Classification of exoplanets according to density
  Considering probability distribution as a function of the average density $\bar{\rho}$ computed for 424 extrasolar planets we identify three log-normal Gaussian population components. The two most populous components at $\bar{\rho}\simeq0.7$ g/cc and $\bar{\rho}\simeq7$ g/cc are the ice/gas giants and iron/rock super-Earths, respectively. A third component at $\bar{\rho}\simeq30$ g/cc is consistent with brown dwarfs, i.e., electron degeneracy supported objects. We note presence of several extreme density planetary objects. 
Andrzej Odrzywolek and Johann Rafelski
astro-ph.EP