From Halopedia, the Halo wiki
Pluto, formal designation 134340 Pluto, is a dwarf planet in the Sol system, and the second-largest known dwarf planet (after Eris) as well as the tenth-largest body observed directly orbiting Sol. It was named after the Roman God of the Underworld. There are no facilities on the dwarf-planet's surface; however, several military and scientific stations orbit Pluto, including the Kelvin Research Station, UNSC Monitoring Station Delta Five, and the remote scanning outpost, Democritus. On November 3, 2552, Democritus detected Cortana's signal in slipspace with Dr. Halsey's message to Fleet Admiral Terrence Hood. In October of 2558, Pluto and its orbital installations came under the control of the Created, along with the rest of the UEG's colonies in the Sol system. This was thanks in part to the smart AI, Cleo, the overseer of UNSC Monitoring Station Delta Five.
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