Rare Earth Hypothesis

The Rare Earth hypothesis argues that the emergence of complex, multicellular life, as opposed to simple microbial life, requires an improbable conjunction of astronomical, geological and biological circumstances, and that such conditions are consequently very uncommon across the universe even if simple life itself is not. Proposed by Peter Ward and Donald Brownlee in their 2000 book of the same name, it cites factors such as a stable star, a protective large moon, plate tectonics, and a suitably placed giant planet shielding the inner solar system from frequent impacts. The hypothesis stands in direct tension with the more optimistic assumptions often underlying the search for extraterrestrial intelligence and the Drake equation.