Chthonian Planet

A Chthonian Planet is a theoretical type of planet which is formed from the stripping away of a hot giant’s atmosphere by the intense radiation of its star(s) (known as hydrodynamic escape), leaving only the rocky or metallic core. Close-orbiting brown dwarfs may also become cthonian planets. Chthonian planets would resemble terrestrial worlds, but would be far denser for many years before they finally decompress (the more massive the original planet, the longer it takes).

The nature of their formation requires them to orbit extremely close to their star. COROT-7b, a possible Chthonian planet, orbits a mere 0.017 AU from its star, COROT-7. However, this might not be true, as chthonian planets takes Time to evaporate all the gas, but COROT-7 is a very young planetary system.

Worldbuilding in Practice
"Geffen is a tiny terrestrial planet orbiting the Delta Cepheid variable star Eta Rodriguez A. Orbiting at a measly 0.027 AU from the star, it is baked by the intense radiation. Astronomical observation indicates that the planet is far denser than any standard terrestrial planet, indicating it could be a Chthonian Planet. With a high mass of 0.58 MJ (184 M⊕), it is likely the remains of a close orbiting brown dwarf."