From Halopedia, the Halo wiki
The Democritus was a remote scanning outpost located in the Sol system, in orbit around Pluto.
The station was undetected by the Covenant and functional as long as November 3, 2552, during the Battle of Earth. It monitored slipspace activity and picked up a message sent from Cortana using Forerunner technology and another signal piggybacked on it by Doctor Catherine Halsey.
The station was likely named after the Greek philosopher and astronomer of the same name.