A class XIX planet is a planetary class mentioned once in Metroid Prime. Zebes is said to be a planet of this class, and is described as inhospitable to most bioforms. Whether any other planets are of this class is unknown.
On the now defunctional Prime section of Metroid.com, Zebes was identified as a Class 5B planet.
"Zebes is a Class 5B planet with an Urthic ore crust ideal for subterranean construction. Though many zones on Zebes are seemingly inhospitible to life, extremely tough organisms have evolved on this planet that can thrive in conditions that would kill ordinary terrestrial creatures.
Mass: 4.8 trillion teratons.
Profile: Planet's crust is primarily Urthic ore, making it ideal for subterranean construction. A class XIX planet, Zebes is inhospitable to most bioforms. The world was considered unremarkable until it became a base for Space Pirate forces."