Rutharian sector L class planet

The Defiant encounters an exogenic field surrounding an L-class planet

An exogenic field was an extremely powerful energy phenomenon which had the capability of posing a hazard to starship navigation.

In 2371, the USS Olympia encountered an exogenic field composed of subspace metreon radiation surrounding an L-class planet in the Rutharian sector, upon returning from an extended, long-range exploration of the Beta Quadrant. Upon probing the energy barrier with their scanners, the Olympia crew experienced an enormous surge of metreon radiation which disabled the ship and caused it to crash. Lisa Cusak, the Olympia's commanding officer, was the only survivor of the crash, and immediately began broadcasting distress calls, which shifted forward in time three years, where it was picked up by the crew of the USS Defiant. Responding to the distress call, the Defiant crew was able to communicate with Cusak, although neither party realized that a time shift was occurring, until the Defiant arrived at the planet, only to find that Cusak had died three years previously. (DS9: "The Sound of Her Voice")

This field, according to the Star Trek Encyclopedia (4th ed., vol. 1, p. 256), existed when a planet's core contained unstable elements.