  Among the most highly valued of exoplanetary discoveries are those of
terrestrial planets found to reside within the Habitable Zone (HZ) of the host
star. In particular, those HZ planets with relatively bright host stars will
serve as priority targets for characterization observations, such as those
involving mass determinations, transmission spectroscopy, and direct imaging.
The properties of the star are greatly affected by the distance measurement to
the star, and subsequent changes to the luminosity result in revisions to the
extent of the HZ and the properties of the planet. This is particularly
relevant in the realm of Gaia which has released updated stellar parallaxes for
the known exoplanet host stars. Here we provide a generalized formulation of
the effect of distance on planetary system properties, including the HZ. We
apply this methodology to three known systems and show that the recent Gaia
Data Release 2 distances have a modest effect for TRAPPIST-1 but a relatively
severe effect for Kepler-186 and LHS 1140.
