P2A-509 was the planet in the Milky Way galaxy, which held a Stargate that SG-1 escaped to from Taldor. SG-3 was performing a standard recon operation on the planet, so SG-1 went there as to meet them and use their G.D.O. to return to Earth (their GDO's having previously been taken by Taldor). (SG1: "Prisoners")