Chthonian planets (sometimes spelled Cthonian) are the hypothetical planets left behind when a Gas Giant loses its gas. These planets are the rocky core left behind after a Star's gravity pulls the gas off a giant, or a Star's heat evaporates the gas off of it. Having lost it's gaseous overcoat, the core will slowly cool and solidify.
Chthonian refers to the subset of ancient greek gods that lived in the "hot infernal underground", much in the same way that these are the planets that used to be beneath the surface of a Hot Jupiter.
Game and Story Use
- The conditions inside a Gas Giant are similar to the Primordial Soup that begat life on earth, so there's some chance that a Chthonian planet might end up with life on it. Anything that survived the atmosphere being stripped off would have to be very hardy and resilient. Single-celled or viral life probably has the best odds, seeing as how it's former environment went away.
- Chthonian Planets may be valuable mining operations - the sort of stuff that's usually inaccessible at a planetary core is right there on the surface - because this is the planetary core!