6000000000 (number)

6000000000 was a number.
The Year That Never Was began when conquered the Earth with a force of six billion Toclafane. At the end of the year all six billion Toclafane advanced on the Valiant in an attempt to protect the paradox machine but were too late as the machine was destroyed by Jack Harkness, resulting in time reversing with the Toclafane being returned to the end of the universe. (TV: The Sound of Drums/Last of the Time Lords)
Onboard the Titanic in 2008, the Tenth Doctor stated that the Earth's population was six billion. (TV: Voyage of the Dammed) The Eleventh Doctor also gave the same number earlier that year. (TV: The Eleventh Hour)
In 2010, Sophie estimated the Earth's population at six billion. (TV: The Lodger)
N f6000000000