Proxima Centauri b is the first planet in the Proxima Centauri system, and is administered as the Republic of Proxima. Located 4.2 light years from Earth, it and its star system are the closet habitable worlds to the Solar System and it was the first exoplanet settled by humans. Its star, Proxima Centauri, is a low-mass red dwarf star with a low average luminosity. The planet orbits roughly every 11.2 days, at a distance of one-twentieth that of Earth to the Sun. In spite of the star's low power, and the planet only receiving 65% the radiation flux as Earth, colonisation was considered a dangerous proposition, as the presumed thin atmosphere of Proxima Centauri b would be insufficient to protect colonists from the higher amount of x-ray radiation. However, further exploration and analysis discovered that its atmosphere was thick enough to provide some protection, and manned exploration proceeded in the early 23rd century. The planet was later subject to terraforming and atmospheric processing, which artificially thickened the atmosphere and made it partially breathable and considerably warmer. The presence of liquid water on its surface enabled ready utilisation of water sources to build functional human settlements, in spite of the cold climate and lack of indigenous life. All life on Proxima Centauri b has been introduced by human settlement. Unforeseen complications arose from the rapid warming of glacial ice, causing runaway mass flooding rendered a great deal of land underwater.

Initially a Trust Territory of the United Nations, the Proxima Centauri colony was granted member status of the Terran Federation as the Proxima Commonwealth in 2380, and it was incorporated as a Province of the Terran Empire in 2600 before being reincorporated into the Inner Sphere Province in 2900 as a Crown County. Today, Proxima Centauri b is a member world of the Central Galactic Union, again constituted as a republic. It has a population of just under 200 million people, with around 20 million of these densely clustered into its oldest major cities. Terraforming opened up large areas of land to development and settlement, creating a fairly even yet sparse distribution of rural populations.

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