It is 400 years since Giordano Bruno was burnt at the stake for heresy. He believed that the earth was not at the centre of the universe and that life on Earth was not unique. He was right about the former, but what about the latter?

Asked the question "are we alone in the Universe?" most people believe that the answer is "no," even when the question specifies that life elsewhere must be able to communicate with us in an intelligible form. Supporters of the "life is unique" brigade have a big hill to climb. Let me start the ascent.

The argument of those who believe that we are not alone runs as follows. There are many stars in the galaxy (many more in the Universe) and many have planets. A large fraction of these will have the conditions necessary for our kind of life: oxygen, water, and so on. It is then only one step to the evolution of intelligent life, et voila: "Life is common." End of problem.

True, planets are quite common around other stars, and their presence has been demonstrated by a number of scientists using the techniques of optical astronomy; it should not be long before planets with the ingredients of life are detected. Moreover, the recent work on the Mars meteorite, with the possible detection of bacteria-style fossils, is intriguing. Although there is considerable argument about the accuracy of these findings, free water was found on the early Martian surface and there may well be elementary life beneath the surface now. It would not be surprising if elementary life ó Martian-style ó were common.

It is the next step ó a step which took 3 billion years on Earth ó which worries me: the transition from elementary biological systems (perhaps extending as far as lower animals) to the polished intelligent beings who now inhabit Earth. My worries are twofold.

The first concerns the factors which can snuff out life before it reaches the intelligent category. Setting aside naturally occurring problems of climate, fertility and so on, there are astronomical hazards associated with the impact of extraterrestrial bodies, enhanced emissions from the Sun, and effects from other stars. This problem is arousing considerable interest, not least because the US military has found a new enemy to replace the Russians. Although it looks as though Earth could last a billion years before the doomsday comet arrives, this period could be much shorter if smaller comets, or even near-misses, were seriously to disturb the climate. The situation may be much worse on other "earths" if the comet rain were denser, which is quite likely because the Earth is protected by Jupiter. Also our Sun is, fortunately, very quiescent ó which is why we are here ó although it is likely that dramatic solar flares occur every 1m years. Many other stars are much less stable, with prospects for life on their planets correspondingly reduced.

When allowance is made for these factors, the odds on "life is common" are considerably reduced. But what clinches the argument is my second difficulty. This concerns the answer to the question: where are they? We, on Earth, have probably only about 1 billion years to go before the solar radiation level renders life unbearable and we have to leave. Certainly, in about 4 billion years, the Sun will run out of fuel and swell alarmingly. When we ask "where are they?" we are not asking "why have we not detected any signals from them?" but rather, "why are they not here in person?" The point is that, if intelligent life were common, there should have been colonisation by the inhabitants of planets around the many other stars which have come to the end of their lives. If the "people" leaving the other planets were similar to us, Earth would have seemed an appealing place to settle. But there is no evidence of their presence.

The simplest conclusion is that these other civilisations do not exist. What, then, is the point of spending large sums searching for their signals using radio receivers (Jodrell Bank, Arecibo, and so on)? My own view is that there is a good case to be made for such spending and for the work of your own institute, dedicated to the Search for Extraterrestrial Intelligence (Seti). For a start, the "we are alone" argument could be wrong. And something unexpected in another area ó perhaps of purely astronomical interest ó might be discovered. Furthermore, there is the stimulus to technology from the development of your listening equipment. But I remain sceptical about hearing "them" out there.

Yours,
Arnold Wolfendale

Dear Arnold Wolfendale
28th April 2000

You do me a disservice. You don't believe the intelligent extraterrestrials are out there, and yet you think that I and my colleagues should continue to bang away searching for them ó not so much because we might succeed, but because we could stumble upon some intriguing spin-off. This is like encouraging James Cook to sail to the South Pacific by appealing to his possible contribution to ship design.

You begin with the currently accepted view that planets capable of incubating life are probably common. I agree, of course. Indeed, estimates of the numbers of planets which populate our galaxy is usually tallied in tens of billions. You then concede that biology will likely spring up on many of these other worlds; once more, I concur. But you then argue (along with Peter Ward and Donald Brownlee in their recent book Rare Earth) that while life may be commonplace, intelligent life ó the kind which could make its presence known via radio signals hurled across the vast spaces between the stars ó is rare, and possibly absent altogether. In this view, we inhabit an enormous galactic zoo populated by lesser creatures. Humans, you suggest, are the smartest things in the Milky Way. This is a nice point of view, at least for humans. But given the enormity of the Universe, and the lesson against cosmic hubris first taught by Copernicus, we should be suspicious of this proposition. You offer two reasons why we should not expect extraterrestrial companions. The first is that the slow progression from simple to sentient life will often be stopped in its evolutionary tracks. Intelligence on Earth was slow in coming. It has been 3.6 billion years since life arose on our planet, and 600m years since the Cambrian explosion of complex, multicellular creatures. This long R&D phase before nature produced humans does suggest that the whole process is vulnerable to even a rare catastrophe. But one didn't happen here ó and Earth is not likely to be an astronomical rarity.

Certainly there have been mass extinctions, the most famous being the destruction of three-fourths of all species ó most notably the dinosaurs ó 65m years ago. This disaster was probably caused by a rock from space ó an asteroid or comet ó about 10km in size. But neither this nor any of the other calamities to befall our planet (including at least one period of global glaciation) have succeeded in terminating biology. The chain of life has snaked unbroken through every disaster for over 3.5 billion years.

Life is durable. And, as you say, we should probably thank Jupiter for keeping the inner solar system relatively free of large comets (although Jupiter also pulls some of these missiles our way). But there is no reason to think that large planets able to deflect lethal rubble from the cradles of life are scarce. Indeed, planet-hunters have found that at least 3 per cent of Sun-like stars have Jupiter-like planets.

Your second argument is that aliens should be in the neighbourhood ó driven to colonisation by imploding stars ó but they are not. This argument, as you know, dates from a remark by the physicist Enrico Fermi in 1950. Indeed, it provoked a cottage industry of research intended to explain how aliens might be plentiful, but poor travellers. Many of these explanations are reasonable. For example, Frank Drake has noted the daunting cost of interstellar travel and concluded that few civilisations would do it, thus limiting the number of colonies. Others have pointed out that colonisation efforts always run out of steam. Even insects which specialise in "colonising" vegetation by chewing it full of holes don't devour the entire forest.

In other words, it could be that the lack of apparent alien presence in our neighbourhood tells us nothing. After all, homo sapiens has been wandering the globe for 100,000 years and yet there are still places where I can go and be out of sight of all humans and their artefacts. Within 200kms of my home is barren desert, devoid of any sign of the hundreds of millions of people who populate the continent. A native of Nevada might logically ó but incorrectly ó infer that his family was the lone clot of humans in America. You are like this putative Nevada native.

It was not long ago ó a matter of decades ó that the existence of life elsewhere was considered a radical idea among astronomers. Nearby planets seemed brutally inhospitable, and the complexity of life suggested that even a simple bacterium was an improbable project that nature would seldom complete. But you now admit that biology may not be such a rare phenomenon after all. I would suggest that evolution to intelligence is a far less daunting proposition than creating the initial living cells. And if the first has taken place on a huge scale, then the second will often occur.

Yours,
Seth Shostak