Are we alone, or does life exist elsewhere in the Universe? We have never found convincing evidence of life on other worlds. But there are reasons to think that extraterrestrial life is possible. On the one hand, the Earth does not appear to be unique in any way: what happened here can very well also happen somewhere else. And on the other hand, there are an awful lot of these “somewhere elses” in the Universe: there are many stars in our Galaxy, and many galaxies in the cosmos.

In our discussion we will deal solely with life as we know it, similar to the form we find on Earth: life that evolves by mutation and natural selection, life based on carbon chemistry and employing DNA as the carrier of genetic information. Other kinds are at least conceivable. There have been speculations about a form of life based on silicon. Science fiction writers have imagined far stranger possibilities. But we will not consider these, for the simple reason that we don't know anything about them. The only life about which we know anything at all is the kind that exists on Earth: concerning other varieties, we can only speculate.

Searches have been conducted for life on Mars, and searches are under way right now for signals from extraterrestrial civilizations. These searches have found nothing persuasive so far, and nobody thinks their task will be easy. But it is no exaggeration to say that the discovery of life elsewhere in the Universe would be one of the greatest scientific triumphs of all time.