  The potentially important role of stellar irradiation in envelope removal for
planets with diameters of $\lessapprox$ 2 R$_{\Earth}$ has been inferred both
through theoretical work and the observed bimodal distribution of small planet
occurrence as a function of radius. We examined the trends for small planets in
the three-dimensional radius-insolation-density space and find that the
terrestrial planets divide into two distinct families based on insolation. The
lower insolation family merges with terrestrial planets and small bodies in the
solar system and is thus Earth-like. The higher insolation terrestrial planet
family forms a bulk-density continuum with the sub-Neptunes, and is thus likely
to be composed of remnant cores produced by photoevaporation. Based on the
density-radius relationships, we suggest that both terrestrial families show
evidence of density enhancement through collisions. Our findings highlight the
important role that both photoevaporation and collisions have in determining
the density of small planets.
