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, Mars, thus a possibility in that of extra-solar planets.
Earth has a carbon cycle that adds and removes carbon dioxide from the atmosphere, as it has a water cycle. In addition to the obvious influence 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 evaluating planets for their life potential and in devising biosignatures.