Exoplanets: Criteria for their Habitability and Possible Biospheres

The word “habitable” is derived from the classical Latin habitabilis (to inhabit, to dwell). As early as 1853, William Whewell introduced the notion of a planet orbiting in the “temperate zone” where liquid water (an essential requirement for all life on Earth) on the surface is favored. A century later, astronomer Harlow Shapley (1953) discussed climate conditions for planets orbiting in the so-called water belt in the context of understanding Earth’s climate change. In the same year, Hubertus Strughold investigated the Solar System “ecosphere” (1953) as part of a physiological study of survival on Mars. Huang (1960) discussed the requirements a star should fulfill to support life in its so-called Habitable Zone (HZ). Dole (1964) then discussed the “complex life HZ,” the region where a planet has surface temperatures from 0 to 30 ;°C over >10 ;% of its surface, an oxygen (O2)-rich atmosphere and