The extraterrestrial hypothesis (ETH) proposes that some

unidentified flying object An unidentified flying object (UFO), more recently renamed by US officials as a UAP (unidentified aerial phenomenon), is any perceived aerial phenomenon that cannot be immediately identified or explained. On investigation, most UFOs are id ...

s (UFOs) are best explained as being physical

spacecraft A spacecraft is a vehicle or machine designed to spaceflight, fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth ...

occupied by extraterrestrial life or non-human aliens, or non-occupied alien probes from other planets visiting

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...

Etymology

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 Ufology ( ) is the investigation of unidentified flying objects (UFOs) by people who believe that they may be of extraordinary origins (most frequently of extraterrestrial alien visitors). While there are instances of government, private, and f ...

Jacques Vallée Jacques Fabrice Vallée (; born September 24, 1939) is an Internet pioneer, computer scientist, venture capitalist, author, ufologist and astronomer currently residing in San Francisco, California and Paris, France. His scientific career beg ...

used 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

James E. McDonald James Edward McDonald (May 7, 1920 – June 13, 1971) was an American physicist. He is best known for his research regarding UFOs. McDonald was a senior physicist at the Institute for Atmospheric Physics and a professor of meteorology at the Univ ...

in a symposium in March 1968 and again by McDonald and

James Harder James Albert Harder, Ph.D., (December 2, 1926 – December 30, 2006) was a professor of civil and hydraulic engineering at the University of California, Berkeley. He was a professor emeritus there. Harder also had interest in ufology. Engineer ...

while testifying before the Congressional Committee on Science and Astronautics, in July 1968. Skeptic Philip J. Klass used it in his 1968 book ''UFOs--Identified.'' In 1969 physicist

Edward Condon Edward Uhler Condon (March 2, 1902 – March 26, 1974) was an American nuclear physicist, a pioneer in quantum mechanics, and a participant during World War II in the development of radar and, very briefly, of nuclear weapons as part of the ...

defined the "extraterrestrial 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."

Chronology

Although the extraterrestrial hypothesis (ETH) as a phrase is a comparatively new concept, one which owes much to the flying saucer sightings of the 1940s–1960s, its origins can be traced back to a number of earlier events, such as the now-discredited

Martian canals During the late 19th and early 20th centuries, it was erroneously believed that there were "canals" on the planet Mars. These were a network of long straight lines in the equatorial regions from 60° north to 60° south latitude on Mars, observed ...

and ancient Martian civilization promoted by astronomer

Percival Lowell Percival Lowell (; March 13, 1855 – November 12, 1916) was an American businessman, author, mathematician, and astronomer who fueled speculation that there were canals on Mars, and furthered theories of a ninth planet within the Solar System. ...

, popular culture including the writings of

H. G. Wells Herbert George Wells"Wells, H. G."
Revised 18 May 2015. ''

Edgar Rice Burroughs, who likewise wrote of Martian civilizations, and even to the works of figures such as the Swedish philosopher, mystic and scientist

Emanuel Swedenborg Emanuel Swedenborg (, ; born Emanuel Swedberg; 29 March 1772) was a Swedish pluralistic-Christian theologian, scientist, philosopher and mystic. He became best known for his book on the afterlife, ''Heaven and Hell'' (1758). Swedenborg had ...

, who promoted a variety of unconventional views that linked other worlds to the afterlife.Swedenborg, Emanuel (1758) ''Concerning the Earths in Our Solar System ...'' In the early part of the 20th century,

Charles Fort Charles Hoy Fort (August 6, 1874 – May 3, 1932) was an American writer and researcher who specialized in anomalous phenomena. The terms "Fortean" and "Forteana" are sometimes used to characterize various such phenomena. Fort's books sold ...

collected accounts of anomalous physical phenomena from newspapers and scientific journals, including many reports of extraordinary aerial objects. These were published in 1919 in ''The Book of the Damned''. In this and two subsequent books, ''New Lands'' (1923) and ''Lo!'' (1931), Fort theorized that visitors from other worlds were observing Earth. Fort's reports of aerial phenomena were frequently cited in American newspapers when the UFO phenomenon first attracted widespread media attention in June and July 1947. The modern ETH—specifically, the implicit linking of unidentified aircraft and lights in the sky to alien life—took root during the late 1940s and took its current form during the 1950s. It drew on

pseudoscience Pseudoscience consists of statements, beliefs, or practices that claim to be both scientific and factual but are incompatible with the scientific method. Pseudoscience is often characterized by contradictory, exaggerated or unfalsifiable claim ...

, as well as popular culture. Unlike earlier speculation of extraterrestrial life, interest in the ETH was also bolstered by many unexplained sightings investigated by the U.S. government and governments of other countries, as well as private civilian groups, such as

NICAP The National Investigations Committee On Aerial Phenomena (NICAP) is an unidentified flying object (UFO) research group most active in the United States from the 1950s to the 1980s. It remains active primarily as an informational depository on th ...

and APRO.

