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 categories. 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
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. A Wanderer planet is not a rogue planet, such as Aether, but rather a small planet that is rich with life.
- "Planet Tallon IV. Mass: 5.1 trillion teratons. Profile: Ecosystem studies indicate that Tallon IV was a biological paradise prior to the impact of an extraterrestrial object. What remains of the biosphere is slowly fading due to exposure to Phazon radiation. At current rate of decay, Tallon IV will be a barren Class XIII wasteland in approximately 25 years." —Metroid Prime Logbook "Tallon IV" (Metroid Prime)
- "Planet Zebes 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." —Scan Data (Metroid Prime)
- "Scans of the Spiral Sector detected a massive energy spike emanating from a Wanderer-class planet identified as Tallon IV." —Space Pirate Data "Contact" (Metroid Prime)
- "--- Mission File 02543 --- 8 days ago, contact with Galactic Federation trooper squad Bravo was lost. --- Contract Agreement --- Locate Federation troopers and render assistance. --- Data Confirmation --- Last transmission received from a rogue planet located in the Dasha region called Aether." —Samus Mission File (Metroid Prime 2: Echoes)