P4X-639 was a planet in the Milky Way galaxy, which held a Stargate. The planet was subjected to constant coronal mass ejections coming from its sun. As a result, the surface of planet is severely scorched and the world is peppered with tornado-like anomalies and geomagnetic storms. These anomalies constantly weakened the planet's magnetic field, increasing the radiation levels on the surface, making the world harmful to organic life. At the same time, these storms are what power an Ancient time loop machine on the world.
Originally the Ancients had settled this planet as one of their bases and colony. When a great plague, perhaps the great plague, arrived they built a time machine to travel back and fix the problem. Unfortunately the device never worked; malfunctioning by creating a perpetual time loop instead of a single jump. They shut it off and the planet was engulfed in the plague, killing any un-ascended ancients that remained. (SG1: "Window of Opportunity")