Rare Earth: Why Complex Life Is Uncommon in the Universe is a 2000 popular science book about xenobiology by Peter Ward, a geologist and evolutionary biologist, and Donald E. Brownlee, a cosmologist and astrobiologist.

They argue that among the essential criteria for complex life are a terrestrial planet with plate tectonics and oxygen, a large moon, magnetic field, a gas giant like Jupiter for protection and an orbit in the habitable zone of the right kind of star.

While initially declaring it a "must read",[2] the geoscientist James Kasting wrote a highly critical reply in the journal Perspectives in Biology and Medicine, challenging its restrictive criteria.

[3] Several books were written in reply including Evolving the Alien by Jack Cohen, who described Ward and Brownlee's assumption as restrictive and unimaginative; and a form of circular reasoning.

While Ward & Brownlee attribute this exceptional unlikeliness to chance, many within the movement regard this as evidence of an intelligent designer.

[8] Rare Earth was succeeded in 2003 by the follow-on book The Life and Death of Planet Earth: How the New Science of Astrobiology Charts the Ultimate Fate of our World, also by Ward and Brownlee, which talks about the Earth's long-term future and eventual demise under a warming and expanding Sun, showing readers the concept that planets like Earth have finite lifespans, and that complex and especially intelligent life is not just rare in space, but also rare in time, and is more likely to die out relatively soon and rather quickly within a short period of time on geological timescales, even more so with the latter, while microbes are expected to survive much longer, and therefore dominate most of the planet's history, and being the first life to appear, will likely also be the last life to ultimately remain and then disappear.