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Guardian of the Solar System was a title held by Mavic Chen in the year 4000. The Guardian ruled Earth and its dominions. He had complete control over the Space Security Service based in Central City on the planet Earth. (TV: The Daleks' Master Plan)
The Eighth Doctor explained to Sam Jones that the Guardian of the Solar System's role in the Galactic Federation in 3999 was analogous to US President Bill Clinton's role in the United Nations in 1997. (PROSE: Placebo Effect)