Origins of the term extraterrestrial hypothesis are unknown, but use in printed material on UFOs seems to date to at least the latter half of the 1960s. French Ufologist Jacques Valleeused it in his 1966 book Challenge to science: the UFO enigma. It was used in a publication by French engineer Aimé Michel in 1967, by Dr. James E. McDonald in a symposium in March 1968 and again by McDonald and James Harder while testifying before the Congressional Committee on Science and Astronautics, in July 1968.SkepticPhilip J. Klass used it in his 1968 book UFOs--Identified. In 1969 physicist Edward Condon defined the "Extra-terrestrial Hypothesis" or "ETH" as the "idea that some UFOs may be spacecraft sent to Earth from another civilization or space other than earth, or on a planet associated with a more distant star," while presenting the findings of the much debated Condon Report. Some UFO historians credit Condon with popularizing the term and its abbreviation "ETH"..]
On June 24, 1947, at about 3.00 p.m. local time, pilot Kenneth Arnold reported seeing nine unidentified disk-shaped aircraft flying near Mount Rainier.
Arnold said the objects moved as if they were a saucer skipping across water, but also described the shape as thin, flat, and disc-like or saucer-like (also like a "pie-plate," "pie-pan," and "half-moon shaped")--see Kenneth Arnold article for detailed quotes. Two to three days later, the terms "flying disc" and "flying saucer" first appeared in newspapers and became the preferred terms for the phenomenon for a number of years, until largely replaced in the 1950s and 1960s by UFO.
Though he was impressed by their high speed and quick movements, Arnold did not initially consider the ETH, stating:
"I assumed at the time they were a new formation or a new type of jet, though I was baffled by the fact that they did not have any tails. They passed almost directly in front of me, but at a distance of about 23 miles, which is not very great in the air. I judged their wingspan to be at least 100 feet across. Their flying did not particularly disturb me at the time, except that I had never seen planes of that type."
However, when no aircraft emerged that seemed to account for what he had seen, Arnold quickly considered the possibility of the objects being extraterrestrial. On July 7, 1947, two stories came out where Arnold was raising the topic of possible extraterrestrial origins, both as his opinion and those who had written to him. In an Associated Press story, Arnold said he had received quantities of fan mail eager to help solve the mystery. A number of them "suggested the discs were visitations from another planet."