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 a star. The remaining rocky or metallic core would resemble a terrestrial planet in many respects.
Chthon (from Greek: Χθών) means "of the Earth". The term was coined by Hébrard et al., since the term chthonian generally refers to Greek deities from the infernal underground.
Transit-timing variation measurements indicate for example that Kepler-52b, Kepler-52c and Kepler-57b have maximum-masses between 30 and 100 times the mass of Earth (although the actual masses could be much lower); with radii about 2 Earth radii, they might have densities larger than that of an iron planet of the same size. As such, exoplanets orbiting very close to their stars could be the remnant cores of evaporated gas giants or brown dwarfs. If cores are massive enough they could remain compressed for billions of years despite losing the atmospheric mass.
As there is a lack of gaseous "hot-super-Earths" between 2.2 and 3.8 Earth-radii exposed to over 650 Earth incident flux, and such lack is not supported by statistics,[clarification needed] it is assumed that exoplanets below such radii exposed to such stellar fluxes could have had their envelopes stripped by photoevaporation.
HD 209458 b is an example of a gas giant that is in the process of having its atmosphere stripped away, though it will not become a chthonian planet for many billions of years, if ever. A similar case would be Gliese 436b, which has already lost 10% of its atmosphere.
COROT-7b is the first exoplanet found that might be chthonian. Other researchers dispute this, and conclude COROT-7b was always a 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.
- Hébrard G., Lecavelier Des Étangs A., Vidal-Madjar A., Désert J.-M., Ferlet R. (2003), Evaporation Rate of Hot Jupiters and Formation of chthonian Planets, Extrasolar Planets: Today and Tomorrow, ASP Conference Proceedings, Vol. 321, held 30 June – 4 July 2003, Institut d'astrophysique de Paris, France. Edited by Jean-Philippe Beaulieu, Alain Lecavelier des Étangs and Caroline Terquem.
- Mocquet, A.; Grasset, O. and Sotin, C. (2013) Super-dense remnants of gas giant exoplanets, EPSC Abstracts, Vol. 8, EPSC2013-986-1, European Planetary Science Congress 2013
- Mocquet, A.; Grasset, O.; Sotin, C. (2014). "Very high-density planets: a possible remnant of gas giants". Phil. Trans. R. Soc. A. 372 (2014): 20130164. Bibcode:2014RSPTA.37230164M. doi:10.1098/rsta.2013.0164. PMID 24664925.
- Lundkvist et al. (2016), "Hot super-Earths stripped by their host stars", arXiv:1604.05220 [astro-ph.EP]
- "Hubble sees atmosphere being stripped from Neptune-sized exoplanet". Nature. 2015-06-24. Retrieved 2015-11-08.
- "Exoplanets Exposed to the Core". AstroBiology Magazine. 2009-04-25. Retrieved 2018-01-07.
- "Super-Earth 'began as gas giant'". BBC News. 10 January 2010. Retrieved 2010-01-10.
- Odert, P. (2010). "Thermal mass-loss of exoplanets in close orbits" (PDF). EPSC Abstracts. 5: 582. Bibcode:2010epsc.conf..582O.
- STRUCTURAL MODELING FOR THE LOW-MASS TRANSITING EXOPLANETS:KEPLER-70B AND KEPLER-70C., P. Futó, 45th Lunar and Planetary Science Conference (2014)