To address factors dictating similarities and differences between solar system radiation belts, we present comparisons between relativistic electron radiation belt spectra of all five strongly magnetized planets: Earth, Jupiter, Saturn, Uranus, and Neptune. We choose observed electron spectra with the highest intensities near ∼1 MeV and compare them against expectations based on the so-called Kennel-Petschek limit (KP). For evaluating the KP limit, we begin with a recently published relativistic formulation and then add several refinements of our own. Specifically, we utilized a more flexible analytic spectral shape that allows us to accurately fit observed radiation belt spectra, and we examine the differential characteristics of the KP limit. We demonstrate that the previous finding that KP-limited spectra take on an E−1 shape in the nonrelativistic formulation is also roughly preserved with the relativistic formulation; this shape is observed at several of the planets studied. We also conclude that three factors limit the highest relativistic electron radiation belt intensities within solar system planetary magnetospheres: (1) plasma whistler mode interactions that limit differential spectral intensities to a differential Kennel-Petschek limit (Earth, Jupiter, and Uranus), (2) the absence of robust acceleration processes associated with injection dynamics (Neptune), and (3) material interactions between the radiation electrons and clouds of gas and dust (Saturn).