Figure 1: Artist's impression of a rocky planet.
Interior models of EPIC212521166 b show that the planet can be comprised of an Earth-like core with 9 times the mass of Earth containing ~70 percent rock and ~30 percent iron, and an overlying layer of water amounting to 9 times the mass of Earth. Alternatively, EPIC212521166 b can be comprised of an iron core and rocky mantle totalling 18.1 times the mass of Earth, with a ~2500 km thick hydrogen-helium atmosphere measuring just 0.2 times the mass of Earth.
The remarkably high mean density of EPIC212521166 b suggests that rocky planets with 10 to 20 times the mass of Earth can form without accreting significant amounts of hydrogen and helium. EPIC212521166 b is unlikely to be the massive core of a gas-giant planet whose hydrogen and helium envelope was stripped away by stellar radiation because radiation from its host star is incapable of removing any substantial amount of hydrogen from the planet. The proximity of EPIC212521166 b from its host star gives it an equilibrium temperature of 640 ± 20 K.
Figure 2: EPIC212521166 b (solid cross, right) compared to other super-Earth, sub-Neptune and Neptunian planets. The mass-radius curves shown here are for planets with 100 percent iron, Earth-like, 50 percent water and 100 percent water compositions (dashed lines from bottom to top). Osborn et al. (2016)
Osborn et al. (2016), "EPIC212521166 b: a Neptune-mass planet with Earth-like density", arXiv:1605.04291 [astro-ph.EP]