This refers to the scientific claim that the earth is endowed with a rare combination of favourable conditions which make it a suitable place to sustain a wide variety of complex living organisms. It is often used to argue that extra-terrestrial life is unlikely as it is highly improbable that planets similar to the earth exist elsewhere in the universe. The term was adopted from the 2000 book, Rare Earth: Why Complex Life Is Uncommon in the Universe, by Peter Ward and Donald Brownlee. Critics have argued that conditions prevalent on the earth are not extraordinary and exist in other places in the universe, which makes extra-terrestrial life probable.