- Class A — Uninhabitable, often without an atmosphere. A lifeless planet.
- Class B — Uninhabitable, usually because of a volatile, unliveable atmosphere.
- Class C — Uninhabitable by man, but certain life forms are present.
- Class I — Cradle-like and fully inhabitable. Oxygen-rich atmosphere and water.
- Naturally Inhabitable (I-NI) — Inhabitable without interference.
- Artificially Inhabitable (I-AI) — Inhabitable only with external stimuli.
- Class X — Generally disregarded as a planet, more of a "found" or "created" planet. Often home only to extremely specific life forms.
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