Extraterrestrial life refers to such life forms (if any) that exist outside planet Earth. It is unknown if any such lifeforms exist.
There are many questions about what extraterrestrial life might be, including:
- Could there be non-carbon based life forms, e.g. life forms based on silicon?
- What is the probability of life evolving? How does it differ depending on the type of lifeform (unicelluar life, multicelluar, intelligent, advanced technological)?
- What conditions are required for the evolution of life?
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 core, which may contain life.
There is some highly controversial 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. Vigorous scientific debate on the correctness of this claim.
Secondly, it is theorised that any technological society will be transmitting information: man-made electromagnetic radiation is alread detectable within an eighty light-year radius of Earth, and is constantly spreading. SETI, the Search for Extraterrestrial Intelligence, analyse the data gathered by the world's largest radiotelescopes and analyse them for artificial patterns using supercomputers and one of the largest distributed computing projects in the world, SETI@home.
It has been theorised that all life on Earth came from space. Evidence to suggest this includes the Cambrian Explosion, where primitive forms of life suddenly appear on the fossil record, and suggestions that comets may contain complex carbon chains, bordering at the level where they may become self-replicating.
Panspermia holds that (extraterrestial) life is prevalent through space in a form analogous to spores.
Extraterrestrial life forms, especially intelligent ones, in fiction and popular speech are often referred to as aliens.