After obtaining his Ph.D. from the University of Chicago in 1960, Sagan taught at the University of California, Berkeley, and at Harvard University and was an astrophysicist at the Smithsonian Astrophysical Observatory (1962–68). There he divided his time between planetary astronomy and work on the Search for Extraterrestrial Intelligence (SETI) project. In 1968 he became director of the Laboratory of Planetary Studies at Cornell University, and he also worked on several U.S. unmanned space missions to Venus and Mars.

With the publication of The Cosmic Connection: An Extraterrestrial Perspective (1973), Sagan gained prominence as a popular science writer and commentator who was notable for his clear writing and enthusiasm for science. His The Dragons of Eden: Speculations on the Evolution of Human Intelligence (1977) won the Pulitzer Prize. He was a coproducer, as well as narrator, of the television series “Cosmos” (1980). In the 1980s he participated in research on the environmental effects of nuclear war and helped popularize the term “nuclear winter.”

Sagan's writings include Atmospheres of Mars and Venus (1961), Planetary Exploration (1970), Broca's Brain: Reflections on the Romance of Science (1979), the novel Contact (1985), Nuclear Winter (1985), and The Demon-Haunted World: Science as a Candle in the Dark (1996).