Prior investigations of planets with high carbon-to-oxygen ratios include Fegley & Cameron 1987. Carbon planets could form if protoplanetary discs are carbon-rich and oxygen-poor. They would develop differently from Earth, Mars, and Venus, which are composed mostly of silicon–oxygen compounds. Different planetary systems have different carbon-to-oxygen ratios, with the Solar System's terrestrial planets closer to being "oxygen planets" with C/O molar ratio of 0.55. In 2020, survey of the 249 nearby solar analog stars found 12% of stars have C/O ratios above 0.65, making them candidates for the carbon-rich planetary systems. The exoplanet 55 Cancri e, orbiting a host star with C/O molar ratio of 0.78, is a possible example of a carbon planet.