Planet One

Planet One was the first planet in the universe.

Astronomical data
Planet One was believed to be the first planet in the universe. One of the most noticeable features was a diamond cliff, which had writing said to be from the dawn of time. Around this cliff were dense jungles with large mushroom-like structures. (TV: The Pandorica Opens)

History
The Eleventh Doctor decided to take Amy Pond to Planet One to see the oldest cliff face in history, so that the TARDIS would translate the writings. However, when they arrived the writing revealed itself to be a message from River: "Hello Sweetie" with "ΘΣ Φ ΓΥΔϟ", a set of temporal co-ordinates, underneath. (TV: The Pandorica Opens)

Behind the scenes
Planeta Uno
 * The message on Planet One was likely a reference to the The Hitchhiker's Guide to the Galaxy series by humourist 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. The message read "WE APOLOGIZE FOR THE INCONVENIENCE."
 * The Theta Sigma in the coordinates could be a reference to an alias commonly used by the Doctor.