A Chthonian planet is a hypothetical class of celestial objects resulting from the stripping away of a gas giant's hydrogen and helium atmosphere and outer layers, which is called hydrodynamic escape. Such atmospheric stripping is a likely result of proximity to a star. The remaining rocky or metallic core would resemble a terrestrial planet in many respects.
HD 209458 b is an example of a planet, which is in the process of having its atmosphere stripped away, although it is not expected to become a Chthonian planet anytime in the near future.
COROT-7b might be a Chthonian planet, and if it is, would be the first one discovered.