9. The One That's Evaporating
Do you know what a chthonian planet is? Spoiler alert: it has very little to do with the Cthulhu mythos of author (and notorious racist) H. P. Lovecraft, and it has even less to do with the Marvel Comics villain, Chthon. The word "chthonian" is defined as "concerning, belonging to, or inhabiting the underworld." So it's only natural that chthonian planets are those that are, in essence, underworlds. That is to say, they are planets that have had their surface layers stripped off by the incredible heat from their parent star, leaving only their inner layers - the world underneath. See? Underworld.
While the gas giant HD 209458b, AKA Osiris, may not be one yet, scientists assume it's on the path to becoming one. Eventually, the "gas" in "gas giant" will be no more.
Osiris is horrifically close to its parent star - so close, in fact, that its star is slowly destroying it. It's evaporating as we speak, and there's really nothing that can be done to mediate or mitigate it. As it stands, Osiris is doomed to slowly, painfully die - a fate befitting a planet named after the Egyptian god of the dead.