Carbon planet

[5] The exoplanet 55 Cancri e, orbiting a host star with C/O molar ratio of 0.78,[6] is a possible example of a carbon planet.

Above that, a layer of carbon in the form of graphite, possibly with a kilometers-thick substratum of diamond if there is sufficient pressure.

[7] A weather cycle is hypothetically possible on carbon planets with an atmosphere, provided that the average surface temperature is below 77 °C.

In 2011, NASA cancelled a mission called TPF, which was to be an observatory much bigger than the Hubble Space Telescope that would have been able to detect such planets.

Carbon planets might also be located near the Galactic Center or globular clusters orbiting the galaxy, where stars have a higher carbon-to-oxygen ratio than the Sun.

[12] In August 2011, Matthew Bailes and colleagues from Swinburne University of Technology in Australia reported that the millisecond pulsar PSR J1719-1438 may have a binary companion star that has been crushed into a much smaller planet made largely of solid diamond.

Further examination revealed that although the planet is relatively small (60,000 km in  diameter, or five times bigger than the Earth) its mass is slightly more than that of Jupiter.

The high density of the planet gave the team a clue to its likely makeup of carbon and oxygen—and suggested the crystalline form of the elements.