The Impact of Stellar Distances on Habitable Zone Planets

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.