Chthonian planet rocky planet and not the eroded core of a gas or ice giant, due to the young age of the star system. Kepler-70b and Kepler-70c are sub-Earth planets that are thought to have been giant planets that were engulfed by the extended envelope of their star in its red giant phase vaporizing their gaseous envelope. Chthonian planet Chthonian planets (, sometimes 'cthonian') are 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
Red giant to the planet when the giant expands out to the orbital distance of the planet. Many of the well known bright stars are red giants, because they are luminous and moderately common. The red giant branch variable star Gamma Crucis is the nearest M class giant star at 88 light years. The K0 red giant branch star Arcturus is 36 light years away. In about 5 to 6 billion years, the Sun will have depleted the hydrogen fuel in its core. It will shrink, with the hydrogen outside the core able to compress enough for hydrogen there to fuse, and