An opening chapter covers the history of cosmological, philosophical, and scientific thinking about life on other worlds, from Democritus down to Lowell. The remaining chapters cover different themes through the 20th century.
Mars has had a central place in the search for life in the solar system. Lowell's canals, the search for vegetation, and then space age observations and the Viking experiments have all been the subject for debates. And more recently there have been claims of fossils in Martian meteorites and speculation about Europa and Titan.
The search for other solar systems has been linked from the beginning with theories of solar system origin. In the first half of the century proponents of uniqueness such as James Jeans dominated, but the fifteen years around 1950 saw a dramatic swing the other way. Recent technical advances have seen the discovery of extrasolar planets — and this is one area where much has happened since Life on Other Worlds was published.
Other worlds and aliens are standard fare in science fiction — and science fiction has inspired science and scientists. In thirty pages Dick can only touch on a few topics: founding fathers Wells, Verne, and Lasswitz, the Golden Age treatment of aliens, and the popular films that have brought aliens into mainstream culture. "By the end of the century the alien, barely invented 100 years before, had come to assume a central role in popular culture and scientific imagination."
Dick gives a brief history of the UFO controversy and the rise and fall of "the extraterrestrial hypothesis". This "brought scientists out of the closet on a subject they otherwise might never have addressed" and was "the public's chief exposure to the subject of extraterrestrial life".
The space program has driven research into the origins of life and raised the possibility of a universal biology. It has also provided a new perspective on questions of chance and necessity in evolution, with evolutionary theorists showing "a diversity of opinion as to whether intelligence existed beyond the Earth, along with virtual unanimity that if it did, the forces of natural selection would produce [vastly different] morphologies".
The search for extraterrestrial intelligence (SETI) began with observational efforts. Around 1960, seminal papers, the Green Bank conference and the Drake equation heralded a theoretical framework for the search. This has inspired — or failed to deter — a broad range of modern observation programs.
The possibility of communication with extraterrestrial intelligence has worried some: there have been concerns about the cultural impact, and the Christian response has seen the development of astrotheology. Other debates have centered on the anthropic principle, the idea that the existence of human life implies physical features of the universe.
In summary, the 20th century extraterrestrial life debates have seen the triumph of cosmic evolution and the widespread acceptance of a biological universe. For science they have brought new problems of evidence and inference, a testing of limits, and a protoscience of exobiology.