What possible life forms could exist on other planets: a historical overview.

Speculations on living beings existing on other planets are found in many written works since the Frenchman Bernard de Fontenelle spoke to the Marquise about the inhabitants of the solar system in his Entretiens sur la pluralité des mondes (1686). It was an entertainment used to teach astronomy more than real considerations about the habitability of our solar system, but it opened the way to some reflections about the possible life forms on other planets. The nineteenth century took up this idea again in a context of planetary studies showing the similarities as well as the differences of the celestial bodies orbiting our Sun. Astronomers attempted to look deeper into the problem of habitability such as Richard Proctor or Camille Flammarion, also well-known for their fine talent in popular writings. While the Martian canals controversy was reaching its height, they imagined how the living forms dwelling in other planets could be. Nowadays, no complex exo-life is expected to have evolved in our solar system. However, the famous exobiologist Carl Sagan and later other scientists, formulated audacious ideas about other forms of life in the light of recent discoveries in planetology. Through these few examples, this paper underlines the originality of each author's suggestions and the evolution and contrast of ideas about the possible life forms in the universe.