Fractional uncertainties in stellar radii (left) and stellar mass (right) vs. distance. Points with blue halos represent transiting-planet host stars. The diagonal lines represent the fractional error expected for a current nominal parallax error floor of 240 μas (black) and for an expected future parallax error of 20 μas (red). In the case of stellar mass (right), there are two diagonal lines representing the dependence of ﹩{\sigma }_{{M}_{\star }}/{M}_{\star }﹩ on σ_d for the transiting planets (where ﹩{M}_{\star }﹩ is determined directly from ﹩{\rho }_{\star }﹩ and ﹩{R}_{\star }﹩; upper line) and radial-velocity planets (where ﹩{M}_{\star }﹩ is determined from ﹩\mathrm{log}g﹩ and ﹩{R}_{\star }﹩; lower line).