  A large number of terrestrial planets in the classical habitable zone of
stars of different spectral types has already been discovered and many are
expected to be discovered in near future. However, owing to the lack of
knowledge on the atmospheric properties, the ambient environment of such
planets are unknown. It is known that sufficient amount of Extreme Ultraviolet
(EUV) radiation from the star can drive hydrodynamic outflow of hydrogen that
may drag heavier species from the atmosphere of the planet. If the rate of mass
loss is sufficiently high then substantial amount of volatiles would escape
causing the planet to become uninhabitable. Considering energy-limited
hydrodynamical mass loss with an escape rate that causes oxygen to escape
alongwith hydrogen, I present an upper limit for the ratio between the EUV and
the bolometric luminosities of stars which constrains the habitability of
planets around them. Application of the limit to planet-hosting stars with
known EUV luminosities implies that many M-type of stars should not have
habitable planets around them.
