As the various sapient races of the Five Systems began to explore, the need to rate various worlds on their habitability arose. While each of the Systems developed their own metrics initially, these days most worlds agree on the Planetary Classification Scale.

The Scale, in brief, looks something like this:

Planetary Classification Scale
Classification Code Brief Meaning
Class 0 A proto-planet, or a planet still forming.
Class 1 A near-molten planet; dead, hot, uninhabitable even with pressure domes (ex. Mercury)
Class 2 A superhot planet with a deadly atmosphere; habitable at extreme risk with domes (ex. Venus)
Class 3 A dead, dry planet; temperatures still inhospitable to life, no native life, but viable with terraforming (ex. Mars)
Class 4 A dying planet, or one that has begun to decay in habitability (ex. Altair Prime)
Class 5 An emerging world, one with life just beginning to form (ex. early Terra)
Class 6 A Cretaceous-type world, with lots of large, wild, dangerous flora and fauna (ex. Terra before the Cretaceous–Tertiary extinction event)
Class 7 A life-abundant world, with a thriving, balanced ecosystem (ex. Terra, Katia, Pyra, etc.)
Class 8 Worlds covered in ice and snow, cold life-bearing worlds; technically able to sustain life, but it is scarce.
Class 9 Jungle worlds, or forest worlds; planets that are dominated by plant cover, with little or no ocean and thick, rainy atmospheres.
Class 10 Desert worlds; distinct from barren worlds insofar that they have abundant life, but little rainfall.
Class 11 Moon-like worlds; worlds cold and dry and barren, easy to put pressure domes on.
Class 12 Gas Giants; worlds with no appreciable surface, but make good centers for space stations or floating cities.