A Carbon planet is a planet (or a moon) which has a surface made basically of carbon compounds. In some cases the entire planet might be made of carbon.
Structure
Depending on the amount of carbon compounds and their nature, a planet might be as follows:
- Thin carbonic layer. This is the case of Saturn's moon, Titan. Its surface and atmosphere contains a huge amount of hydrocarbures, spread over the surface, in the oceans and in the atmosphere.
- Carbonic crust. In this scenario, the planet has all its crust made of pure carbon or organic compounds, but its core has a different composition. In case of an Outer Planet, the crust can be floating on an ocean of liquid water. Since lighter organic materials have a lower melting point then heavy carbonic molecules, heavier compounds will move deeper in the crust, while lighter molecules will be on the surface, possibly creating oceans and atmosphere.
- Complete carbon planet. In this 3rd scenario, the planet is almost entirely made of carbon. Most probably, in its core you will find atomic carbon (possibly molten), surrounded by a crust of carbohydrates.
Depending on what other chemical elements exist on the planet, it is possible to find:
- inorganic carbon compounds (carbonates of alkali metals, for example) - they are found in large amounts on Earth, are solid and usually inert to chemical reactions
- Carbon dioxide and monoxide - all organic compounds burn and many of them can extract oxygen from water, so if the planet is closer to its sun, as a result of a lightning, this is what should happen)
- Natural organic molecules (for example tholins, created from solar or cosmic radiation, carbohydrates like those found in oil <from light methane or ethane to heavy molecules with more then 50 atoms of carbon> or molecules that include nitrogen or sulfur)
- Life (if it exists, the planet should be protected as a reservation)
- Atomic carbon (carbon powder, looking probably like a good quality coal). In this case, there is a high probability that the planet also has huge deposits of diamonds and graphite.
Depending on its hosting star, in theory, a carbon planet should be as follows:
Inner planet: Close enough to its sun or around a strong X-ray source (like Neutron stars or B - type stars), there is a higher chance that molecules will break apart and we will find atomic carbon. Also, since carbon is the end-sequence element in atomic fusion of some stars, ending as White dwarfs, there is a high chance that planets around them would have huge amounts of carbon.
Comfort zone: In the comfort zone (middle of Habitable Zone, where an Earth - like planet would be expected), we would expect to find temperatures similar to what is on Earth. However, many organic molecules (like methane) are known to have a greenhouse effect, so the place where temperatures would be suitable for a human would be found further away. Depending on average temperature, we would find oceans made of various things. Oceans are not supposed to be made by a single substance, but of many different ones, with different melting and boiling points. Depending on composition, some fluids might be flowing fast, like alcohol, some will be like gasoline, some might look like oil and finally some would be flowing very slow, like honey. Also, the ground would be very different. Depending on temperature and chemical composition, in some places the ground would be slimy, like hot wax or tar, but in other places it can be hard, as old asphalt or coal is. Also, the atmosphere is expected to contain a high amount of gasses, including methane. Clouds, hazes, rains and snows are expected to occur. Since water requires a higher amount of energy to evaporate then many organic compounds, on a carbon planet, rains should be more often. Also, it is expected that, since oceans are made not of a single liquid, rains will be of different compositions. One major difference is that the seas will not have icebergs. Water has this unique feature, to have a lower density as ice then it has as a liquid, but the vast majority of substances are more heavy as solids then as liquids.
Outer planet: In case of an Outer Planet, temperatures are lower, The greenhouse effect might be too small to keep many compounds liquid. There will be a lower amount of fluids and gasses. The lower amount of radiated energy from the sun, the lower will be meteorological events. There will be less winds and less rains, even if the average humidity (of organic materials, not water) will be higher then in a comfort zone.
Free-floating planet: In case of a Rough planet, without any source of heat from a star, temperatures will be close to absolute zero. Without an additional source of heat (like tidal heating), everything will be frozen on the surface, maybe except a tenuous atmosphere of hydrogen and helium.
Colonization and terraforming
Terraforming a carbon planet is a very hard task. The main problem is that if we bring oxygen to that planet, its atmosphere, its lakes and its rocks might ignite. In case of a thin carbonic layer, terraforming is possible. If the planet has a carbon crust or is entirely made of carbon, it will be much harder. Only in the far future, scientists might find ways to cover the carbon with inert materials (like silicon) from other planets or from the core. Transporting the carbon to outer space would also be a solution.
Paraterraforming would be a better solution. This would allow settlers to have a dome city or a flying city (if atmospheric pressure is too high).
But would settlers want to terraform a carbon planet? These worlds have resources that can be used in industry. Carbon planets will be a cheap source of raw materials for plastic, medicine and much more. If we look around us, we see organic materials derived from oil in anything, from clothes to cell phones and from pesticides to wall paints. Future civilizations will not burn carbon as we do today, because they will find it too expensive and they will also need oxygen for this, but they will still need a source for plastic when the oil will end.
On a planet containing atomic carbon, there is another catch. Settlers will find there graphite and diamonds in large amounts. Also, carbon (atomic) powder is useful in industry, from rubber and paints, to nanotubes and carbon fiber.
So, it is more probably that on carbon planets we will see mining facilities and rapidly expanding industrial centers, then new terraformed environments. It remains to see if we will also see pollution, as seen on Earth from chemical industry.