Chthonian planet

Artist's conception of COROT-7b.

Artist's conception of HD 209458 b transiting its star.

A chthonian planet ( /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 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 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-7 b COROT-7 b may be the first chthonian planet discovered, followed by Kepler-10b.^{[citation needed]}

Chthonia (from Greek: χθων) means "of the Earth". The term was coined by Hébrard et al., since the term chthonian generally refers to Greek deities from the infernal underground.

See also

• Hot Jupiter
• COROT-7b: diameter 1.7 x Earth, mass 4.8 x Earth, orbital period = 20.5 hours (super-Earth candidate)
• Hypothetical planet