The Drake Equation In the autumn of 1961 Frank Drake, who had made the very first SETI observations in Project OZMA, hosted a three-day conference at Green Bank, West Virginia, USA, to discuss the likelihood that other civilisations in our galaxy might be trying to make contact with us. Drake had realised that what, on the face of it, seemed an impossible task, could be broken down into a number of parts, each of which could be looked at separately. The individual parts could then be combined to enable an estimate to be made. He thus set out the agenda of the conference in the form of an equation, now famously known as the Drake Equation. The concept is based on the assumption that other lifeforms require similar conditions to those here on Earth. Each part of the equation comprised either a number or a factor and during the conference the team of scientists discussed each term in detail and made their best estimate of its value. The individual terms were then placed within the Drake equation to evaluate the number, N, of civilisations with whom we might communicate: N = ( R* x fp x ne x fl x fi x fc) x L The terms of the Drake Equation Initially, let the terms in brackets (which will be discussed later) be combined together in a value Rcc so that the equation reduces to just: N = Rcc x L Rcc is the rate at which civilisations desiring to communicate with others come into being, and L is the average time for which they remain attempting to do so. Imagine that one was looking down on the Galaxy and each time a communicating civilisation arose, a light began to shine. It would then remain shining, on average, for a time L years and then turn off. As other civilisations arise further lights would come on and then eventually go off again. We want to know how many lights, on average, will be on at any one time. For example, let us suppose that such a civilisation comes into being about every 10,000 years. If such civilisations attempt to make contact with others for 1,000,000 years on average, then we would expect there to be around 100 in our galaxy from whom we might possibly detect signals. On the other hand, if they were to attempt to make contact for only 1000 years then there would be a one in ten chance of another civilisation attempting to communicate at the present time. It might be noted that our civilisation here on Earth would not be included in a similar calculation for elsewhere in the Galaxy, as we are not yet making any serious attempts to communicate with others. The Rcc term Now let us look at the term Rcc a little more closely. It starts with a numerical term R* which is the rate at which suitable stars are forming in the Galaxy (More on R*). These are stars which are hot enough and live long enough to allow time for an intelligent civilisation to evolve. Not surprisingly our Sun fits the bill, so we often refer to these as Sun-like stars. In fact we really want the value of this term as it was a few billion years ago when it was almost certainly larger than now and had a value close to 1. The other terms The remaining factors that make up the term Rcc generally reduce this initial value as it is not expected that every suitable star would eventually give rise to a communicating civilisation. In turn these are: fp = the fraction of those stars which have a planetary system This value is, at present, unknown, but other solar systems are now being discovered around Sun-like stars and on dynamical grounds we expect that they should be common. A possible value might be 1 in 5 - that is 0.2. More on fp. ne = the number of "earth-like" planets in a solar system. This is the average number of planets suitable for life that you would find in a typical solar system. This was set, perhaps optimistically, at 1 by the Green Bank scientists. More on ne. fl = the fraction of these planets on which life arises. Life arose here on planet Earth virtually as soon as conditions were suitable for it to do so. We thus hope that this factor might be reasonably high - perhaps between 0.1 and 1. More on fl. fi = the fraction of these life forms that evolve into intelligent civilisations like ours. This took a long time on our planet - over 3.5 billion years. So it cannot be that easy and cometary and asteroid impacts may prevent it happening very often. This is a tough one to estimate. More on fi. fc = the fraction of these civilisations that choose to attempt to communicate across the Galaxy. Again this is very hard to estimate. We could be, but are not yet, seriously attempting to communicate. More on fc. How accurate is the current estimate of N? The problem is that while some of the factors involved in the evaluation of Rcc are reasonably well known, we can only make educated guesses for others. Neither do we have any real idea of the typical value for L (More on L), so our final estimate for N is not expected to be accurate. In fact it has been said that the Drake Equation is a way of encapsulating a lot of ignorance in a small space! Evaluations of N in the early days of SETI were probably on the optimistic side with values of up to 1,000,000 considered possible. Some now say that intelligent civilisations will arise only rarely and thus that we might be the only one existing in our Galaxy at the present time. The true answer will no doubt lie somewhere in between and the SETI projects could perhaps be regarded as an experimental way of finding the answer of how often advanced civilisations arise.