P3X-289 was a planet in the Milky Way galaxy, which had completely toxic atmosphere consisting of sulfur dioxide, carbon monoxide, methane and ammonia but with a small haven not far from the Stargate that was safe from the toxic atmosphere. The planet were originally controlled by the System Lord Morrigan. Residents of the planet lived under this dome, though in very small numbers. They accessed what they called the Link for getting all the information they needed and survived in this manner for 400 years, because of the dome's energy barrier. P3X-289 is approximately 6,000 light years away from Earth.
The planet is now abandoned since the dome didn't have enough energy to sustain the people in the controlled environment. With the help of SG-1, the surviving residents were evacuated to another planet through the Stargate. (SG1: "Revisions")