Are we alone in the Universe?
The Drake Equation

Previous “Habitable Planets” – Next “Astrobiology
There is on average one planet orbiting every star in the Universe (Swift et al., 2013; Cassan et al., 2012). If this sounds exciting, you might wanna read the previous post in this series. Our Galaxy (the Milky Way) is an immense disk of gas and stars with a diameter of about 100 000 light years, hosting about 100 billion stars and, therefore, also about 100 billion planets. Take a deep breath. Now, it turns out the Milky Way is just one of 100 billion galaxies that populate our Universe, a colossal expanding stretch of spacetime with an age of 13.7 billion years. The math is trivial: There are about 10 000 000 000 000 000 000 000 = \(10^{22}\) planets out there. This number is extremely large. Apparently larger than the number of grains of sand found in every beach and every desert on Earth.

But how many of these planets host life? And in particular, how many planets host intelligent life we might be able to communicate with?

In order to estimate the number of technological civilizations that might exist among the stars, in 1961 Frank Drake proposed the following simple equation:

The Drake equation: it estimates the number \(N\) of civilizations in The Milky Way Galaxy whose electromagnetic emissions are detectable. Interactive version here.

It is a product of factors giving the number \(N\) of civilizations in the Milky Way Galaxy with whom we could make contact. The terms in the equation are:

  • \(R\) is the rate of star formation, which tells how many stars are born every year in our Galaxy.

  • \(n_e\) is the average number of habitable planets in any planetary system

All the \(f\) terms are factors \(\le 1\):

  • \(f_p\) is the fraction of stars that have planets

  • \(f_l\) is the fraction of habitable planets hosting life

  • \(f_i\) is the fraction of life-bearing planets that develop an intelligent life-form

  • \(f_c\) is the fraction of intelligent life-forms that decide to communicate

Finally \(L\) is the longevity of a communicative civilization (in years). Humankind has been “communicative” only for a few decades and it’s hard to predict what \(L\) will be for us. However I think that the current lack of detection of E.T. communications could be used to put some limits on the maximum longevity of a communicative civilization. More on this in a follow up post.