Figure 1: Artist’s impression of a rocky planet.
The transition from a predominantly rocky composition to a volatile-rich composition occurs for planets roughly 1.6 to 1.8 times the radius of Earth. Planets around this size and larger are known as super-Earths since they are larger in size than Earth, but smaller than Neptune. For comparison, Neptune is about 3.8 times the size of Earth. Studies have shown that hot-super-Earth sized planets that orbit close to their host stars can have their volatile-rich envelopes stripped by photo-evaporation due to high incident fluxes. This results in a lack of super-Earth sized planets with high incident fluxes.
A study by Lundkvist et al. (2016) shows there appears to be no super-Earth sized planets between 2.2 and 3.8 times the size of Earth that receive incident flux above 650 times the incident flux on Earth. This is consistent with the prediction that the majority of planets between 2.2 and 3.8 times the size of Earth are expected to be volatile-rich and would be stripped of their outer envelopes when subjected to high incident fluxes. As a result, some fraction of hot-super-Earth sized planets smaller than 2.2 times the size of Earth started out as larger planets whose volatile-rich envelopes were stripped away by high incident fluxes.
Figure 2: Radius-flux diagram showing the distribution of exoplanets. The location of the four rocky solar-system planets; Mercury (Me), Venus (V), Earth (E) and Mars (M) is indicated with the green writing (no points). The location of the hot-super-Earth desert has been shaded. Lundkvist et al. (2016)
Lundkvist et al. (2016), "Hot super-Earths stripped by their host stars", arXiv:1604.05220 [astro-ph.EP]