Humanity is fast approaching the point of being able to answer the age-old question: Are we alone in the cosmos? The answer will almost certainly turn out to be 'no'. This is virtually guaranteed by two discoveries made in the last hundred years or so: that the Milky Way is just one of countless galaxies; and that the number of planets beyond our solar system -- 'exoplanets' -- is vast.But what is extraterrestrial life actually like? What kinds of...