Sontar

Sontar was the home planet of the Sontarans.

History
The name Sontar was included in the Sontaran battle chant, "Sontar, ha." (DW: The Sontaran Stratagem) Sontar was a reddish planet with gravity six times higher than that of Earth's; this is said to be how the Sontarans develop such strong physiques and muscles. (Captain Jack's Monster Files: Sontarans) (DW: The Two Doctors)

Circa the 25th or 26th century, the Sontarans' enemies, the Rutans destroyed Sontar with photonic bombs, leading to the dispersal of Sontaran troops across their home galaxy of Mutter's Spiral. (DWM: Pureblood).

Having lost their main breeding facility the Sontarans moved to turn Earth into a huge breeding farm but with the help of the Doctor, Martha Jones, Donna Noble and UNIT they managed to stop the Sontarans from turning the earth into a breeding planet. (DW: The Poison Sky  / The Sontaran Stratagem)

Donna's World
In the parallel world where Donna Noble never met The Doctor, Captain Jack Harkness and Torchwood moved to stop the Sontaran fleet. Torchwood 3 were killed while Jack Harkness was transported to Sontar. (DW: Turn Left)

Other information
The supreme military commander and founder of the Sontaran Empire, General Sontar, had the same name as the planet.