Historical reports of extraterrestrial visits

An early example of speculation over extraterrestrial visitors can be found in the French newspaper ''Le Pays'', which on June 17, 1864, published a story about two American geologists who had allegedly discovered an alien-like creature, a mummified three-foot-tall hairless humanoid with a trunk-like appendage on its forehead, inside a hollow

egg-shaped An oval () is a closed curve in a plane which resembles the outline of an egg. The term is not very specific, but in some areas (projective geometry, technical drawing, etc.) it is given a more precise definition, which may include either one o ...

structure.

H. G. Wells Herbert George Wells"Wells, H. G."
Revised 18 May 2015. ''

The War of the Worlds ''The War of the Worlds'' is a science fiction novel by English author H. G. Wells, first serialised in 1897 by ''Pearson's Magazine'' in the UK and by '' Cosmopolitan'' magazine in the US. The novel's first appearance in hardcover was i ...

, popularized the idea of Martian visitation and invasion. Even before Wells, there was a sudden upsurge in reports in "

Mystery airship Mystery airships or phantom airships are a class of unidentified flying objects best known from a series of newspaper reports originating in the western United States and spreading east during late 1896 and early 1897.. According to researcher ...

s" in the United States. For example, ''The Washington Times'' in 1897 speculated that the airships were "a reconnoitering party from Mars", and the ''Saint Louis Post-Dispatch'' wrote: "these may be visitors from Mars, fearful, at the last, of invading the planet they have been seeking." Later, there was a more international airship wave from 1909-1912. An example of an extraterrestrial explanation at the time was a 1909 letter to a New Zealand newspaper suggesting "atomic powered spaceships from Mars." From the 1920s, the idea of alien visitation in space ships was commonplace in popular comic strips and radio and movie serials, such as

Buck Rogers Buck Rogers is a science fiction adventure hero and feature comic strip created by Philip Francis Nowlan first appearing in daily US newspapers on January 7, 1929, and subsequently appearing in Sunday newspapers, international newspapers, books ...

and

Flash Gordon Flash Gordon is the protagonist of a space adventure comic strip created and originally drawn by Alex Raymond. First published January 7, 1934, the strip was inspired by, and created to compete with, the already established '' Buck Rogers'' ad ...

. In particular, the Flash Gordon serials have the Earth being attacked from space by alien meteors, ray beams, and biological weapons. In 1938, a radio broadcast version of ''

'' by

Orson Welles George Orson Welles (May 6, 1915 – October 10, 1985) was an American actor, director, producer, and screenwriter, known for his innovative work in film, radio and theatre. He is considered to be among the greatest and most influential f ...

, using a contemporary setting for H. G. Wells' Martian invasion, created some public panic in the United States.

The 1947 flying saucer wave in America

On June 24, 1947, at about 3:00 p.m. local time, pilot

Kenneth Arnold Kenneth Albert Arnold (March 29, 1915 – January 16, 1984) was an American aviator, businessman, and politician. He is best known for making what is generally considered the first widely reported modern unidentified flying object sighting in ...

reported seeing nine unidentified disk-shaped aircraft flying near

Mount Rainier Mount Rainier (), indigenously known as Tahoma, Tacoma, Tacobet, or təqʷubəʔ, is a large active stratovolcano in the Cascade Range of the Pacific Northwest, located in Mount Rainier National Park about south-southeast of Seattle. With a ...

. 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 Fan mail is mail sent to a public figure, especially a celebrity, by their admirers or " fans". In return for a fan's support and admiration, public figures may send an autographed poster, photo, reply letter or note thanking their fans for t ...

eager to help solve the mystery. Some of them "suggested the discs were visitations from another planet." When the 1947 flying saucer wave hit the United States, there was much speculation in the newspapers about what they might be in news stories, columns, editorials, and letters to the editor. For example, on July 10, U.S. Senator

Glen Taylor Glen Allen Taylor (born April 20, 1941) is an American billionaire business magnate and politician from Minnesota. A self-made businessman, Taylor made his fortune from being the founder and owner of Minnesota-based Taylor Corporation, one of t ...

of Idaho commented, "I almost wish the flying saucers would turn out to be space ships from another planet," because the possibility of hostility "would unify the people of the earth as nothing else could." On July 8, R. DeWitt Miller was quoted by UP saying that the saucers had been seen since the early nineteenth century. If the present discs weren't secret Army weapons, he suggested they could be vehicles from Mars, or other planets, or maybe even "things out of other dimensions of time and space." Other articles brought up the work of

, who earlier in the 20th Century had documented numerous reports of unidentified flying objects that had been written up in newspapers and scientific journals. Even if people thought the saucers were real, most were generally unwilling to leap to the conclusion that they were extraterrestrial in origin. Various popular theories began to quickly proliferate in press articles, such as secret military projects, Russian spy devices, hoaxes,

optical illusions Within visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and