Exoplanets: Criteria for their Habitability and Possible Biospheres

Kurzfassung

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 <1.5 Earth’s gravity. Hart (1979a, b) applied a numerical climate model to estimate the width of the HZ for liquid water and showed that runaway climate processes implied a thin HZ extending from 0.95 to 1.01 astronomical units (AU). Schneider and Thompson (1980), however, argued that understanding of complex climate feedbacks (e.g., between atmosphere and glaciation) suggested that such climate estimates are only “order of magnitude.” Kasting et al. (1988) showed that including long-term negative climate feedbacks such as the carbonate-silicate cycle could stabilize a planet’s climate and expand the HZ width calculated by the Hart et al. studies. A key study by Kasting et al. (1993) subsequently investigated the HZ width for a range of main sequence stars. In the modern literature, the HZ is widely studied, including models with, for example, complex climate feedbacks, interactive atmospheric climate-chemistry (e.g., Segura et al., 2003; Grenfell et al., 2007a), radiative effects of clouds (Kitzmann et al., 2011), climate dependence on planetary orbit (e.g., Williams and Pollard, 2003), the effect of 3D planetary properties such as ocean mass and albedo (e.g., Abe et al., 2011), and investigation of climate and evolution (e.g., Selsis et al., 2007; Wordsworth et al., 2011).