Chthonian planet

File:Artist’s impression of Corot-7b (alternative).jpg

Artist's conception of CoRoT-7b.

File:Transiting planet HD 209458b.png

Artist's conception of HD 209458 b transiting its star.

Chthonian planets (Template:IPAc-en, 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.<ref name="hebrard">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.</ref>

EtymologyEdit

Chthon (from Template:Langx) means "earth". The term chthonian was coined by Guillaume Hébrard et al. and generally refers to Greek chthonic deities from the infernal underground.

Possible examplesEdit

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 two Earth radii,<ref>Template:Cite journal</ref> they might have densities larger than that of an iron planet of the same size. These exoplanets orbit very close to their stars and 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.<ref>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</ref><ref>Template:Cite journal</ref>

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, it is assumed that exoplanets below such radii exposed to such stellar fluxes could have had their envelopes stripped by photoevaporation.<ref>Template:Cite journal</ref>

HD 209458 bEdit

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 436 b, which has already lost 10% of its atmosphere.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

CoRoT-7bEdit

CoRoT-7b is the first exoplanet found that might be chthonian.<ref name="core">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="bbccthonian"> {{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Other researchers dispute this, and conclude CoRoT-7b was always a rocky planet and not the eroded core of a gas or ice giant,<ref> Template:Cite journal</ref> due to the young age of the star system.

TOI-849 bEdit

In 2020, a high-density planet more massive than Neptune was found very close to its host star, within the Neptunian desert. This world, TOI-849 b, may very well be a chthonian planet.<ref>Template:Cite journal</ref>

ReferencesEdit

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