Planet One

Planet One was, as the Eleventh Doctor claimed, the "first planet," believed to be the oldest planet in the universe,

One of the most noticeable features was a diamond cliff, the oldest cliff face in history, which had writing said to be from the dawn of time. There were also dense jungles with large mushroom-like structures.

The Eleventh Doctor took Amy Pond to Planet One to see the cliff face so that the TARDIS would translate the writing. When they arrived the writing revealed itself to be a message from River Song reading "Hello Sweetie" with a set of temporal co-ordinates, "ΘΣ Φ ΓΥΔϟ". (TV: The Pandorica Opens)

Another account claimed Chrone was the oldest planet in the universe, (COMIC: The Vortex Code) while another referred to Etra Prime as the oldest planet in the known universe. (AUDIO: The Apocalypse Element)

Behind the scenes

 * 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 reads "WE APOLOGIZE FOR THE INCONVENIENCE".
 * The "Theta Sigma" ("ΘΣ") in the coordinates is likely a reference to the Doctor's Academy nickname, which became a commonly used alias.

Planeta Uno