Mars is the fourth planet of the solar system.
Mars is one of the most popular settings of science fiction. It was long believed to be habitable - cold and dry as compared to Earth, but still habitable - and many people even imagined intelligent life there.
Examinations with interplanetary probes shattered this myth, however. The planet is very cold, and its atmosphere very thin and composed amost entirely of carbon dioxide. Higher life forms could not exist there. Yet, the notion that the planet could be terraformed in the future remains popular.