Argus X

Argus X was the uninhabited tenth planet of its star system. This system was located over a thousand light years from the Tycho system.

In 2268, the USS Enterprise (NCC-1701) conducted a planetary survey of Argus X, discovering a large vein of tritanium ore. James T. Kirk sent a recommendation to Starfleet for a survey vessel to be dispatched to this planet as a result of the rich find.

Also discovered on the planet by the Enterprise landing party was the dangerous dikironium cloud creature, which attacked and killed (or nearly killed) five security guards. 

Background information
For the original production of the episode, the planet model for Argus X was first used for Delta Vega. It was used again for Alfa 177 (, Tantalus V, Planet Q , Benecia , Gothos , Lazarus' planet , Beta III , Janus VI , Organia , and Mudd . It was later used for Ardana.

For the remastered episode, a new planet model was created by CGI artist Max Gabl.

According to Star Trek: Star Charts ("United Federation of Planets I") and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385"), the Argus system was located in the Alpha Quadrant. The system's primary was a M-class star.