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] also called alien life (or, if it is a sentient or relatively complex individual, an "extraterrestrial" or "alien"), is life that does not originate from Earth. These as-yet-hypothetical life forms may range from simple bacteria-like organisms to beings with civilizations far more advanced than humanity. Although many scientists expect extraterrestrial life to exist, there is no unambiguous evidence for its existence so far. The science of extraterrestrial life is known as exobiology.
The science of astrobiology considers life on Earth as well, and in the broader astronomical context. In 2015, "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia, when the young Earth was about 400 million years old. According to one of the researchers, "If life arose relatively quickly on Earth, then it could be common in the universe."
Since the mid-20th century, there has been an ongoing search for signs of extraterrestrial intelligence, 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 have increased the public's interest in the possibility of extraterrestrial life. Some encourage aggressive methods to try to get in contact with life in outer space, whereas others argue that it might be dangerous to actively call attention to Earth.