THERE are some 100 billion galaxies in the observable universe, with about 100 billion stars in each of those galaxies. And in recent years, we have discovered that there are probably more planets than there are stars. In fact, there are more planets in the universe than there are sand grains on all the beaches of all the coastlines of all the continents. Yet, in all this immensity, there is only one place where we know there is life – the tiny, fragile "blue dot" we call Earth.

This rather handicaps our speculations about life elsewhere. Not that you would know it, to judge by the hundreds of books published every year about extraterrestrial life, its prospects and possible forms.