E-10 is a planet and the birthplace of the Alphabetrians and Numbericons as well as the home of the Googas and where the "I" of Harmony originated. It first appeared in "Rise of the Numbericons: The Movie" where Water-T, Mr. Goldenfold, Morty, and Lithium-P travel and crash land to the planet along with Sinistar-7. They encounter the Googas and join them for a "Ewok party" as they find out that the "I" of Harmony originated near the Googas' village.
On E-10, they are then captured by Emperor Dread Nought and his forces as Eight uses the "I" of Harmony to transform into Infinity and plans to destroy Alphabetrium. Water-T, Mr. Goldenfold, Morty, and Lithium-P escape and leave E-10 with the help of the Googas improving their ship.