Carbon dioxide (CO2) is a compound with molecules each consisting of one carbon and two oxygen atoms. It is a constituent of the atmosphere of Earth, a major constituent on Venus and Mars, thus a possibility in that of extra-solar planets.
Like Earth's water cycle, Earth has a carbon cycle that adds and removes carbon dioxide from the atmosphere. In addition to the obvious elements of animal and plant life, it includes an oceanic carbon cycle that leads to long-term and a deep CO2 cycle that takes carbon dioxide into the mantle and back out, analogous to and related to the deep water cycle. These are of interest in evaluation planets for their life potential and in devising biosignatures.