observations with the Hubble Space Telescope and other instruments have shown that planetary formation is not a fluke. It happens with other stars besides our own Sun. Some estimates of f p range up to 0.5; that is, half of all new star systems include planets. Let’s be more conservative and estimate that only 1 in 5 stars have planets. Thus f p = 0.2.

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