Is the Earth the only planet to harbour life?

Pending the results of any expedition beneath Europas icy exterior, and of investigations into the possibility of life on Mars the Earth is currently the only planet that we know houses life. This seems obvious if you glance about the solar system, where everything you see is too hot, too cold, too poisonous or in some other way too unwelcoming to let life get going or continue.

But think about it a little more and there seems to be something odd going on here. In the sixteenth and seventeenth centuries, it was finally proved that the Earth is just another planet, not the centre of the universe. Later it became apparent that despite being unique in many ways, humans are just another species, produced by natural selection like all the others. Now it is generally accepted that even the universe is probably one of many.

Can we really be sure that there is no other life in the universe? Of course we cannot. But in the 1960s the British scientist James Lovelock devised a neat way of detecting life on any planet with an atmosphere whose composition can be determined. He reasoned that if the atmosphere contains two gases that would normally react together and deplete one another, there must be some living creature on the planet to go on producing one or both of them. The obvious example is the oxygen in the Earths atmosphere. Oxygen is highly reactive, which is why firefighters deserve to be well paid. It exists in the atmosphere alongside methane, even though the two react together. The only reason they coexist is that the oxygen is constantly replenished by fresh supplies generated by plants. The methane comes partly from living organisms and partly from inorganic sources such as volcanoes.

This argument is not completely foolproof. It is possible to imagine a world in which two conflicting atmospheric gases are generated in different places without the involvement of life and fight it out in the atmosphere. But we are now getting to the stage at which it is becoming possible to measure the atmospheres of planets of other stars. Maybe, as Lovelock foresaw, Earth-like incompatibilities will emerge that argue for life on these planets. The real issue, perhaps, is that our thinking about life on other planets is determined by what we know here. After all, why do science fiction writers usually put their aliens on planets in the first place? And solid, rocky ones at that? Obviously this makes life simple tor the human protagonists, but is there any reason why deep clouds such as those of Jupiter, or a gas cloud in deep space, could not be a handy spot for life? Thinking about life in the universe is a little like thinking about languages if the only one you speak is English. Even one more example would expand our wisdom enormously.

With this in mind, there are a number of approaches now being mined to find out whether there is any detectable life out there. The title of this endeavour is SETI – the Search for Extraterrestrial Intelligence – which gives the game away. What is being sought is not life, but signals generated by intelligent life not too unlike us. At the moment the emphasis is on trawling radio signals, but there are also proposals to detect signals of extraterrestrial life in visible light or even atomic particles.

The real question, however, is that posed by the Italian physicist Enrico Fermi and now generally called the Fermi Paradox. If the aliens exist, where are they? They have had plenty of time to get here, after all, if you compare the speed of a possible spacecraft to the size of the galaxy. But despite the loose talk about UFOs, there are hundreds of thousands of skilled amateur astronomers looking at the sky every night and they never see anything they cannot identify.

A number of answers have been thought up for this question. One is that there are indeed no aliens. Another is that because of the crude state of human development, the Earth has been set aside as a kind of wildlife park which aliens have agreed not to visit. But perhaps this explanation also relies too much on the assumption that other species are like us, with a restless urge to explore new worlds. Maybe they are happy just where they are.