Nuffield Radio Astronomy Laboratories, Jodrell Bank

The Probability of contacting other Civilisations

In order to see how likely it is that we might be able to communicate with some other civilisation within our galaxy we need to be able to estimate how many such civilisation there are within it.   This then allows us to calculate th probability of one being within the range of our search; in the case of the current Phoenix programme this is 200 light years as all the chosen target stars are within this distance.

In 1961 Frank Drake, who conducted the very first SETI search, produced the now famous Drake Equation . This derives an estimate of the number of communicative civilisations N by multiplying together a number of factors which together determine the overall probability. It is important to note the word estimate because few of the factors are known with any degree of certainty. The trouble is that we are basing many of our estimates on the basis of a single example - us.   However the situations is improving a little.   For example other Solar Systems are now being discovered and the continuation of this work will eventually give us quite a good estimate of the number of suitable stars which have solar systems, this being one of the factors involved. The equation is :-

N   =   R *   x   f p   x   n e   x   f l   x   f i   x   f t   x  L


N is the number of communicative civilizations.

This in the number of civilisations in our Galaxy, the Milky Way, whose radio emmissions are detectable.

R *   is the rate of formation of suitable stars.

The rate of formations of stars with a sufficiently large zone around them in which the surface temperatures would allow the development of lifeforms and whose lifetime is sufficiently long so that they could then evolve into intelligent life.   These, not surprisingly, tend to be like our own Sun, so are called Sun-like stars.  This rate is approximatly 1 star per year.

f p   the fraction of these stars with planets.

The fraction of Sun-like stars with planetary systems is not yet known precisely, but already around a dozen other planets have been found around such stars and so it is likely to be a minimum of 5%, probably nearer 10%.   Lets say a probability of 1 in 10.

n e   the number of suitable planets per planetary system.

These are those where the surface temperatures would allow liquid water to exist.  This implies that it must be sufficiently large to be able to keep an atmosphere.   For example both the Earth and the Moon are within the habitable zone of our Sun but the Moon has too little gravity to be able to keep an atmosphere so is lifeless.   In our Solar System, Venus is too hot and Mars is now too cold but would have been warmer when active volcanoes gave it an atmosphere.   Lets say a probability of 1.

f l   the fraction of those planets where life develops.

Life may well not develop on all suitable planets, but there is no good reason to believe that it won't - it appeared here on Earth virtually as soon as conditions became suitable!   Lets say a probability of 1.

f i   the fraction of these where intelligent lifeforms evolve.

This is one of the major uncertainties!   On Earth it took a further 3000 Million years before multicellular life appeared.   This suggests that the transition from very simple lifeforms to more complex ones is not easy.   So complex lifeforms capable of reason and the other attributes of intelligence could be quite rare.   This is a real guess.   Lets say a probability of 1 in 1000.

f t   the fraction of these where technology develops.

Given that the planet has a benign period long enough to allow unhindered development this should be quite likely.   Lets say a probability of 1 in 1.

L   the "Lifetime" of communicating civilisations.

This estimate is a real problem.   We have no idea how long our civilisation will last.   Assuming that we learn how to protect our planet from stray asteroids or comets and can learn to live within the limitations of our Planet, we could survive for 100's of millions of years.   So L could be very large.

What is the result?

Multiplying up the factors apart from L gives us the equation:-

N   =   L / 10,000

So you can see that the result depends critically on L.   If we are conservative and give a lifetime of 100,000 years then we would expect perhaps 10 other civilisations to exist at the present time.   But, optimistically, with a lifetime of 100,000,000 years, N could be 10,000.

You can see that depending on how optimistic or pessimistic we are in estimating the probability that a technological civilisation will arise and similarly for its lifetime, our Galaxy could now contain many technological civilisations or, alternativly, we could be the only one.   The key is really the lifetime of a civilisation once it has reached a stage when it could communicate with us.   There is no reason not to believe that other such civilisations have arisen in the past and will do so in the future, but they have to exist now for any chance of contact with them.   They also have to have both sufficient spare resources and the will to transmit a message for us to receive - it would be pointless if civilisations all made attempts to receive signals but none was prepared to transmit them!   It does seem sensible, however, for those civilisations who, like us, have just acheived the ability to communicate across space to attempt to receive signals in the first instance.

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