- This article is about the planet. For other articles with the same name, see Utopia
Utopia was (apparently) a planet, one of the last remaining inhabitable worlds in the universe, after which the Utopia Project was named; the purpose of the project was to send the remaining survivors of Humanity to the planet based on a modulating (non-automatic) signal beaming "Come to Utopia". Legends spread about this world, such as that the skies were made of diamonds.
In the year 100,000,000,000,000, several thousand years after the foundation of the Project, and not long before the anticipated end of the universe, the remainders of Humanity took off from Silo 16 on Malcassairo to Utopia in a rocket, where they hoped to live the rest of their lives. (DW: Utopia)
However, the planet they crashed on was cold, dark, and inhospitable, and the humans were forced to survive by combining themselves into spherical shells, becoming the Toclafane. As part of the Master's scheme to take over the planet Earth in the 21st century, the Time Lord shuttled back and forth between that time period and Utopia (using a TARDIS stolen from the Doctor); it is known that he took his companion-come-wife, Lucy Saxon, to the planet at one point (DW: The Sound of Drums/Last of the Time Lords).
The Toclafane, with the Master's help, eventually left Utopia before it went nova and travelled through a time rift created by the Master to 21st century Earth and took shelter and control of the planet for a year. Ultimately, however, the efforts of the Doctor, Martha Jones, and Jack Harkness resulted in not only the Toclafane invasion being defeated, but time itself was rewound, creating The Year That Never Was. As a result, the Toclafane, in the new timeline, presumably never left Utopia and eventually died out, along with their cousins on Malcassairo, when the planet Utopia itself likely destroyed at some point thereafter as the universe came to its natural end. (DW: Last of the Time Lords) However, the Toclafane may have simply flown away from Utopia as they did when they invaded Earth.
Whether or not this planet was truly the source of the signal, or merely the nearest when the ship began to fail, was never made clear.