The "frozen planet" is a planet in Pegasus galaxy where Atlantis moved after the departure from Earth in 2009. Before Atlantis arrived, the planet had no Stargate, having lost its original long before and could only be accessed with a spaceship. The planet is on the edge of the Pegasus galaxy and is the secret location that the Wraith were originally created at.
The planet has an Earth-like atmospheric composition that can support life, and ninety percent of the surface consists of frozen seas. The temperature on the surface is like that of Antarctica and its climate depends on its sun which is slightly further from the planet. There are some large ice sheets on the surface. The planet's land mass is under ice. There are some islands on the planet and the planet's equator is slightly warmer. The planet has a squid-like lifeform native to its seas and a flock of pigeons inadvertently transplanted to the Pegasus Galaxy by Atlantis.
Six months after Atlantis had landed on Earth they had International Oversight Advisory decided that Atlantis would remain on Earth. Woolsey trying to get Atlantis back but when the expedition thought was the end of the expedition. Meanwhile, General Jonathan O'Neill took temporary command of the city. But when Atlantis expedition found out everything was a plan that they would return to Pegasus galaxy one again with. O'Neill gave the command back to the Richard P. Woolsey. The expedition began repairing Stardrive so they could leave the Earth. Atlantis had used cloak active for five months and then started running out of power and risk revealing the city.
Atlantis left Earth and a few days later it was found Atlantis expedition an ice covered planet in the outskirts of the galaxy and their Stardrive was disabled so they prepared Atlantis' sublight engines of the land. The city had enough power to land on the planet. (SGA: "Legacy: Homecoming")
While investigating strange life-signs within Atlantis, Doctor Radek Zelenka and Major Evan Lorne discover that Atlantis has inadvertently introduced a flock of pigeons to the planet's ecosystem, having not realized the pigeons were within the city before takeoff. Despite the biologists worries about what introducing pigeons to the planet will do, it is decided to leave them alone. (SGA: "The Lost")
The team eventually located an Ancient facility on the planet that they determined to be a prison. Doctor William Lynn found records of a prison break where the prisoners escaped to Athos as the planet once had a Stargate but it was long gone. However, he couldn't find more than that. He also found no records of the planet of any kind in the database including a Stargate address or even a survey despite the secret prison facility. Investigating the facility, the Atlantis team found the facility's ZPM gone. Eventually, the expedition learns that the facility was where the Ancients experimented on humans as a path to making themselves immortal in case they couldn't Ascend. These experiments, the prisoners that escaped, would eventually become the very first Wraith. (SGA: "The Furies", "Secrets")
After Doctor Rodney McKay was rescued from the Wraith and the expedition located the weapon the Ancients had created to kill the Wraith, Queen Death launched all-out attack on the planet which was defended by Atlantis, the George Hammond, an Ancient ship repaired by the Genii called the Pride of the Genii and Guide's Wraith faction. After Queen Death was killed, Sheppard landed Atlantis on the planet once more, but closer to the equator this time where they'd get warmer weather. (SGA: "Inheritors")