|Alternate name:||Planet Questionmark, Alpha Z-21 (Federation nomenclature)|
|Classification:||class M planet|
|Location:||Alpha or Beta Quadrant|
|Dominant Species:||Krill's people|
|Population:||A few thousand (as of 2265)|
Zarta (also known as Alpha Z-21) was a class M planet located in the space of the galaxy's Alpha or Beta Quadrants, the homeworld of native inhabitants. On Zarta, machines had achieved dominance over the native humanoid population. Because the planet was still a mystery to the Federation by 2265, it had earned the nickname Planet Questionmark.
The prosperous humanoid civilization of Zarta began experimenting in 2115 with machines assuming more roles in society. Eventually, this led to utopia for the planet until the creation of the city builder, a machine designed to construct entire cities. Soon the machines began directing the planet's resources to themselves, building structures without stopping.
About the time this perpetual building process began, the planet was the subject of a cursory survey by a deep galaxy ship in 2165, using radio film. The ship exploded before returning to its mother ship. Its brief record showed the planet's populace as having a high level of technology, not knowing the dire situation that would develop.
The planet was visited by the USS Enterprise in 2265. The crew discovered that the planet's population was threatened with extinction by the city builders. Captain James T. Kirk and his crew assisted local leader Krill in devising a means to defeat the city builders and reclaim the planet. (TOS comic: "Invasion of the City Builders")