Planets are classified by the features commonly found on similar planets, such as oceans, chasms, and deserts. For example, on a world classified as a "Terra", one would find sprawling oceans that can go for hundreds of miles, caves, lakes, and occasionally both poles of the planet will be covered in thick ice sheets. Earth would classify as a planet such as this, but it is not alone in it's features. Many worlds have features similar to this, and are generally sought for for colonization.
Rocky Worlds Edit
These are one of the two main branches of planets and are primarily the most sought after by civilizations. They can be defined as having a rocky surface and likely can be found with an atmosphere ranging from Nitrogen-Oxygen, to gaseous sulfuric acid. It isn't uncommon to find a rocky world completely stripped of an atmosphere.
Easily the most common planet type, these are usually recognized by their endless expanses of desert. Little to no water can be found on the surface unless you are in polar regions, where occasionally you can find a thin layer of ice for a polar glacial region. Temperatures vary wildly due to the lack of humidity. The hottest worlds discovered have had temperatures upwards of 1500°C, while some of the coldest can be near absolute zero.
Typically the most life-friendly planet, however Terra class worlds can also be very hostile. Earth would be considered very life friendly, but the life found on Earth is very hostile and very few extra-terrestrial life can survive for long. These worlds contain the majority of the galaxies populations, and also have the highest number of life-bearing worlds.
A selena type world is a world usually lacking an atmosphere or having a very thin atmosphere. They are usually covered in thousands to millions of craters. While being completely useless for colonizing, selena worlds are a great source for mining and produce much of the galaxies supply for copper, iron, magnesium, tin, and other metals in high demand.
Gas Giants Edit
Gas planets are the largest in both mass and size in the scale of planets. There are two types of gas giants, ice giants and gas giants.