Messaline

Messaline was a planet colonised by humans and Hath.

Structure
Messaline was an Earth-like planet with a similar atmospheric composition, with a roughly 80:20 ratio of nitrogen and oxygen and high ozone levels. The main differences were the occasional radiation spikes on the surface and the lack of native life.

The surface of the planet, before terraforming, was cold and rocky with the occasional tar pit. (TV: 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 terraforming was planned to make the planet hospitable for the two species. However, when the mission commander died, there was a power vacuum. The two species then engaged in a seven-day battle, although approximately one hundred forty generations passed during the week. (TV: The Doctor's Daughter)

There was an outbreak of pacifism among the humans in the Eastern Zone three generations before the Tenth Doctor's arrival.

Terraforming
When the Tenth Doctor destroyed the terraforming device, and released the terraforming gases into the air, a peace was established between the two races as they realised what the battle actually was about. The colonisation of the planet continued as planned. Subsequently its terrain was transformed with the earlier release of gas from the terraforming device. (TV: The Doctor's Daughter)

Behind the scenes

 * In the roleplaying game Worlds in Time, the planet was invaded by the Zygons, only to be stopped by the Eleventh Doctor and his companion(s). (GAME: Worlds in Time)

Messaline