Carbon-rich exoplanets real and hypothetical

Artist’s impression of a carbonaceous exoplanet, taken from From the website:

A hypothetical extrasolar carbon planet. You are seeing the south pole of the planet to the left, where methane has condensed into ice. The white dots on the surface are reflections from layers of diamonds on the surface. The seas found at the center of the image are made of oil of various hydrocarbons. The dark areas on the planet are marks of tar-like precipitation. The clouds are also made of various hydrocarbons.

Carbon and oxygen are of roughly equal abundance in the Universe. However in most stars oxygen predominates: for example the solar C:O ratio is 0.57. Observationally, a great deal can depend on whether the ratio of carbon to oxygen is large (C:O > 1) or small in a given system. Most evolved red giants are richer in oxygen, with spectra very different – showing bands of oxygen-rich molecules and possibly silicate-rich dust – to those in which carbon is predominant. The latter are the rarer carbon stars, whose optical spectra are dominated by bands of C2 and which may also show infrared features arising from polycyclic aromatic hydrocarbons (PAHs). Systems richer in carbon may host carbon-rich planets. Where the C:O ratio is solar carbon and oxygen combine to form the CO molecule, but if C:O > 1 CH4 is formed instead, and it is this which has been observed in the planet WASP-12b.

Similarly, for the recently announced carbon-rich exoplanet 55 Cancri e, the C:O ratio as measured in the stellar spectrum is 1.12 ± 0.19. With this in mind, modelling of the possible carbon-rich interior, complete with diamond layer, is discussed here.

Update: High C:O ratios in planet-hosting stars called into question


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