Is there anyone out there?

It's very unlikely, says Arnold Wolfendale.

When lecturing on the search for extra-terrestrial life and the future of life on earth, I usually start by asking "how many of you believe that there is intelligent life elsewhere in the galaxy - and how many believe that we are unique?" On average, only 3 per cent of the audience are convinced we are alone. This is fascinating in view of the complete lack of evidence for "anyone out there".

The early church taught that we are unique and thus the early scholars who thought about these things and concluded that there was no reason for us to be so, had a very difficult time. The 16th-century monk Giordano Bruno is a case in point. His famous statement: "Innumerable suns exist; innumerable earths revolve about the suns in a manner similar to the way the seven planets revolve around our sun. Living beings inhabit these worlds I" led us to his being burnt at the stake.

The point about the earth not being the centre of the universe is, of course, an important one. Copernicus is credited with the first realisation that the earth revolves round the sun and his view got a great boost from Galileo's observations in 1609 using the newly invented telescope. Galileo, too, had a hard time from the church, although the "house arrest" he received was a much lesser fate than that meted out to Bruno.

Accepting we are on a planet orbiting a rather ordinary sort of star and, in view of the very large number of stars in the galaxy and the similar number of galaxies in the universe, there would be thought to be many, many sites where life could - and perhaps should - have developed. Hence the popular view that "we are not alone".

Care is needed, however, for a number of reasons. The first, and most obvious, question is "if life is common, where is it?" There is no evidence for aliens having visited the earth and, despite efforts, no radio signals have (yet) been received that can be attributed to such beings. The likelihood of extraterrestrial life must, therefore, be studied from the theoretical standpoint.

It is necessary, first, to consider what is meant by "life". Here, I use it to mean some sort of "beings", not too dissimilar from ourselves, which are "intelligent" and are able to communicate. A second question is "for how long will life on earth continue?" The factors that enter into the problem will have relevance to other planets, around other stars, which may be hypothesised to harbour intelligent life.

Man-made "disasters" are the first consideration. These are manifold, ranging from population and disease to global warming and ozone depletion. But it is hard to envisage such factors leading to the complete annihilation of life on the planet. A more significant threat is from astronomical phenomena.

An obvious parameter for the earth is the life expectancy for the sun and thereby the earth. The sun was formed about 4.5 billion years ago and it has a similar time left. After a few billion years the swelling of the sun will start to make the earth uninhabitable and it will then be time to go. It is just possible that giant solar flares will occur every few million years but they would be most unlikely to destroy all life on the earth.

Another potential astronomical hazard is that of cometary impact. Calculation shows that comets probably strike the earth every hundred million years or so and they could be life-threatening. The "death of the dinosaurs" as the result of one or more comets striking the earth 65 million years ago - a theory that is probably correct - is an example of a near-fatal encounter.

A further potential hazard is planetary instability. Resonances could build up that would cause another planet to impact with the earth, with catastrophic results, which is very unlikely indeed.

The perspective for other planets that harbour life of the hazards that face our earth is interesting. In our case, the terrestrial situation is much better than that for many of these other planets. For example, our low rate of cometary impacts is due largely to the presence of the very massive planet Jupiter, which deflects many potentially dangerous comets. Our planetary system is very stable indeed, a situation not commonly found in Monte Carlo attempts to generate artificial planets from pre-stellar material. There are thus good reasons to believe life elsewhere is not as common as might have been thought.

It has taken us more than four billion years to get where we are, starting the clock when the earth was formed. It might be thought that this was long enough for any planet with roughly the right conditions of temperature, gas content and so on for life to develop, but this is by no means obvious. Indeed, many biologists feel the probability of (intelligent) life creation is astronomically small, even on the galactic scale.

It is apparent I am not in the "life is common" camp. As well as the arguments advanced already, there is the "where are they?" question. Why is there no trace of past alien visitors or obvious radio beacons in the sky? The argument that sensible advanced civilisations will keep to themselves does not hold water for reasons to do with the finite life of the parent star. Many stars have been and gone in the galaxy and the (postulated) inhabitants of their planets will have had to leave. Intelligent life somewhat further advanced than ours should have had little difficulty in constructing and servicing the necessary spacecraft for their colonising journeys. Thus, if life were common, say many thousand inhabited planets in the galaxy so far, we would surely have some record of its existence.

Intelligent, recognisable life is rare, with less than a few locations in the galaxy. Indeed, perhaps three per cent of the population is right and we are unique.

Sir Arnold Wolfendale is president, the Institute of Physics, and professor of experimental physics, the Royal Institution. He is emeritus professor of physics, Durham University, where he carries out his research. On February 8, he will deliver the 15th Harland lecture on "The Search for Extraterrestrial Life and the Future of Life on Earth" at Exeter University.