ET may have phoned home, but is there any hard evidence for life out there? And why, asks Carole Stott in the third of our series looking at today's hot topic, have extraterrestrials not been in touch?
The ancient Greeks were perfectly reasonable when they placed the flat, unspinning, stationary Earth at the centre of the Universe. Basking in the balmy Mediterranean nights, they had no way of telling that our huge rocky globe was hurtling around the Sun at 30km a second, and turning on its axis every 24 hours. The Earth was a special place. And mankind was special, too. In 1543, a scholarly Polish monk changed everything. In De Revolutionibus Orbium Coelestium (On the Revolutions of the Heavenly Spheres) Nicolas Copernicus suggested that the Earth was just one of a group of planets in orbit around the Sun. Sixty or so years later, the Italian Galileo Galilei insisted that the Sun and the stars were the same kind of object. The only fundamental difference was distance.
Further blows to our special ranking came in the 1920s. Not only was the Sun shown to be a member of a huge flattened group of stars known as the Milky Way Galaxy, but the small, faint, spiral nebulae, seen through the Earth's largest telescopes, were found to be galaxies too. The Universe had become huge. And Earth had become extremely small.
Earth was now an ordinary planet in orbit around an ordinary star which was in an ordinary galaxy. If there was nothing special about where we lived, it seemed reasonable to assume that there was nothing special about human life itself.
Today's astronomers have concluded that about 4 per cent of the stars in the sky have planetary systems. Other stars were either formed too soon after the Big Bang (and do not have the metallic elements essential for planet formation), or are in binary pairs, where the complicated gravitational field produced by the two orbiting stars is too complicated for planets to form. The remaining stars are surrounded by doughnut-shaped planetary nebulae which are too new for planets.
In a typical galaxy of a million million stars, 4 per cent adds up to 10,000 million planetary systems. And each system should contain at least one planet that lies within that specific band of temperatures where H2O is liquid. Too close to its star and a planet's water will boil away; too far away and it will freeze. Warm, wet (ie, liquid) water is essential for the generation of life, and there are about 10,000 million warm, wet planets in each galaxy.
Biologists tell us that three billion years ago, as soon as conditions on Earth became conducive to sustaining simple organisms, life broke out. This intimates that simple, single-cell life is ubiquitous. Our galaxy and the whole universe should be teaming with it.
So why do we Earth-dwellers seem so lonely? Assuming there is absolutely nothing unusual about our planet, or our central star, or our galaxy, we should leap to the conclusion that there is nothing unusual about ourselves, our schools, or our spacecraft. As you read this, just wonder how many other people on other planets are doing similar things.
Yet we remain alone. No one has been to visit us and no one has transmitted a detectable radio message in our direction.
This paradox troubled the nobel laureate, Italian physicist Enrico Fermi. Fifty years ago, he posed the simple question: "Where are they?" Maybe they don't want to travel. But this is most unlikely. As soon as we Earthlings became capable of launching intercontinental ballistic missiles we also started to explore our neighbouring planets. Not only is the solar system, a mere 40 years after the dawn of the space age, awash with orbiting spacecraft, but many have left the system altogether and are on their way into the galactic disc through the nearby stars. Twelve men have walked on the Moon. The fact that the evolution of the Sun will eventually make the Earth uninhabitable means that space travel will be essential for survival.
Assuming that galaxy dwellers are commonplace, we would be extremely presumptuous to suggest that we earthlings are the most advanced civilisation. The best we can hope for is that we are average.
As to the radio messages, these have been radiating away from our planet at a speed of 300,000km per second since we started transmitting radio and television programmes. From the radio standpoint, Earth is a very noisy place and our radio waves are already passing over the nearest 250 warm, wet planets. If advanced people live on these planets, they know we are here. American astronomer Frank Drake suggested that our galaxy contains a million civilisations capable of "talking" to us. Scientists working for the Search for Extraterrestrial Intelligence programme have been scanning the radio spectrum for messages they may have sent. The search started in 1960, and since then larger telescopes, better radio receivers and more sophisticated computer search programmes have been used, all to no avail. Even after a multitude of reports of sightings of "flying saucers", no one has come forward with any tangible physical artefact from an extraterrestrial body that has landed.
We are left with two facts. First, we know the planet Earth is inhabited with intelligent life-forms. Second, most of us are convinced that so far we have no evidence of any other intelligent life in the universe having visited us. No one has left a calling card and no one has even bothered to send a message.
What can we conclude? One possibility is that the Greeks were right to think of Earth and its inhabitants as special - albeit for the wrong reasons. Perhaps we are unique. Maybe primitive life in the universe is commonplace but the transition to intelligent life is extremely difficult. We might be the only planet on which that has happened. On the other hand, maybe we are not yet ready to be visited.
Or, perhaps, "they" are here already, and we just haven't looked closely enough to recognise them.
Carole Stott is an astronomy writer Next week: philosophy