  The population of small, close-in exoplanets is bifurcated into super-Earths
and sub-Neptunes. We calculate physically motivated mass-radius relations for
sub-Neptunes, with rocky cores and H/He dominated atmospheres, accounting for
their thermal evolution, irradiation and mass-loss. For planets $\lesssim
10~$M$_\oplus$, we find that sub-Neptunes retain atmospheric mass fractions
that scale with planet mass and show that the resulting mass-radius relations
are degenerate with results for `water-worlds' consisting of a 1:1
silicate-to-ice composition ratio. We further demonstrate that our derived
mass-radius relation is in excellent agreement with the observed exoplanet
population orbiting M-dwarfs and that planet mass and radii alone are
insufficient to determine the composition of some sub-Neptunes. Finally, we
highlight that current exoplanet demographics show an increase in the ratio of
super-Earths to sub-Neptunes with both stellar mass (and therefore luminosity)
and age, which are both indicative of thermally driven atmospheric escape
processes. Therefore, such processes should not be ignored when making
compositional inferences in the mass-radius diagram.
