The observational detection of a localized reduction in the small planet occurrence rate, sometimes termed a ‘gap’, is an exciting discovery because of the implications for planet evolutionary history. We examined the trends of a real sample of observed small close-in planets in the three-dimensional radius-insolation-density space and found 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, suggesting a common formation mechanism. 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. We investigate the transition between sub-Neptunes and super-Earths by applying envelope evolution models of the H/He envelope together with the mass-radius diagram and a photoevaporation model. Our analysis also shows that Sub-neptunes are found to have more massive cores, and lost on average 30% of their envelope, while super-Earths lost on average 100%, shrinking their radii significantly.