History [of extraterrestrial life]
Although Greek philosophers such as Democritus believed that there must be an infinite number of worlds all with inhabitants like us, the prevailing view in antiquity—later supported by theological arguments—was that the Earth is special, and the only abode of life. However, with the development of Copernican ideas in the 1600s it came to be widely though not universally believed, even in theological circles, that other planets—as well as the Moon—must have inhabitants like us. Many astronomers attributed features they saw on the Moon to life if not intelligence, but in the late 1800s, after it was found that the Moon has no atmosphere, belief in life there began to wane. Starting in the 1870s, however, there began to be great interest in life on Mars, and it was thought—perhaps following the emphasis on terrestrial canal-building at the time—that a vast network of canals on Mars had been observed. And although in 1911 the apparent building of new canals on Mars was still being soberly reported by newspapers, there was by the 1920s increasing skepticism. The idea that lichens might exist on Mars and be responsible for seasonal changes in color nevertheless became popular, especially after the discovery of atmospheric carbon dioxide in 1947. Particularly in the 1920s there had been occasional claims of extraterrestrial radio signals (see page 1188), but by the 1950s interest in extraterrestrial intelligence had largely transferred to science fiction (see page 1190). Starting in the late 1940s many sightings were reported of UFOs believed to be alien spacecraft, but by the 1960s these were increasingly discredited. It had been known since the mid-1800s that many other stars are much like the Sun, but it was not until the 1950s that evidence of planets around other stars began to accumulate. Following a certain amount of discussion in the physics community in the 1950s, the first explicit search for extraterrestrial intelligence with a radio telescope was done in 1960 (see page 1189). In the 1960s landings of spacecraft on the Moon confirmed the absence of life there—though returning Apollo astronauts were still quarantined to guard against possible lunar microbes. And despite substantial expectations to the contrary, when spacecraft landed on Mars in 1976 they found no evidence of life there. Some searches for extraterrestrial signals have continued in the radio astronomy community, but perhaps because of its association with science fiction, the topic of extraterrestrial intelligence has generally not been popular with professional scientists. With the rise of amateur science on the web and the availability of low-cost radio telescope components the late 1990s may however have seen a renewal of serious interest.