Smertrios, HD 149026b, a hot Hyperbarian world with a dense core of nearly 100 x Earth's mass, mostly water ice and silicates but with an innermost core of iron
These worlds form in systems with high metallicity (that is to say, high levels of elements heavier than helium). They can have very high gravity (ten or twenty times Earth's). These planets almost always retain a dense but compact atmosphere, as atmospheric escape is very slow for even the lightest gases unless the world is extremely near to the local star. Hot hyperbarian worlds are more common than cooler examples, since they tend to form in the denser inner parts of protoplanetary clouds. Example Smertrios, HD 149026b
Beneath a layer of metallic hydrogen, these worlds generally have a mantle of high-pressure water ice, an outer core of silicate and an innermost core of iron, although the proportions of each constituent vary greatly. Some very dense worlds are extraordinarily dry, and are believed to have formed after a collision between very large protoplanets.