Extraterrestrial life is life beyond planet Earth (other than humans travelling in space, and living organisms they bring along or send).
There are many questions about extraterrestrial life, including:
Does it exist?
Could there be non-carbon based life forms, e.g. life forms based on other elements like silicon (see carbon chauvinism)?
How does life differ depending on the type of structure (unicellular life, multicellular, intelligent, advanced technological)?
What range of conditions are required for the evolution of life?
Could it pose a threat to life on earth?
The scientific study of extraterrestrial life is called xenobiology (or exobiology or astrobiology).
Scientists are searching for extraterrestrial life in two very different ways. Firstly, they are searching for evidence of unicellular life within the solar system: searching Mars and meteors which have fallen to Earth, and a proposed mission to Europa, one of Jupiter’s moons with a liquid water layer under its surface, which may contain life.
There is some evidence for the existence of microbial life 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, though the lack of corroborating evidence from other experiments on the Viking indicates that a non-biological reaction is a more likely hypothesis. Indepedently, in 1996 structures resembling bacteria were discovered in a meteorite known to be formed of rock ejected from Mars. Again, this evidence is vigorously disputed.
Secondly, it is theorised that any technological society will be transmitting information: man-made electromagnetic radiation is already detectable within an eighty light-year radius of Earth, and is constantly spreading. SETI, the Search for Extraterrestrial Intelligence, takes the data gathered by the world’s largest radiotelescopes and analyses it for artificial patterns using supercomputers and one of the largest distributed computing projects in the world, SETI@home.
Some scientists believe that some UFOs are the spacecraft of intelligent extraterrestrials; however since these scientists are currently very much in the minority, work such as SETI continues in the hopes that a signal will be detected.
Astronomers also search for extrasolar planets that would be conducive to life. Current radiodetection methods have been inadequate for such a search, as the resolution afforded by recent technology is inadequate for detailed study of extrasolar planetary objects. Future telescopes should be able to image planets around nearby stars, which may reveal the presence of life (either directly or through spectrography revealing, for instance, the presence of free oxygen in a planet’s atmosphere).
Panspermia holds that (extraterrestrial) life is prevalent through space in a form analogous to spores.
Extraterrestrial life forms, especially intelligent ones, are often referred to as aliens.