  The recent detection of Earth-sized planets in the habitable zone of Proxima
Centauri, Trappist-1 and many other nearby M-type stars has led to
speculations, whether liquid water and life actually exist on these planets. To
a large extent, the answer depends on their yet unknown atmospheres, which may
though be within observational reach in the near future by JWST, ELT and other
planned telescopes. We consider the habitability of planets of M-type stars in
the context of their atmospheric properties, heat transport and irradiation.
Instead of the traditional definition of the habitable zone, we define the
bio-habitable zone, where liquid water and complex organic molecules can
survive on at least part of the planetary surface. The atmospheric impact on
the temperature is quantified in terms of the heating factor (a combination of
greenhouse heating, stellar irradiation, albedo etc.) and heat redistribution
(horizontal energy transport). We investigate the bio-habitable domain (where
planets can support surface liquid water and organics) in terms of these two
factors. Our results suggest that planets orbiting M-type stars may have
life-supporting temperatures, at least on part of their surface, for a wide
range of atmospheric properties. We apply this analyses to Proxima b and the
Trappist-1 system. Finally we discuss the implications to the search of
biosignatures and demonstrate how they may be used to estimate the abundance of
photosynthesis and biotic planets.
