The star system containing Earth, Mars and Jupiter Station, which serves as the political and military heart of the United Federation of Planets and the home of Starfleet Command.
The Sol system, in Sector 001 of the Alpha Quadrant, is the birthplace of humanity. Earth holds Starfleet Headquarters and the Federation's seat of government, the Moon and Mars are long-settled colonies, and the Utopia Planitia yards above Mars build many of Starfleet's best-known ships.
The system is threatened again and again: by V'Ger in The Motion Picture, the whale probe in The Voyage Home, the Borg in The Best of Both Worlds and First Contact, the Xindi weapon in Enterprise and the Breen raid in Deep Space Nine. Picard shows the synthetic attack that devastated Mars in 2385.
In Discovery, 32nd-century Earth has left the Federation and guards the system alone until it returns to the fold after the events of the fourth season.