Aliens, UFOs, little green men: Whatever you call it, life that did not start on the Earth has yet to be found, but a number of respected scientists speculate that extraterrestrial life exists. Some such as Frank Drake have even attempted to estimate the number of civilisations in our galaxy based on what we know about the Universe and life on Earth.
Image: Could an advanced civilisation exist in one of the thousands of galaxies shown in this Hubble image? (credit: NASA, ESA, S. Beckwith/STScI, HUDF Team)
Is life out there?
A Swiss team finds the first planet orbiting a star similar to the Sun.
In 1995 a Swiss team of astronomers found the first planet orbiting a star similar to the Sun, 51 Pegasi. The first exoplanets were found around a neutron star in 1992.
Lynn Rothschild explains how the Earth's orbit makes it special.
Dr Lynn Rothschild explains how the Earth is located in the habitable zone of the Sun, an orbit that permits liquid water to exist. Astronomers hope to find planets similar to the Earth in the habitable zones of other stars as part of their search for places where life could exist.
NASA's William Borucki explains Kepler's mission.
Launched in 2009, the Kepler space telescope's mission is to find Earth-like worlds orbiting distant stars. In this clip, NASA's William Borucki explains how it will work.
Patrick Moore discusses the search for life with Sagan.
Sir Patrick Moore spoke to Dr Carl Sagan in 1974 about the search for other civilisations.
Frank Drake explains how to estimate the number of civilisations in our galaxy.
The Drake equation is designed to estimate the number of detectable alien civilisations in the Milky Way, as Frank Drake explains in this clip.
Extraterrestrial life[n 1] is life that does not originate from Earth. It is also called alien life, or, if it is a sentient and/or relatively complex individual, an "extraterrestrial" or "alien" (or, to avoid confusion with the legal sense of "alien", a "space alien"). These as yet hypothetical forms of life range from simple bacteria-like organisms to beings far more complex than humans. The possibility that viruses might exist extraterrestrially has also been proposed.
The development and testing of hypotheses on extraterrestrial life is known as "exobiology" or "astrobiology", although astrobiology also considers Earth-based life in its astronomical context. Many scientists consider extraterrestrial life plausible, but there is no direct evidence of its existence. Since the mid-20th century, there has been an ongoing search for signs of extraterrestrial life, from radios used to detect possible extraterrestrial signals, to telescopes used to search for potentially habitable extrasolar planets. It has also played a major role in works of science fiction. Over the years, science fiction works, especially Hollywood's involvement, has increased the public's interest in the possibility of extraterrestrial life. Some encourage aggressive methods to try and get in contact with life in outer space, while others argue that it might be dangerous to actively call attention to Earth. In the past, the clash between civilized culture and indigenous people has not gone well.
On 13 February 2015, scientists (including Geoffrey Marcy, Seth Shostak, Frank Drake and David Brin) at a convention of the American Association for the Advancement of Science, discussed Active SETI and whether transmitting a message to possible intelligent extraterrestrials in the Cosmos was a good idea; one result was a statement, signed by many, that a "worldwide scientific, political and humanitarian discussion must occur before any message is sent".