  It may be possible to detect biosignatures of photosynthesis in an
exoplanet's atmosphere. However, such a detection would likely require a
dedicated study, occupying a large amount of telescope time. It is therefore
prudent, while searching for signs of life that we may recognise, to pick the
best target possible. In this work, we present a new region, the
``photosynthetic habitable zone'' \textemdash the distance from a star where
both liquid water and oxygenic photosynthesis can occur. It is therefore the
region where detectable biosignatures of oxygenic photosynthesis are most
likely to occur. Our analysis indicates that in the most ideal conditions for
life and no atmospheric effects, the photosynthetic habitable zone is almost as
broad as the habitable zone. On the other hand, if conditions for life are
anything less than excellent and atmospheric effects are even moderate, the
photosynthetic habitable zone is concentrated at larger separations around more
massive stars. Such cases are also not tidally locked to their host star, which
could result in planetary rotation periods similar to the Earth's. We identify
five planets, Kepler-452 b, Kepler-1638 b, Kepler-1544 b and Kepler-62 e and
Kepler-62 f, that are consistently in the photosynthetic habitable zone for a
variety of conditions, and we predict their day lengths to be between 9 and 11
hours. We conclude that the parameter space in which we should search for signs
of life is much narrower than the standard habitable zone.
