Alpha Geminorum was a star. To fool Luke Rattigan and the students at the Rattigan Academy into helping the Sontarans (and later, being used as target practice), General Staal told Rattigan he would take the group to "Castor 36," an Earth-like planet well beyond the system, after "Planetfall". Only Rattigan fell for it. (TV: The Poison Sky)
Behind the scenes
In the real world, Alpha Geminorum is the same star as Castor.