  (abridged) A method for simulating light curves containing stellar
micro-variability for a range of spectral types and ages, based on
parameter-by-parameter scaling of a multi-component fit to the solar irradiance
power spectrum (from VIRGO/PMO6 data), and scaling laws derived from ground
based observations of various stellar samples, is presented.
  We observe a correlation in the Sun between the amplitude of the power
spectrum on weeks timescales and the BBSO Ca II K-line index of chromospheric
activity. On the basis of this evidence, the chromospheric activity level,
predicted from rotation period and B-V colour estimates according to the
relationship first introduced by Noyes(1983) and Noyes et al. (1984), is used
to predict the variability power on weeks time scale. The rotation period is
estimated on the basis of a fit to the distribution of rotation period versus
$B-V$ observed in the Hyades and the Skumanich (1972) spin-down law. The
characteristic timescale of the variability is also scaled according to the
rotation period.
  We use this model to estimate the impact of the target star type and age on
the detection capability of missions such as Eddington and Kepler. K stars are
found to be the most promising targets, while the performance drops
significantly for stars earlier than G and younger than 2.0 Gyr. Simulations
also show that \emph{Eddington} should detect terrestrial planets orbiting
solar-age stars in most of the habitable zone for G2 types and all of it for K0
and K5 types.
