An alien (or an extraterrestrial) is a creature from another planet. Due to the fact that the word "alien" also means "foreigner", the phrase space alien is sometimes used in order to make the distinction. The word Martian is sometimes used to refer to any space alien, although it properly means something or someone from the planet Mars. Contact with aliens has been a recurring theme in 20th and 21st century popular culture, often featuring in movies, novels and other forms of fiction. As a result, there are many well-known alien characters who often provide a source of inspiration for trick-or-treaters, Halloween party-goers and other costume-wearers.
It is currently unknown whether or not space aliens exist, although such eminent scientists as Carl Sagan and Stephen Hawking have said that they probably do. They have argued that Earth is unlikely to be unique, given the vastness of space. A search for extraterrestrial intelligence (known by the acronym SETI) has been on going since 1960, although the idea of using radio to search for transmissions from alien civilizations was first suggested by the scientist Nikola Tesla in 1895. Organizations which are currently searching for signs of intelligent life elsewhere in the universe include the SETI Institute, Harvard University and the University of California, Berkley. This search is now largely privately funded, although in its early days it was heavily subsidized by the United States government.
Some people claim that aliens regularly visit Earth, sometimes taking people and carrying out experiments on them. Unidentified Flying Object (UFO) sightings and claims of alien abduction are generally not taken seriously by the scientific community.
The earliest known story of alien life, travel into outer space and interplanetary warfare is True Story, written in Greek by the Syrian author Lucian in the 2nd century CE. The author says that his ship is carried up to the Moon by a waterspout. He finds that there are no women on the Moon, children growing inside men's legs, and the King of the Moon is at war with the King of the Sun over who has the right to colonize the Morning Star. The Moon King's army contains such odd creatures as mushroom-men and dog-faced men who fly on acorns.
The Tale of the Bamboo Cutter, a Japanese folktale which dates back to the 10th century, is an early story of extraterrestrial visitors to Earth. The story tells how a childless couple find a tiny girl inside a stalk of rice and adopt her. They later discover that she is really a princess from the Moon.
Prior to the 19th century, accounts of extraterrestrial life, such as Cyrano de Bergerac's The Other World: The Societies and Government of the Moon, were often intended as satirical commentaries on life on Earth.
H.G. Wells 1898 novel The War of the Worlds tells of the invasion of Earth by aliens from the planet Mars. It has been enormously influential on later works of science fiction.
Aliens have figured prominently in the popular culture of the 20th and 21st centuries, appearing in novels, comic books, movies and television programs. They have been portrayed as everything from monsters that destroy everything and everyone that stands in their way to angelic beings who come to protect the Earth. Some well known extraterrestrials include the lovable E.T. from Spielberg's 1982 movie, the strictly logical Mr. Spock from the Star Trek franchise and Superman, the last son of the dead planet Krypton. Aliens are usually portrayed in popular culture as being much more technically advanced and more intelligent than humans. Popular themes for science fiction movies have involved aliens from a dying planet coming to Earth in search of a precious resource, such as water or women to breed with; those aliens are usually depicted as being unconcerned about the consequences of their actions on the people of Earth, whom they usually consider to be inferior. Many such familiar science fiction conventions have been parodied in Treehouse of Horror episodes of The Simpsons in which the alien characters of Kang and Kodos appear.
In movies and on television, the main alien characters are usually depicted as being somewhat humanoid in appearance, usually having a face with two eyes and a mouth and a body with two arms and two legs. The reason most often given for this is that it makes it easier for an audience to accept and empathize with the alien characters. However, science fiction stories sometimes hint at intelligent aliens who bear no resemblance to any form of life on Earth.