NASA/Earth Observatory

The River Nile from space highlights the intelligent species that lives on Earth. But how many other civilizations are in our galaxy that we could communicate with?NASA/Earth Observatory

OK, I've had enough. I've been looking up at the night sky for 20 years and not once have I ever seen anything that has aroused my suspicion that an alien visitor has popped by Earth to take a look.

The thing is, I am contacted far too often by people saying they have seen an unidentified flying object, or UFO. Being terribly literal, they probably have seen something "unidentified," and it may look like it's flying; whatever it is, it certainly is an "object," but it doesn't mean it's aliens.

I've never seen anything that makes me think UFOs are alien in origin. But ask me if I think aliens exist, somewhere, anywhere, and I answer with a loud and affirmative yes. Whether UFOs are alien or not will be a discussion that rages for decades, but the question about microbial life in the solar system or intelligent life in the Universe at large, is a different and fascinating one. If "they" are out there — and for the purpose of this article let's assume they are — then how many alien civilizations can we expect to come across?

In the 1960's, U.S. astronomer Frank Drake started the first real search for alien radio signals being beamed across the Universe. Off the back of this work, he came up with an equation that is at best, an educated stab at the number of civilizations in our Galaxy with which we might actually be able to communicate right now.

This is a fascinating possibility, and one that has an interesting relationship with the Fermi Paradox. This paradox explains that there seems to be a distinct lack of evidence of such communicating civilizations regardless of the final number that drops out of the bottom of the equation. Perhaps technologically advanced civilizations have a habit of blowing themselves up before getting the signal out?

Let's get down to some mathematics. The equation, in its pure form, looks like this:

N = R* × fp × ne × fl × fi × fc × L

It looks horrible, but it's really easy to understand. Its purpose is to take some numbers we can estimate from our existing knowledge rather than totally guess, plug them all together and come up with the magic number "N" — an estimate on the number of contactable civilizations.

We can start by taking the average rate of star formation (R*) in our Galaxy, the Milky Way. A strange factor to consider perhaps, but understanding how many stars form each year allows us to ultimately consider how many civilizations are born each year. Current theory suggests this number, in the Milky Way, is about 10 new stars per year.

The second item fр defines the fraction of those stars which have planets and, according to current scientific values, is something like 50 percent.

Of the remaining values, we can actually look closer to home and learn from our very own solar system. Taking ne and fΙ, which explain the number of planets that can support life and the fraction of those planets where life actually evolves, and applying to the Solar System, will suggest a couple of possible options.

Obviously, Earth can support life (as we know it) and perhaps in the past so has Mars, this gives us an answer of 2 for the first point but we should also perhaps consider Europa and Titan as they, too, may harbor conditions where life, however primitive or weird, may be able to evolve, giving us an answer of 4 instead. The fraction where life evolves is at least 1 but it's thought that if the conditions are right, then life will evolve to some degree, at some time, leaving f

We can then apply some decent and pretty reliable numbers to the final factors in Drake's equation. fⁱ is the fraction of those life bearing planets where intelligent life evolves. From our own Solar System we know of only one place where intelligent life has evolved (although sometimes I do wonder) leaving us with 25 percent.

According to the SETI organization, fⁱrefers to the "fraction of civilizations that develop a technology that releases detectable signs of their existence into space" and comes in at a meager 1 percent.

Finally comes "L," the length of time that the civilization releases detectable signals into space, which, using humans as an example, this technological advancement can be estimated at 10,000 years.

So this is the exciting bit, plug those numbers all in to the equation and you come up with... (drum roll please) ...50!

Is that really it?

The number of contactable civilizations in our Galaxy, right now, that we might communicate with, is just 50... fifty? It's estimated that there is around 400 billion stars in our Galaxy and, according to my numbers (which, by their nature are educated guesses), there are just 50 alien civilizations that we could communicate with.

You only have to think about the vast scale of the Galaxy spanning 100,000 light years diameter (that's big) to realize the chances of us actually being within communicable distance seems pretty small. A depressing thought.

But does it mean we should give up our attempts at finding ET? After all, with so few of them in our Galaxy, finding a needle in a haystack would be relatively easy by comparison. No, quite the opposite, I think we absolutely should, indeed must, continue our search for our alien cousins (and before you infer it, I'm not suggesting we are related).

One of the great things about our species is the unswerving desire to explore and discover. Though the task is daunting, it is in our nature to try. If the alternative is to sit on our bums and mope, assuming we are alone (or worse, not even caring) then I know which direction I would choose.