It’s the Dewey Decimal Classification number for Pluto, sitting happily in the “Planets of the Solar System” schedule. So there are more implications of Pluto being demoted to dwarf planet. The folks are Dewey Towers have noticed:

We’re still looking at what this means for 523.4 Planets of solar system and for 523.48 Trans-Uranian planets in Dewey. In particular, we will need to decide exactly where Charon and other KBOs (“Plutonian objects”) go in the classification, and whether “Trans-Uranian planets” is still the best caption.

Of course, Trans-Uranic Heavy Planets may not be used where there is life.