A Chthonian planet (pronounced /ˈkθoʊniən/, sometimes misspelled Cthonian), 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 hydrodynamicescape. Such atmospheric stripping is a likely result of proximity to a star. The remaining rocky or metallic core would resemble a terrestrialplanet in many respects.
HD209458b is an example of a planet that is in the process of having its atmosphere stripped away, though it is not itself a Chthonian planet nor is it expected to become one in the near future.
COROT-7b may be the first Chthonian planet discovered.
Chthonia (Greek: χθων) means "of the Earth". The term was coined by Hébrard et al., since the term Chthoniangenerally refer to Greek deities from hot infernal underground.