Extraterrestrial life is defined as life that does not originate from Earth. 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. Many scientists consider extraterrestrial life to be plausible, but there is no conclusive evidence of the existence of extraterrestrial life.