  Oxygenic photosynthesis is the most important biochemical process in Earth
biosphere and likely very common on other habitable terrestrial planets, given
the general availability of its input chemical ingredients and of light as
source of energy. It is therefore important to evaluate the effective
possibility of oxygenic photosynthesis on planets around stars as a function of
their spectral type and the planet-star separation. We aim at estimating the
photon flux, the exergy and the exergetic efficiency of the radiation in the
wavelength range useful for the oxygenic photosynthesis as a function of the
host star effective temperature and planet-star separation. We compute
analytically these quantities and compare our results with the estimates for
the small sample of known Earth-like planets. We find that exergy is an
increasing function of the star effective temperature, within the range
2600-7200 K. It depends both on the star-planet separation and the star
effective temperature. Biospheres on exoplanets around cool stars might be
generally light-limited. So far, we have not observed terrestrial planets
comparable to Earth in terms of useful photon flux, exergy and exergetic
efficiency.
