Efficiency Of The Oxygenic Photosynthesis On Earth-like Planets In The Habitable Zone Source: astro-ph.EP Photons flux in two differently defined PAR ranges at the surface of planets at the the two edges of the HZ (dark blue lines for an upper limit of 800 nm and light blue for an upper limit of 750 nm), as a function of the star effective temperature, in units of 1020 photons s−1 m−2 (HZ inner edge: continuous line; HZ outer edge: dotted line). The green dot and circle show the photon flux in PAR range on the Earth surface, yellow dots and circles the estimated photon flux on the surface of known Earth analogs (see Table 1) respectively with an upper limit for the PAR range of 800 nm (dots) and 750 nm (circles). The red dotted line shows the average photon flux which is necessary to sustain the Earth biosphere. The green dotted line shows the typical lower threshold for OP on Earth.