Gas core

A gas core, also known as a chthonian planet, was once a gas giant, but due to some catastrophic incident in the past (most probably the death throws of what was formerly the system's primary star), was cooked quite hot and thereby stripped of most of its atmosphere, leaving behind a huge (super-terrestrial) core surrounded by heavier gasses. Such a world is quite large, with a high gravity and a poisonous, possibly even corrosive, atmosphere.