A chthonian planet (//, sometimes '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.[clarification needed] A similar case would be Gliese 436b, as it has already lost 10% of its atmosphere. 
Chthonia (from Greek: Χθών) 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.
- COROT-7b: diameter 1.7 × Earth, mass 4.8 × Earth, orbital period = 20.5 hours (super-Earth candidate)
- Hot Jupiter
- Hypothetical astronomical object
- Hébrard G., Lecavelier Des Étangs A., Vidal-Madjar A., Désert J.-M., Ferlet R. (2003), Evaporation Rate of Hot Jupiters and Formation of chthonian Planets, Extrasolar Planets: Today and Tomorrow, ASP Conference Proceedings, Vol. 321, held 30 June – 4 July 2003, Institut d'astrophysique de Paris, France. Edited by Jean-Philippe Beaulieu, Alain Lecavelier des Étangs and Caroline Terquem.
- "Hubble sees atmosphere being stripped from Neptune-sized exoplanet". Nature. 2015-06-24. Retrieved 2015-11-08.
- "Exoplanets Exposed to the Core". AstroBiology Magazine. 2009-04-25. Retrieved 2009-07-13.
- "Super-Earth 'began as gas giant'". BBC News. 10 January 2010. Retrieved 2010-01-10.
- "Planet : CoRoT-7 b". Retrieved 2009-06-13.
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