The term extraterrestrial life encompasses all life, ranging from the lowest possible forms to those with suprahuman intelligences,
that may exist beyond the Earth. In the most general sense such life may be based on principles far different from those which operate on Earth, thus extending the traditional definitions of life and providing insight into the nature of all living things. Although no compelling evidence has been presented to confirm the existence of extraterrestrial life, its possible occurrence has long been the subject of debate and continues to generate intense controversy.HISTORYThe subject of life beyond the Earth was touched upon by the Pythagoreans in the 5th century © and by the Greek atomists Democritus and Epicurus in the 4th century ©. Only in the 17th century, however, when the widespread acceptance of the heliocentric world system of Nicolaus Copernicus provided a framework in which the other planets of the solar system could be apprehended as similar to the Earth, did a serious debate arise about the
possibility of planetary life. Johannes Kepler, in the Somnium (Dream, 1634), and John Wilkins, in The Discovery of a World in the Moone (1638), defended the idea of life on the Moon. Later in the century Bernard le Bovier de Fontenelle, in his popular treatise Entretiens sur la pluralitŽ des mondes (1686; trans. as A Plurality of Worlds, 1688), and Christiaan Huygens, in his Cosmotheoros (1698), postulated the existence of life on the planets revolving around the Sun and around other stars.Although many theological and philosophical
objections remained, such 18th-century Newtonians as Immanuel Kant and Pierre Simon de Laplace placed the benefits of the belief to natural theology above all Scriptural objections. A strong measure of skepticism introduced by William Whewell's essay Of the Plurality of Worlds (1853) was countered by numerous writers, notably Sir David Brewster, Camille Flammarion, and, above all, Percival Lowell, with his claims of canals on Mars. When, in the 20th century, improved telescopes and spectroscopy provided strong evidence against the possibility of intelligent life in the solar system, attention increasingly turned to the possibility of planets outside the solar system.