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
Circumstellar disc star (protostar) is formed by the gravitational collapse of a pocket of matter within a giant molecular cloud. The infalling material possesses some amount of angular momentum, which results in the formation of a gaseous protoplanetary disc around the young, rotating star. The former is a rotating circumstellar disc of dense gas and dust that continues to feed the central star. It may contain a few percent of the mass of the central star, mainly in the form of gas which is itself mainly hydrogen. The main accretion phase lasts a few million years, with accretion rates typically between 10