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?
As the Sun ages, Europa will warm and may support life.
Jupiter's moon Europa will warm as the Sun ages and may support life. Scientists speculate it could become a watery world.
Patrick Moore discusses ET life with comedian Michael Bentine.
Sir Patrick Moore and comedian Michael Bentine discuss the possibility of extraterrestrial life.
Patrick Moore speaks to Lovell.
In 1979 Sir Patrick Moore asked the great radio astronomer Sir Bernard Lovell whether he thought we could ever contact other advanced civilisations.
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.
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.
Extraterrestrial life (from the Latin words: extra ["beyond", or "not of"] and terrestris ["of or belonging to Earth"]) is defined as life that does not originate from Earth. It is often also referred to as alien life, or simply aliens (or space aliens, to differentiate from other definitions of alien or aliens). These hypothetical forms of life range from simple bacteria-like organisms to beings far more complex than humans.
The development and testing of hypotheses on extraterrestrial life is known as exobiology or astrobiology; the term astrobiology, however, includes the study of life on Earth viewed in its astronomical context. Nonetheless, scientists at the National Institutes of Health reported studies that life in the universe may have begun "9.7±2.5 billion years ago", billions of years before the Earth was formed, based on extrapolating the "genetic complexity of organisms" [from "major phylogenetic lineages"] to earlier times. Many scientists consider extraterrestrial life to be 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.