  The dozens of rocky exoplanets discovered in the Circumstellar Habitable Zone
(CHZ) currently represent the most suitable places to host life as we know it
outside the Solar System. However, the presumed presence of liquid water on the
CHZ planets does not guarantee suitable environments for the emergence of life.
According to experimental studies, the building blocks of life are most likely
produced photochemically in presence of a minimum ultraviolet (UV) flux. On the
other hand, high UV flux can be life-threatening, leading to atmospheric
erosion and damaging biomolecules essential to life. These arguments raise
questions about the actual habitability of CHZ planets around stars other than
Solar-type ones, with different UV to bolometric luminosity ratios. By
combining the "principle of mediocricy" and recent experimental studies, we
define UV boundary conditions (UV-habitable Zone, UHZ) within which life can
possibly emerge and evolve. We investigate whether exoplanets discovered in
CHZs do indeed experience such conditions. By analysing Swift-UV/Optical
Telescope data, we measure the near ultraviolet (NUV) luminosities of 17 stars
harbouring 23 planets in their CHZ. We derive an empirical relation between NUV
luminosity and stellar effective temperature. We find that eighteen of the CHZ
exoplanets actually orbit outside the UHZ, i.e., the NUV luminosity of their
M-dwarf hosts is decisively too low to trigger abiogenesis - through
cyanosulfidic chemistry - on them. Only stars with effective temperature >3900
K illuminate their CHZ planets with enough NUV radiation to trigger
abiogenesis. Alternatively, colder stars would require a high-energy flaring
activity.
