X-tonic rays were a form of radiation that emanated from X-tonic stars. Professor Hobbes averred that such rays could also be thought of as "raw Galvanic Radiation".

He further claimed that this radiation was so deadly to living things that there "couldn't be" life in the star system that included the planet Midnight. Shielding from the radiation was possible though, even by glass. For example, Finitoglass could stand the radiation for several minutes, and the Leisure Palace had large domes fifteen feet thick for more effective shielding.

Hobbes' assertions, though generally correct, were too absolute for the Tenth Doctor's liking. Indeed, at least one being, the so-called "Midnight entity", was able to live and apparently thrive on the surface of the X-tonically-bombarded planet. (TV: Midnight)