Planetary Classifications

Planets in the Metroid universe are classified differently by different cultures. A universal classification does not exist. The Galactic Federation classifies planets in at least 19 different categpories. Very few of them have been mentioned in the games. Going by the way Federation Classes are referred to in Samus's logs, it would seem that the class of the planet determines the possibility the planet has of supporting life. It would seem that the smaller the number of the class, the less suitable it is to support life.

Federation Class XIII

In Metroid Prime Samus estimates that Tallon IV would have eventually turned into a Class XIII planet, a barren wasteland, by the Phazon radiation. A planet of this class has the mass of 5.1 trillion teratons, being slightly smaller than Earth (5.9 trillion teratons).

Federation Class XIX

Zebes is described to be a Class XIX planet, that is inhospitable to most bioforms. The planet's crust is primarily Urithic ore. Zebes has a the mass of 4.8 teratons.

Space Pirate Wanderer-class

When the Space Pirates first arrived on Tallon IV to harvest Phazon, they described the planet to be a Wanderer-class planet. Evidently, Wanderer does not refer to rouge planets (planets outside star systems) like Aether, but rather to small planets rich with life.