X-tonic ray

Xtonic rays (alternatively spelled as x-tonic) were a type of ray spreading galvanic radiation extremely harmful to any known form of life. It would reduce humans to dust within a second if exposed. Shielding from the radiation was possible though, even in glass surfaces. For example Fenito glass could stand the radiation for several minutes, and the Leisure Palace had large domes 15 feet thick for more permanent shielding. The sun that Midnight orbited spread huge amounts of xtonic light, and thus no life was thought possible in the entire system. (DW: Midnight) This was however proven wrong as the Midnight Entity is shown to be able to survive and even live on the surface of the planet Midnight.

It is likely that Xtonic rays are actually very high frequency gamma rays, suggested by the fact that Donna Noble was shown to be sunbathing under them with a fifteen foot thick wall of Fenito Glass between her and the rays, possibly slowing them down enough so that they become Ultraviolet Radiation or even Visible Light and hence would be good for sunbathing, an activity that would be appropriate for the Leisure Palace.