Extrasolar planets hosted by stars with sufficiently high carbon-to-oxygen ratios could be made of diamonds and silica, according to new research by Arizona State University and the University of Chicago.
When stars and planets are formed, they do so from the same cloud of gas, so their bulk compositions are similar.
A star with a lower carbon to oxygen ratio will have planets like Earth, comprised of silicates and oxides with a very small diamond content.
But exoplanets around stars with a higher carbon to oxygen ratio than our Sun are more likely to be carbon-rich.
Arizona State University researcher Dr. Harrison Allen-Sutter and colleagues hypothesized that these carbide exoplanets could convert to diamond and silicate, if water were present, creating a diamond-rich composition.
To test this hypothesis, the scientists needed to mimic the interior of carbide exoplanets using high heat and high pressure.
To do so, they used high pressure diamond-anvil cells in a lab.
First, they immersed silicon carbide in water and compressed the sample between diamonds to a very high pressure.
Then, to monitor the reaction between silicon carbide and water, they conducted laser heating, taking X-ray measurements while the laser heated the sample at high pressures.
As they predicted, with high heat and pressure, the silicon carbide reacted with water and turned into diamonds and silica.
Therefore, if water can be incorporated into carbide planets during their formation or through later delivery, they could be oxidized and have mineralogy dominated by silicates and diamond in their interiors.
The reaction could produce CH4 at shallower depths and H2 at greater depths that could be degassed from the interior, causing the atmospheres of the converted carbon planets to be rich in reducing gases. Excess water after the reaction can be stored in dense silica polymorphs in the interiors of the converted carbon planets.
Such conversion of mineralogy to diamond and silicates would decrease the density of carbon-rich planet, making the converted planets distinct from silicate planets in mass–radius relations for the 2–8 Earth mass range.
While Earth is geologically active, the team’s results show that carbide planets are too hard to be geologically active and this lack of geologic activity may make atmospheric composition uninhabitable.
References: Allen-Sutter et al. 2020. Oxidation of the Interiors of Carbide Exoplanets. Planet. Sci. J 1, 39; doi: 10.3847/PSJ/abaa3e