Planet One
From Tardis Wiki, the free Doctor Who reference
Planet One was the first planet in the universe.
History
The Doctor decided to take Amy Pond to Planet One to see the oldest cliff face in history, made of pure diamond, to decipher the writings on the face that up until then had not been translated. However when they arrived, the writing revealed itself to simply say "HELLO SWEETIE" with a set of temporal co-ordinates underneath. The words were written by River Song (who likely teleported to the location via a vortex manipulator). (DW: The Pandorica Opens)
