The Toxic Planet was a planet in the Pegasus galaxy and the home of the Vanir until its toxicity became too much for them.
History
Following the Vanir's defeat and destruction of their intergalactic ships in the first battle with the Wraith, they made their way to the Toxic planet. There, they could hide from the Wraith.
For the next 10,000 years the Vanir was able to survive on this hostile world wearing simple breathing apparatus.
Over time the planet's toxic atmosphere became more and more extreme, so the Vanir replaced their breathing apparatuses with their Armored exoskeletons, but even this measure failed to completely solve the problem. Due to their inability to counter the planet's increasingly extreme conditions, the Vanir were forced to venture outside of their star system into Wraith-dominated space. (SGA: "The Lost Tribe")