The core would likely be iron-rich like a terrestrial planet, but that would be covered by a thick, hot silicon carbide and titanium carbide layer, solid graphite within the crust, and possibly diamonds, not silicate minerals and rocks. The surface would be covered with hydrocarbons and the atmosphere is likely to have carbon monoxide in large amounts. At milder temperatures, with plenty of light, methane in the atmosphere would break apart and combine together, turning into heavier hydrocarbons. These would rain down as oils and tars. In the coldest temperature ranges, the methane could be a liquid and flow as oceans and rivers. It has been suggested that volcanic activity might allow the diamond and silicon carbide under the crust to form mountains.
If there is liquid water on the planet, it may have life. Since the ratios of silicon and carbon have been reversed, it's possible that the planet would evolve Silicon-Based Life. As the ratios of oxygen and carbon in the atmosphere have been reversed, it's possible oxygen-compounds would be used as food and carbon would be respired. The biosphere may be able to support Methane-Breathing Life. A species could breathe the carbon monoxide for energy by processing it with water to carbon dioxide and hydrogen. Another species might ingest carbon compounds and use the hydrogen to convert them to methane, yielding energy. A final species might eat an oxidant in their food supply and respire it with the methane to make energy.
Stars in the Galactic Core have more carbon in them than stars near our own, so it's possible that carbon planets are the norm in those regions. The lack of oxygen means they will be quite dry, and their carbon-rich atmospheres will lead to a major greenhouse effect, as well as blocking light. They might be similar to a Desert Planet with bitumen sands and graphite rocks. At larger distances from the star, carbon planets might have life with Hydrocarbons As Biological Solvent. Another variant might be a Gas Giant with an atmosphere rich with carbon monoxide, methane and heavier hydrocarbons.
Game and Story Use
- They'd be hell for humans to try to live on - and you can forget terraforming - but you could mine them. They'd be a source not only of diamond, but also of other carbon compounds we have yet to discover.
- With all that carbon, it's possible you'd end up with a planet that's just teeming with life.