  A stellar evolution computer model has been used to determine changes in the
luminosity L and effective temperature T(e) of single stars during their time
on the main sequence. The range of stellar masses investigated was from 0.5 to
1.5 times that of the Sun, each with a mass fraction of metals (metallicity, Z)
from 0.008 to 0.05. The extent of each star's habitable zone (HZ) has been
determined from its values of L and T(e). These stars form a reference
framework for other main sequence stars. All of the 104 main sequence stars
known to have one or more giant planets have been matched to their nearest
stellar counterpart in the framework, in terms of mass and metallicity, hence
closely approximating their HZ limits. The limits of HZ, for each of these
stars, have been compared to its giant planet(s)'s range of strong
gravitational influence. This allows a quick assessment as to whether
Earth-mass planets could exist in stable orbits within the HZ of such systems,
both presently and at any time during the star's main sequence lifetime. A
determination can also be made as to the possible existence of life-bearing
satellites of giant planets, which orbit within HZs. Results show that about
half of the 104 known extrasolar planetary systems could possibly have been
housing an Earth-mass planet in HZs during at least the past billion years, and
about three-quarters of the 104 could do so for at least a billion years at
some time during their main sequence lives. Whether such Earth-mass planets
could have formed is an urgent question now being investigated by others, with
encouraging results.
