In planetary astronomy and astrobiology, the Rare Earth hypothesis argues that the origin of life and the evolution of biological complexity, such as sexually reproducing, multicellular organisms on Earth, and subsequently human intelligence, required an improbable combination of astrophysical and geological events and circumstances. According to the hypothesis, complex extraterrestrial life is an improbable phenomenon and likely to be rare throughout the universe as a whole. The term "Rare Earth" originates from Rare Earth: Why Complex Life Is Uncommon in the Universe (2000), a book by Peter Ward, a geologist and paleontologist, and Donald E. Brownlee, an astronomer and astrobiologist, both faculty members at the University of Washington.

In the 1970s and 1980s, Carl Sagan and Frank Drake, among others, argued that Earth is a typical rocky planet in a typical planetary system, located in a non-exceptional region of a common galaxy, now known to be a barred spiral galaxy. From the principle of mediocrity (extended from the Copernican principle), they argued that the evolution of life on Earth, including human beings, was also typical, and therefore that the universe teems with complex life. In contrast, Ward and Brownlee argue that planets which have all the requirements for complex life are not typical at all but actually exceedingly rare.

There is no reliable or reproducible evidence that extraterrestrial organisms of any kind have visited Earth. No transmissions or evidence of intelligent life have been detected or observed anywhere other than Earth in the Universe. This runs counter to the knowledge that the Universe is filled with a very large number of planets, some of which likely hold the conditions hospitable for life. Life typically expands until it fills all available niches. These contradictory facts form the basis for the Fermi paradox, of which the Rare Earth hypothesis is one proposed solution.

The Rare Earth hypothesis argues that the evolution of biological complexity anywhere in the universe requires the coincidence of a large number of fortuitous circumstances, including, among others, a galactic habitable zone; a central star and planetary system having the requisite character (i.e. a circumstellar habitable zone); a terrestrial planet of the right mass; the advantage of one or more gas giant guardians like Jupiter and possibly a large natural satellite to shield the planet from frequent impact events; conditions needed to ensure the planet has a magnetosphere and plate tectonics; a chemistry similar to that present in the Earth's lithosphere, atmosphere, and oceans; the influence of periodic "evolutionary pumps" such as massive glaciations and bolide impacts; and whatever factors may have led to the emergence of eukaryotic cells, sexual reproduction, and the Cambrian explosion of animal, plant, and fungi phyla. The evolution of human beings and of human intelligence may have required yet further specific events and circumstances, all of which are extremely unlikely to have happened were it not for the Cretaceous–Paleogene extinction event 66 million years ago removing dinosaurs as the dominant terrestrial vertebrates.

In order for a small rocky planet to support complex life, Ward and Brownlee argue, the values of several variables must fall within narrow ranges. The universe is so vast that it might still contain many Earth-like planets, but if such planets exist, they are likely to be separated from each other by many thousands of light-years. Such distances may preclude communication among any intelligent species that may evolve on such planets, which would solve the Fermi paradox which wonders: if extraterrestrial aliens are common, why aren't they obvious?

Rare Earth suggests that much of the known universe, including large parts of the Milky Way galaxy, are "dead zones" unable to support complex life. Those parts of a galaxy where complex life is possible make up the galactic habitable zone, which is primarily characterized by distance from the Galactic Center.

Item #1 rules out the outermost reaches of a galaxy; #2 and #3 rule out galactic inner regions. Hence a galaxy's habitable zone may be a relatively narrow ring of adequate conditions sandwiched between its uninhabitable center and outer reaches.

Also, a habitable planetary system must maintain its favorable location long enough for complex life to evolve. A star with an eccentric (elliptical or hyperbolic) galactic orbit will pass through some spiral arms, unfavorable regions of high star density; thus a life-bearing star must have a galactic orbit that is nearly circular, with a close synchronization between the orbital velocity of the star and of the spiral arms. This further restricts the galactic habitable zone within a fairly narrow range of distances from the Galactic Center. Lineweaver et al. calculate this zone to be a ring 7 to 9 kiloparsecs in radius, including no more than 10% of the stars in the Milky Way, about 20 to 40 billion stars. Gonzalez et al. would halve these numbers; they estimate that at most 5% of stars in the Milky Way fall within the galactic habitable zone.