Messaline was a planet. (DW: The Doctor's Daughter)
Structure
Messaline is an Earth-like planet with a similar atmospheric composition. The main differences were the occasional radiation spikes hitting the surface, the lack of native life and the apparent lack of any gases other than nitrogen, oxygen and ozone in the atmosphere.
The surface of the planet was, before terraforming, cold, rocky and with the occasional tar pit. (DW: The Doctor's Daughter)
Colonisation
Messaline was colonised by Humans and Hath in July 6012 of the New Byzantine Calendar. It was a barren world, so terraformation was planned to occur there to make the planet hospitable for the two species. However, when the mission commander died, there was a power vacuum. This power vacuum resulted in the two species taking part in a seven-day battle, although approximately 140 generations passed during the course of it.
When the Doctor destroyed the terraforming device and released the terraforming gases into the air, a peace was formed between the two races as they realised what the battle actually was about. The colonisation of the planet continued as planned. (DW: The Doctor's Daughter)