Sigma 957 is a Class-4 world with an atmosphere composed mainly of nitrogen and oxygen. Located in Sector 305, a contested region of space close to the Narn Regime, it is known as a dangerous place where the Walkers of Sigma 957 dwell.
In 2258, after an initial flyby scout detected traces of deridium (a by-product of Quantium 40), Universal Terraform commissioned Catherine Sakai to do a more thorough planetary survey. Her ship was disabled when it got too close to the Walkers as they opened a jump point, though she was rescued by a pair of Narn, sent there on orders from Ambassador G'Kar of the Third Circle.
A year later in 2261, the Army of Light returned along with Lorien one last time to call them forth for the final battle. After the Walkers departed for the Rim with the Vorlons, Shadows and all the remaining First Ones, Sigma 957 has presumably become an empty and abandoned world.
It's unknown if any further attempts have been made to exploit the planet's natural resources or search for leftover technology, but any endeavor could prove to be extremely dangerous as other abandoned worlds of the First Ones have been found to be booby trapped, or protected by ancient defense systems.