Carbon planets may harbour graphite plains, diamond mountains

Carbon planets are hypothetical worlds that may literally contain diamonds strewn across their surfaces. As against a crust rich in silicates on Earth or Mars, carbon planets are small worlds with a composition dominated by graphites and carbides. Scientists theorise that these exoplanets may form in carbon-rich stellar environments, where the carbon-to-oxygen ratio exceeds that of our Solar System. In such environments, carbon dominates over silicates during the process of planet assembly, creating cores and mantles rich in carbon compounds. Instead of rocky terrains and water oceans, the landscapes on these worlds may be made up of vast plains of graphite or crystalline diamond, forged under immense pressure. Models indicate that the atmospheres of carbon planets could be thick in methane or carbon monoxide.

Carbon planets were first proposed by the American planetary scientist Marc Kuchner in 2004, building on the idea that planetary compositions is influenced by the makeup of the star. Humans do not know of a confirmed carbon planet, but there are some candidates, including 55 Canri e, a Super Earth orbiting a carbon-rich star at a distance of 40 lightyears from the Earth. The world was initially called a ‘diamond planet’. The temperature of the planet is extremely high, at 2,400°C. Despite the high temperature the planet is dense, indicating a carbon-rich composition. The nature of the world is debated, and a carbon planet is just one of the possible explanation.

Carbon planets are compelling and of scientific interest because if confirmed, they can reshape our understanding of planetary evolution and habitability. Carbon is the backbone of life on Earth, but on these worlds, it is the very ground. Exotic lifeforms could thrive in a methane rich atmosphere. Detecting carbon planets is a challenge though. The James Webb Space Telescope is just about the only astronomical instrument around that is capable of probing the composition of alien atmospheres. However, determining the bulk composition of a planet requires more indirect methods, such as finding out the density of the planet and the chemistry of the host star. Future missions, such as the European ARIEL telescope is expected to survey thousands of exoplanets, and can potentially identify carbon planets with spectral signatures.

Due to an abundance oof carbon and limited oxygen, carbon planets may have atmospheres rich in methane. There might be subsurface or surface liquids on the surface, made up of methane or ethane. The landscapes would be made up of hydrocarbon soots. Instead of water, life could use methanes as solvents, with the metabolic processes based on the chemistry of hydrocarbons. Organisms might metabolise complex hydrocarbons, while releasing carbon dioxide or acetylene as byproducts. Microbial lifeforms exploiting redox reactions involving methane could resemble methanogenic archaea on Earth. If discovered, such worlds could push the boundaries of astrobiology.