The term alien abduction or abduction phenomenon describe "subjectively real memories of being taken secretly and/or against one's will by apparently non-human entities and subjected to complex physical and psychological procedures." People claiming to have been abducted are usually called "abductees". Claims involve the person involved being subjected to a forced medical examination which emphasises their reproductive system. They also claim to have been warned against environmental abuse and the dangers of nuclear weapons.
Extraterrestrial life is defined as life that does not originate from Earth. It is unknown whether any such life exists or ever existed in the past, although many scientists think it likely that on Mars, for instance, life either exists or has existed. Various claims have been made for evidence of extraterrestrial life, such as those listed in a 2006 New Scientist article, which the magazine describes as "hints" rather than proof. A less direct argument for the existence of extraterrestrial life relies on the vast size of the observable universe. According to this argument, endorsed by Carl Sagan and Stephen Hawking, it would be improbable for life not to exist somewhere other than Earth.
The development and testing of theories about extraterrestrial life is known as astrobiology, exobiology, or xenobiology. One possibility is that life has emerged independently at many places throughout the universe. Another possibility is panspermia or exogenesis, in which life would have spread between habitable planets. These two hypotheses are not necessarily mutually exclusive. Possible forms of extraterrestrial life range from simple bacteria-like organisms to sapient beings far more advanced than humans.
Suggested locations on which life might have developed, or which might continue to host life today, include the planets Venus and Mars; moons of Jupiter and Saturn such as Europa, Enceladus and Titan; and extrasolar planets such as Gliese 581 c and d, recently discovered to be near Earth mass and apparently located in their star's habitable zone, with the potential to have liquid water.
Beliefs that some unidentified flying objects are of extraterrestrial origin (see extraterrestrial hypothesis), along with claims of alien abduction, are considered spurious by most scientists. Most UFO sightings are explained either as sightings of Earth-based aircraft or known astronomical objects, or as hoaxes. Some sightings have remained unexplained, in some cases having been reported by trained professionals.
Search for Life
Scientists are directly searching for evidence of unicellular life within the Solar System, carrying out studies on the surface of Mars and examining meteors which have fallen to Earth. A mission is also proposed to Europa, one of Jupiter's moons with a possible liquid water layer under its surface, which might contain life.
There is some limited evidence that microbial life might possibly exist (or have existed) on Mars. An experiment on the Viking Mars lander reported gas emissions from heated Martian soil that some argue are consistent with the presence of microbes. However, the lack of corroborating evidence from other experiments on the Viking indicates that a non-biological reaction is a more likely hypothesis. Independently, in 1996, structures resembling nanobacteria were reportedly discovered in a meteorite, ALH84001, thought to be formed of rock ejected from Mars. This report is also controversial, and scientific debate continues.
Electron micrograph of martian meteorite ALH84001 showing structures that some scientists think could be fossilized bacteria-like lifeforms.
In February 2005, NASA scientists reported that they had found strong evidence of present life on Mars. The two scientists, Carol Stoker and Larry Lemke of NASA's Ames Research Center, based their claims on methane signatures found in Mars' atmosphere resembling the methane production of some forms of primitive life on Earth, as well as on their own study of primitive life near the Rio Tinto river in Spain. NASA officials soon denied the scientists' claims, and Stoker herself backed off from her initial assertions.
Though such findings are still very much in debate, support among scientists for the belief in the existence of life on Mars seems to be growing. In an informal survey conducted at the conference at which the European Space Agency presented its findings, 75 percent of the scientists in attendance were reported to believe that life once existed on Mars, and 25 percent reported a belief that life currently exists there.
The Gaia hypothesis stipulates that any planet with a robust population of life will have an atmosphere not in chemical equilibrium, which is relatively easy to determine from a distance by spectroscopy. However, significant advances in the ability to find and resolve light from smaller rocky worlds near to their star are necessary before this can be used to analyze extrasolar planets.