A Chthonian planet (sometimes misspelled Cthonian), is a gas giant with its hydrogen and helium atmosphere stripped away due to its closeness to its star. The remaining rocky or metallic core would resemble a terrestrial planet in many respects.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.

HD 209458b (Osiris) 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. Some scientists suggest that Mercury is a Chthonian planet due to its high density and metal-rich core, but other than this possibility no known Chthonian planets have been discovered.

The word chthon (as in "chthonic deity") is from chthonia, which means "of the Earth" in Greek. There are at least three different Chthonias in Greek mythology, and they are all closely associated with the goddess Demeter.

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