During the middle of the 21st century, when the first intentional interstellar probes were being sent out to inform humanity about the world's of the Local Cluster, scientists devised a rating system for the colonization potential of any discovered extrasolar planets. This system was later, after almost a century of alteration and improvement, adopted by the Alliance to determine the worth of different planets for colonization.
The system was only designed to evaluate the worth of a world for human habitation, not resource extraction. Due to this, worlds are also rated on their richness in resources when their value is determined.
The system technically has six classes, Earth-Like (Class-1), Near-Earth (Class-2), Marganably-Habitable (Class-3), Uninhabitable (Class-4), Hostile (Class-5), and Highly-Hostile (Class-6).
Earth-Like worlds, called Class-1 worlds, are planets that would be commonly called Garden Worlds, planets humans could live on with no or limited technological support for indefinite periods. These were the most sought after worlds for colonization, and land on them was commonly bought in large amounts as an investment, as the land's value was guaranteed to go up in the years to come. This actually limited colonization on these worlds as much of the best land was left undeveloped. This led to many colonists getting land on Class-2 or even Class-3 worlds instead of fighting in the intense bidding wars over land on Class-1 Worlds.
Near-Earth worlds, called Class-2 worlds, are planets that are habitable to humans in most ways, but have limitations that prevent them from sustaining human populations without technological assistance. The mass land purchases that plagued the Class-1 worlds also affected the Class-2 worlds, though not to as great a degree. This allowed for more small independent groups to gain land on these worlds than on most of the Class-1 worlds, at least when they started looking there instead of the Garden Worlds. This has led to the creation of colonies on suboptimal worlds when the Garden Worlds of Alliance space remain lightly populated, as the land on those worlds is held by groups who are either waiting for the land to appreciate in value before selling it, or selling the land now for prices far greater than that on Class-2 worlds.
Marginally-Habitable worlds, called Class-3 worlds, are planets which, while colonizable, require far more technological support than a Class-2 world, with it in some cases being not too far off the support needed for spacecraft. These worlds usually have highly different gravities (either too strong or too weak compared to Earth's), atmospheres that are unbreathable to humans (and commonly requiring fully sealed suits), limited radiation protection, and sometimes other problems that need technological solutions. Few colonies exist on these worlds, and most are considered only good for resource extraction.
Uninhabitable worlds, called Class-4, are worlds that require technological support not much different from a spacecraft for humans to survive on. Most of these are barren rocks, asteroids and gas giants, valuable only through the resources that can be extracted from them.
Hostile worlds, called Class-5 worlds, are worlds that are actually more dangerous than the vacuum of space. High radiation, heat, pressure, gravity or many other things can lead to a world gaining this classification. Most of these worlds are of no use, with only valuable resources or extenuating circumstances giving them any worth to humanity.
Highly-Hostile worlds, called Class-6 worlds, are worlds that are significantly more dangerous than even Class-5 worlds. Super Jupiters near the level of Brown Dwarfs, Super Earths with gravities 10, 15, or even 20 times that of Earth's, worlds so close to their stars that ships not designed to reach them may not make it through the trip, these worlds are so dangerous any attempts at colonization are considered fool's errands, while even resource extraction is only profitable in the rarest circumstances, like worlds in orbit of Neutron Stars, filled with eezo to the point that even the extreme costs inherent with Neutron Stars are worth it.
Worlds are not just rated by whole numbers, worlds are given a rating that includes 2 decimal places within it. For example, Terra Nova was given a rating of 1.07, while Shanxi was given a rating of 1.04 and Babylon a rating of 0.98. Earth has a rating of 1.00 and is the benchmark for the habitability of all worlds. The lowest rating given as of 2157 is for Midgard, which has a rating of 0.92, while the the highest (and thus most dangerous) rating is held by the world of Mors, in orbit of the Neutron Star Kindjal.