An uninhabited planetoid carrying an automatic communications and astrogation station, from whose orbit Kirk, Uhura and Chekov are snatched away in The Gamesters of Triskelion.
Gamma II is a lifeless planetoid on which the Federation maintains an unmanned communications and astrogation station. In the Original Series episode The Gamesters of Triskelion the Enterprise calls there for a routine inspection of the equipment.
Kirk, Uhura and Chekov step onto the transporter platform to beam down, and vanish before the transporter is even switched on. They have been seized by the Providers of Triskelion, more than eleven light years away, to be trained as gladiators.
Spock searches the surface of Gamma II and the space around it and finds nothing. A faint ionisation trail is his only clue, and he follows it against the advice of McCoy and Scott, who want to keep looking nearer to the planetoid.