A world close to a star which has been scoured by hot solar winds. They are often the cores of Hermian or similar worlds which have been left behind after their mantle was removed in a collision early in formation. As such, they tend to be small and dense.
Mostly Silicates, Ferric and Heavy Metals. Will have large amounts of radioactive elements and precious metals, making them a oft sought target for mining. Their close proximity to a star makes such mining a tricky proposition though.