A carbon planet, also referred to as a diamond planet or carbide planet, is a theoretical type of planet proposed by Marc Kuchner that could form if protoplanetary discs are carbon-rich and oxygen-poor. According to planetary science, it would develop differently from Earth, Mars and Venus, planets with crusts made up mostly of silicon-oxygen compounds.
Such a planet would probably have an iron-rich core like the known terrestrial planets. Surrounding that would be silicon carbide and titanium carbide. Above that, a layer of carbon in the form of graphite, possibly with a kilometers-thick substratum of diamond if there is sufficient pressure. The surface would contain hydrocarbons and carbon monoxide. Extraterrestrial life might be possible if water is present, but the highly reducing environment could result in metabolism taking the opposite approach to that of terrestrial life, with oxygen-bearing compounds being eaten as food to react with the carbon-based atmosphere.
Geological features could arise that would be analogous to those on Earth, but with different contents. Lakes and seas could be made of methane, tar or other oils.
The pulsar PSR 1257+12 may possess carbon planets that formed from the disruption of a carbon-producing star. Carbon planets might also be located near the galactic core, where stars have more carbon.
Carbon planets are predicted to be of similar diameter to silicate and water planets of the same mass, potentially making them difficult to distinguish.