Arean worlds are small silicate rich worlds with a mostly dormant metallic core. They lose their magnetosphere, atmosphere and liquid water over time. Sometimes such worlds go through cycles of cooling and heating, but ultimately they are too small to retain enough energy to continue this indefinitely.
They normally begin as Proto Lithian worlds, which rapidly cool to enter the Arean class.
EoArean worlds are early Arean worlds with liquid oceans, thick atmospheres and hot temperatures.
MesoArean worlds have cooled significantly, and lost much of their oceans and atmosphere, but still have significant amounts of both. They might have very basic life, often in the form of bacteria.
EuArean worlds have lost their surface oceans and nearly all of their atmosphere, and are frozen dead worlds.
If conditions are just right, a MesoArean world can become an Arean-Lacustric world. These are often slightly larger or denser, and manage to cling onto their atmospheres and oceans for much longer. Life has a chance to evolve beyond basic bacteria.
If the world remains too warm, due to closer proximity to its star, then it can become an AreanXeric world which is hot and completely dry. They are even more lifeless than EuArean worlds, but aren't so cold.
AreanTundral are at the other end of the scale, and are cold but with large deposits of water ice.