Carbon planet

A carbon planet is a hypothetical type of world that contains more carbon than oxygen. In our own Solar System, terrestrial planets like Earth, Mars, and Venus are primarily composed of silicon-oxygen compounds. These are often called "oxygen planets." Carbon is the fourth most abundant element in the universe by mass, following hydrogen, helium, and oxygen. If a protoplanetary disc is carbon-rich and oxygen-poor, it could form a planet with a very different chemistry.

The internal structure of a carbon planet would differ greatly from Earth. It would likely possess an iron-rich core similar to known terrestrial planets. Surrounding this core, there might be layers of molten silicon carbide and titanium carbide. Above these layers, a large amount of carbon could exist as graphite, which is the soft form of carbon. If the pressure is high enough, a thick substratum of diamond might form beneath the graphite. Volcanic activity could potentially push these diamonds toward the surface. This process might result in mountains made of diamond and silicon carbide.

The surface and atmosphere of these worlds would be quite extreme. Instead of water, the surface might contain liquid or frozen hydrocarbons, such as methane or tar. These planets would likely lack water because any oxygen delivered by comets or asteroids would react with the surface carbon. If the average surface temperature stays below 77 degrees Celsius, a weather cycle might be possible. The atmosphere of a cool carbon planet would consist mainly of carbon monoxide or carbon dioxide. It might also contain a significant amount of carbon smog.

The concept of the carbon planet was formally developed in 2005. Scientists Marc Kuchner and Sara Seager coined the term. They investigated these possibilities following a suggestion by Katharina Lodders regarding Jupiter's formation. While the idea is controversial, recent surveys provide interesting data. In 2020, a survey of 249 solar analog stars was conducted. Researchers found that 12% of these stars have carbon-to-oxygen ratios above 0.65. This makes them strong candidates for hosting carbon-rich planetary systems. One possible example is the exoplanet 55 Cancri Ae, which orbits a star with a ratio of 0.78.

Scientists have identified several objects that may serve as examples. The planet Janssen, announced in 2012, shows evidence of being a carbon planet. It has eight times the mass of Earth and twice the radius. Research suggests it is covered in diamond and graphite rather than granite and water. It orbits the star Copernicus once every 18 hours. Other objects, like the pulsar planets Draugr, Poltergeist, and Phobetor, might have formed from the disruption of carbon-producing stars.

Some researchers have also found evidence of carbon-rich objects near pulsars. In 2011, Matthew Bailes and his team reported on PSR J1719-1438 b. This is a small companion orbiting a pulsar. It has a diameter of 60,000 kilometers, which is five times larger than Earth. However, its mass is slightly more than that of Jupiter. This high density suggests a makeup of carbon and oxygen in a crystalline form. Some scientists hypothesize this object is the remaining inner core of an evaporated white dwarf star.

Understanding carbon planets helps scientists learn about the diversity of the universe. These worlds may be more common near the Galactic Center or in globular clusters. In these areas, stars have higher carbon-to-oxygen ratios than our Sun. As old stars die, they release large amounts of carbon into space. Over many generations of stars, the concentration of carbon in the universe increases. This suggests that the number of carbon planets could grow over time. Studying these hypothetical worlds allows us to map the chemical possibilities of space.