Artist’s impression of Corot-7b (alternative)

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 An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...

and outer layers, which is called hydrodynamic escape. Such atmospheric stripping is a likely result of proximity to a

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

. The remaining rocky or metallic core would resemble a terrestrial planet in many respects.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.

Etymology

''Chthon'' (from el, Χθών) means "earth". The term ''chthonian'' was coined by Hébrard, et al., and generally refers to Greek

chthonic The word chthonic (), or chthonian, is derived from the Ancient Greek word ''χθών, "khthon"'', meaning earth or soil. It translates more directly from χθόνιος or "in, under, or beneath the earth" which can be differentiated from Γῆ ...

deities from the infernal underground.

Possible examples

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 An Earth mass (denoted as M_\mathrm or M_\oplus, where ⊕ is the standard astronomical symbol for Earth), is a unit of mass equal to the mass of the planet Earth. The current best estimate for the mass of Earth is , with a relative uncertain ...

(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 ] An iron planet is a type of planet that consists primarily of an iron-rich core with little or no mantle. Mercury is the largest celestial body of this type in the Solar System (as the other terrestrial planets are silicate planets), but larger ...

of the same size. These exoplanets are orbiting 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. 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. 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. 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.

References

{{exoplanet Hypothetical planet types

Etymology

Possible examples

See also

References