Somewhere Over the Rainbow

by Nicholas Mee on July 15, 2017

In 1960, the astronomer Frank Drake weighed up the options for his first attempt to find extraterrestrial intelligence. Two nearby stars Epsilon Eridani and Tau Ceti, 10 and 12 light years distant, seemed like promising candidates. Both are similar to the sun with around four fifths of the sun’s mass. They were ideal targets for Drake’s quest for alien civilizations Project Ozma, named after the queen of the land over the rainbow in the books of L. Frank Baum. Nothing was known about any planetary systems that Epsilon Eridani and Tau Ceti might have; thirty years would pass before the first discovery of an exoplanet in any system. Tau Ceta is now thought to have a retinue of at least five planets with masses ranging from two to six Earths. Drake pointed the 26 metre radio telescope at Green Bank, West Virginia towards each of the two stars with great anticipation and listened for anything unusual that might indicate the presence of alien communications. Nothing was heard. This was the beginning of SETI (the search for extraterrestrial intelligence).

The Green Bank Radio Observatory.

Quantifying Our Ignorance

Undeterred, Drake attempted to estimate the number of alien civilizations in our galaxy and rally support for a wider search. As Drake was well aware, determining the likely evolution of intelligent beings on other worlds is an almost hopeless task given our limited knowledge of fundamental questions such as the origin of life and the nature of intelligence. Nonetheless, he set these difficulties aside and devised a formula, now known as The Drake Equation, for calculating the number of detectable civilizations. It combines all the conditions that are necessary for alien civilizations to develop. Even today, sixty years later, we remain in the dark about several of the terms. But Drake believed that the equation would focus attention on where our ignorance lies, which would ultimately help to resolve these issues.

The numbers that Drake required to determine whether SETI might succeed are:

R* – the average rate of star formation in our galaxy

fp – the fraction of stars that have planets

ne – the proportion of planets that could potentially support life

fl – the fraction of those planets that actually develop life

fi – the fraction of life-bearing planets where intelligent civilizations have developed

fc – the fraction of such civilizations to have developed communications technologies, such as radio, that are detectable from other star systems

L – the length of time for which such civilizations emit detectable signals

When multiplied together these terms give a figure N representing the current number of alien civilizations in our galaxy whose signals we might hope to detect:

N = R* · fp · ne · fl · fi · fc · L

A colourful representation of the Drake equation. Credit: SETI Institute.

Drake knew that there was great uncertainty in most terms in his equation, but he was keen to show that there was good reason to continue the search for extraterrestrial signals. He convened a meeting of distinguished scientists at Green Bank Observatory, with expertise in all the diverse fields that were relevant to the equation. After considering each term, the team concluded that given current knowledge of astrophysics and the evolution of life, their best estimate was that the galaxy should be home to around 10,000 advanced civilizations, therefore a serious search for extraterrestrial intelligence stood a real chance of success. Drake reckoned that if around ten million stars were monitored, eventually detection was assured, although this might take decades or even centuries. At the end of the conference the delegates raised their champagne glasses and Otto Struve, the director of Green Bank, offered up a toast:

‘To the value of L. May it prove to be a very large number.’

Finding Your Very Own Alien

In the decades since the Green Bank conference there have been ever more sophisticated attempts to scan the heavens with radio telescopes on the lookout for signals from extraterrestrials. Much of this work is now coordinated by the SETI Institute based in California. Project Phoenix, which began in 1995, has used some of the world’s largest radio telescopes at Green Bank, Arecibo in Puerto Rico and the Parkes Observatory in Australia to target 800 sun-like stars within a range of 200 light-years for signs of alien communications. So much data has been collected that processing the data is now the bottle neck. To tackle this issue the SETI@Home project was set up in 1999. It has become one of the biggest distributed computing projects in the world. Everyone is welcome to get involved by visiting the website: Once the SETI@Home software has been installed, any computer downtime will be utilised to analyse data from Arecibo and Green Bank while a screensaver runs in the foreground.

The SETI@Home screensaver.

Is There Anybody Out There?

The time available for SETI research is quite limited at the long established observatories. But now a dedicated radio observatory is under construction. The Allen Telescope Array (ATA) is named after Microsoft co-founder Paul Allen who is funding the project. It is intended to perform full sky surveys and will combine pure astronomy research with SETI. The array began operations in 2007 with 42 six metre radio dishes and when complete there will be 350 dishes. The ATA will increase the targeted analysis of star systems by a factor of at least 100. It is hoped that over the next two decades stellar reconnaissance will increase to a million or more nearby stars that are deemed possible hosts of life-bearing planets.

In the years since Drake devised his equation there is at least one relevant area where our knowledge has advanced significantly. We now know that most stars have planetary systems and the results of the Kepler Space Telescope suggest that the galaxy contains at least 40 billion rocky planets. So there is no shortage of planetary real estate.

With so many rocky planets available we might assume that life is abundant in the galaxy and that technological civilizations abound. But is this really true? The great nuclear physicist Enrico Fermi thought not. He argued that it could not be so, but that is another story.

Further Information

The SETI Institute website can be found at:

Details about the Allen Telescope Array can be found here:




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