From Academic Kids
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.
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, 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.