## At Last, How Many Alien Civilizations are There? — Astrobiology Magazine

on November 30, 2012 at 4:30 pm

Claudio Maccone revisits the Drake Equation. Image Credit: SETI League

## At Last, How Many Alien Civilizations are There?

Posted: 11/30/12

Author: Bruno Martini

**Summary:** Frank Drake’s famous equation for estimating alien life has been altered by one scientist to create the “Statistical Drake Equation”. The results provide good news for the SETI community.

During the space age, 1961 was a special year: the Russian cosmonaut Yuri Gagarin became the first man to orbit Earth, while the American astronomer Frank Drake developed the now famous Drake Equation. This equation estimates the number of detectable extraterrestrial civilizations in our Milky Way galaxy, supposing our present electromagnetic detection methods. The Drake equation states:

N = Ns x fp x ne x fl x fi x fc x fL

N = number of alien civilizations in the Milky Way

Ns = estimated number of stars in the Milky Way;

fp = fraction or percentage of these stars with planets on its orbits;

ne = average number of these planets with potential to host life as we know it;

fl = percentage of these planets that actually develop life;

fi = percentage of these planets that actually develop intelligence on human level;

fc = percentage of these civilizations that actually develop electromagnetic radiation emitting technologies;

fL = percentage of these civilizations that keep emitting electromagnetic signals to space. This factor is extremely dependent on the lifetime a civilization remains electromagnetic communicative.

Looking to the Drake equation factors, it is obvious that none can be precisely determined by modern science. More than that, as we move from the left to right in the equation, estimating each factor becomes more controversial. The later terms are highly speculative, and the values one may attribute to each of them might tell more about a person’s beliefs than about scientific facts.

But the Drake equation must not be evaluated only by the numerical values it produces. Some say the Drake equation is a way to organize our ignorance. By exposing the extraterrestrial intelligence hypothesis mathematically, we limit the real possibilities to each term and approach the final answer: how many alien civilizations are there?

Read more: At Last, How Many Alien Civilizations are There? — Astrobiology Magazine.