Classification of Planets:
While many ways to classify planets, moons and large asteroids exist, when exploring, a way to classify the use humans could derive from a planet was needed. Saying that a planet was made of this rock and had such and such weather system and temperature and so forth was good, but a quick classification system analysis was very helpful. Here is the one created by the US Committee for Interstellar Exploration and adopted by the rest of the world.
Class A - These are planets in the beginning stage of forming. This class has four subclasses designated by number to signify what stage it is in. All of these plants have no value for life, minerals, water, or any other value to humans.
Class B - These are barren planets that have no redeeming mineral value, no atmosphere, no life, and so forth. Luna, Earth’s moon, and Rhea, Saturn’s, are prime examples of Class B entities, with no redeeming value.
Class C - These planetary bodies are also barren, but have great wealth in terms of minerals. They tend to be very hot, and this class has two subclasses. The first one is for bodies similar to Luna but instead having a large amount of mineral wealth, such as metals. The second is like Mercury, that have significant wealth, but also are lightning hot and hostile to life.
Class E - These planets represent gas planets. They are sub divided into 5 categories divided by number. 1 is the smallest and 5 the largest. Jupiter is an E4 and Saturn an E3. They tend to have large moons that may have a lot of mineral, water or other value to them. Class E planets are quite common.
Class F - These bodies represent ones that are quite cold and a long distance from the star(s) in their system. They are completely or virtually frozen, and rarely with water, but instead other elements, like nitrogen, methane and ammonia. They are literally dirty iceballs in space. They are different from a water world that is cold and has ice on the surface, because those worlds are habitable, just a little on the cold side, and have tons of H2O. These worlds, such as Uranus’s moon Titania, are quite inhospitable to life. Some of these bodies, such as Triton, can have vast resources under its nitrogen ice. Pluto is Class F. These are quite common.
Class G - The very rare Gaia Class planet is a hypothetical planet that is even more well suited for life than Earth. It would lack the extreme areas of polar ice and hot deserts, and encompass life throughout. It would have a larger percentage of land surface area to water for habitation. None have been met.
Class H - The harsh arid deserts of this dry planet are not impossible to live in, but quite difficult. With a breathable atmosphere, they have some life. Imagine Earth, but with maybe 25% of the total water content, and you have a Class H planet. They still tend to have small ice caps and temperature variation like other planets. They are actually fairly uncommon, despite common portrayal in media.
Class K - The Class K world resembles Mars, Calisto or Ganymede, in that it has water, an atmosphere, perhaps small levels of life, and has a lot of potential for terraforming. It requires a dome and suits initially, but that can be changed, often very quickly.
Class M - This is the rare Earthlike class of planet. It is hard to find, usually inhabited by natives or space faring people, and has a lot of value. Wars are fought over Class M planets.
Class O - This represents a planet that is exclusively or virtually covered in water. They are often teeming with life that is unlike land dwelling humanity. They can still offer food, water, a base of operations, certain minerals, and more. Class O planets have two subsets. The first one is for water in liquid form and the second, O2, for water in solid form covering most of the planet. They are pretty uncommon. Europa, prior to colonization, is a good example of an O2 body. (Please note that an O2 planet is not formed of ice anymore than an O1 planet is formed of water, it simply has a lot of ice on the surface).
Class Q - These are rogue planets that do not orbit a star, and thus never have weather, atmospheres, or water. They may have valuable minerals or other resources.
Class T - The toxic planet has a caustic atmosphere that eats away at metals and plastics, and usually is quite hot, with torrid storms. While it certainly could offer some value in various ways, in reality, it’s generally best to stay far away. Venus and Titan are good examples. These are uncommon.
Class U - These planets resemble Uranus and Neptune. They are a mixture of Class E gas planets, such as Saturn, and Class F ice ones, like Titania. They are fairly common. Since they tend to be smaller, they have fewer moons. The ice can be sieved off with the right tools, developed in the last few years by humans.
Class V - This is a planet ruled by volcanic and seismic activity, and the landscape is roiling. It may be rich in metals, precious minerals, and such, but it is quite hostile to life. Io is barely in this class, and many other planets and moons have been discovered that are even a greater hotbed of activity. These are fairly common.
Class X - In theory, there may be planets out there that defy classification in the normal system, due to unknown conditions. Those planets, if any exist, are proactively assigned the Class X. This is reserved for undefined planets.
Any classification type may have a small R above it to signify that this planet requires the use of a rebreather. Many atmospheres will support life, but are slightly off from a traditional Earth atmosphere. They require the use of a rebreather which goes over the mouth and alters the atmosphere to one fully fit to humans.
Note that this is a classification system for humans, not for other races. Also note that the inner classes are the good ones, and the outer ones at the beginning and end of this list are the bad ones. G, H, K, M and O are the only ones capable of sustaining a permanent base outside of a protective dome.