Planet One

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 "ΘΣ Φ ΓΥΔϟ", 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)

Background
The message on Planet One was likely a reference to the Hitchhiker's Guide series by humorist and former Doctor Who writer Douglas Adams. In the fourth book, So Long, And Thanks For All The Fish, Arthur Dent, Marvin, and Fenchurch search for God's Final Message to His Creation, which they find written in letters of fire, thirty feet high, on a cliff-face in the Quentulus Quazgar Mountains. According to Marvin, the original message read, "WE APOLOGIZE FOR THE INCONVENIENCE."