There are 13 different levels of habitability for planets in Master of Orion.
These only apply to how easily people can survive on the planet and the level of technology needed by a civilization to create colonies on the planet.
The size of the planet and the resources of the planet are not directly related to the habitability as they sometimes are in other versions of Master of Orion.
The levels of habitability are:
Gaia[edit]
The best beyond question. These are the perfect worlds in terms of the game. Try to imagine what would constitute the perfect planet, that is what these worlds are. Population maximum capacities and population growth rates are much greater than average.
Fertile[edit]
Second best. Halfway between a regular planet and a Gaia planet. They tend to have higher population maximums than average and they grow people at a rate halfway between standard and gaia planets.
Standard environments[edit]
Third best. These include Terran, Jungle, Ocean, Arid, Steppe, Desert, and Minimal. These planets are generally in order of highest to lowest max population.
Hostile environments[edit]
Fourth best. These all require advanced levels of technology to research and are listed from easiest to research to hardest. They all have generally low maximum populations and grow population at half the normal rate. The only difference between these is how the level of technology required to inhabit them. They are Barren, Tundra, Dead, Inferno, Toxic, Radiated.
Note: unlike Master of Orion 2 where Toxic is the worst, Radiated is the worst in this game. Toxic was special in Orion 2 and in a bad way. It is not like that in Orion 1, it is no different than other types that require advanced technology to inhabit other than its difficulty to inhabit.
Note also, all of these below Gaia can be upgraded through terraforming with advanced levels of Planetary research. Unlike in Orion 2 where Toxic cannot be terraformed, it can be terraformed like any other type in Orion 1.