In 1960 Dr Frank Drake attempted to create a way to guess how many intelligent civilisations like our own would arise in our galaxy and in the universe. The question of if we're alone has driven astronomy and space exploration for centuries and is one of the greatest unanswered questions we have left.
What Drake wanted to do was to try and work out how likely it was that we'd ever make contact with another civilisation, or if the probability of intelligence arising was so remote that we'd spend our time very much alone. The result of his attempts is the now famous Drake Equation, the formula that essentially powers SETI and helps astronomers to focus their searches for artificial signals in the universe.
The Drake equation works backwards, looking at each factor that has lead to intelligent life evolving on Earth and then trying to predict how likely each event is to occur again elsewhere. By breaking down the factors leading to the evolution of intelligent life it is believed that we'll be able to narrow down the search, hopefully with greater chances of success.
The Drake Equation is composed of seven parts, each thought to be a contributing factor as to how easy it is for an intelligent species to arise. A crucial factor to remember is SETI's definition of intelligent life. The search is focusing on finding another species in the galaxy that is capable and willing to communicate through radio transmission or some other form of electromagnetic radiation. Using Earth as an analogy for life elsewhere it is assumed that as we leak radio transmission into the galaxy, then so would another species.
The problem with looking for radio transmissions is that it assumes two things. One is that an intelligent race will use radio for communication and be leaking it into the galaxy. The other is that they will continue to use it. It is actually very cost-ineffective to broadcast in all directions, and we can see this on Earth where broadband and fibre optic cables are now the new standard.
Soon we will leak very little information into space as we focus all communication in its intended directions. If we again use Earth as an example it could be said that any species just a few decades ahead of us in technological development would no longer leak radio into the cosmos, and a species a hundred years younger would not have developed radio yet. By looking for radio transmissions we're looking for a narrow space in a species' development, however it is assumed for the purpose of the equation that the alien life we're looking for are willing communicators and would let themselves be known, much like us.
The Drake equation is written as N = R starred x fp x ne x fl x fi x fx x L. N is the number of civilisations in the Milky Way with which communication may be possible. The other figures are the factors that are considered vital for the appearance of intelligent life, based on observations in our own Solar System and what we know of life on Earth.
R starred is the rate of star formation in the galaxy. Currently the figure for this is put at seven per year. Life cannot exist without a star, and so this is seen as the first step. Fp is the fraction of stars that form that support planets. As we discover more exoplanets by the week this number is creeping up and up, current estimates put the figure at around 0.5, but the figure is certain to rise. Our current technology is only reliable at detecting Jupiter size planets, but once we start finding Earth analogs in other solar systems we'll have a much better grasp of how many planets there are for potential aliens to evolve on.
Ne is the number of those planets where life can be supported. Life as we know it requires liquid water, which requires a certain temperature band known as the habitable zone. Life also requires an atmosphere, which only a planet meeting certain criteria will be able to hold. Based on observations of the Solar System we see only one planet orbiting that is capable of supporting liquid water on its surface and therefore, life. It is thought that Mars may once have had water on the surface, but due to its low mass it has lost must of its atmosphere and so water cannot stay as a liquid on the surface.
The next figure, Fl, is for how many of those planets that are capable of supporting life will develop life. On Earth we find life everywhere there's water. Even without light at the bottom of the sea, or buried deep in layers of arctic ice, if we find water there's at the very least bacteria. This suggests that if conditions are right then life is certain to arise, putting Fl at a value of 1.
Fi and Fc look at the chance of intelligent life evolving and then being willing to communicate with other planets. On Earth only one species in the four and a half billion year history of the planet has evolved intelligence and radio technology, which puts these figures quite low. Again, these guesses have a certain degree of bias as we only have one example to look at. It may prove that if life starts on a planet that the path to sentience is inevitable. It may also prove an extremely rare event, we won't know for sure and so these figures are largely guesswork.
L is the number of years that the civilisation will last. Currently this figure is set at 10,000 years. Given that mankind has already developed the capability to destroy ourselves through nuclear weapons (developed just seven years after radio astronomy) it might be guessed that the self destructive ability goes hand in hand with interstellar communication. Our civilisation teetered on the brink of nuclear war during the Cold War, and the threat, while greatly reduced, has not entirely evapourated.
Given that some civilisations that develop the capability to destroy themselves will destroy themselves then it cannot be assumed that once intelligent life has evolved that it will last indefinitely. L is a guess, but is used to guess the window in which we have to communicate with a species from their development of communication to their eventual downfall.
Using Drake's estimations the figure for the number of species able to communicate with us in the Milky Way is 10. The Milky Way contains between 200 and 400 billion stars, so the chances of finding an alien species are around one in thirty billion. Different estimates of parts of the equation will give higher values. For example if L is set to 50,000 years then there are 50 civilisations in the Milky Way. More optimistic outlooks can set the number of communicating aliens at 5,000 per galaxy.
The figure can change wildly depending on the number plugged into the equation, and at SETI they're working on minimising the guesswork for each of the factors to help them narrow their search. With any luck we'll find our first alien broadcast within the next few decades, and if we do, a lot of the success will be attributed to the Drake equation.
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