Rare Earth hypothesis

In the book Rare Earth: Why Complex Life is Uncommon in the Universe, Peter Ward, a geologist and paleontologist, and Donald Brownlee, an astronomer and astrobiologist, propose that life as we know it is rare in the universe.[19][20] They suggest that microbial life, however, is probably common in the universe, because of recently discovered extremophiles.[21] The book argues that the chances of all the conditions that occurred to create the Earth occurring again would be rare; thus intelligent life would be rare. One important factor focused on in the book is planetary habitability (see section below).

Peter Ward, one of the authors, said the following:[1]

How do we define life as we do know it? Life on Earth has DNA, a specific genetic code. It also uses only 20, and the same 20, amino acids. Life is always cellular according to some people, but I think not. I personally define a virus as alive. As for other life, what could it be? Could there be non-DNA life? If such life does exist, what does chemistry permit? Certainly chemistry permits certain types of life on our planet and others not. But once we move out in the solar system, especially in the vast realm of cold, chemistry changes. There could be different information systems, different solvents, different membranes. And as we go from hotter to colder, when we go to Venus, out to Mars, to Europa, and to Titan, we really should expect radically different chemistries.