| Class A:
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This is a “hot” gas giant such as Jupiter which generates internal heat. There is no “surface” per se, just an increasingly dense atmosphere. See Class-S.
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| Class B:
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This is a “cold” gas giant (does not generate internal heat), smaller and father from the star, such as Uranus. It also has no surface.
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| Class-C:
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This is a “hothouse” planet such as Venus, with a corrosive and/or toxic atmosphere.
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| Class-D:
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This is an airless rock such as Luna. Classes D, I, and J are three different points on the same spectrum of airless rocks floating in space.
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| Class-E:
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This is a “super-terrestrial” world, basically a larger-than-earth planet with an atmosphere and biosphere and at least marginally habitable.
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| Class F:
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This planet has a sulfurous atmosphere.
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| Class-G:
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This is a dry desert planet with no oceans, such as Mars. It may be cold or hot. It will have an atmosphere, but this may be thin or without oxygen. There is rarely a biosphere.
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| Class H:
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This is an “early Earth” planet with a developing biosphere (mostly bacteria, some low-grade plants). The atmosphere will be unbreathable.
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| Class-I:
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This is a rocky planet that is hot enough to melt lead (daytime temperature of 620 °F), such as Mercury. The atmosphere will probably be thin, (if the is one at all) and there will be no recognizable biosphere.
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| Class-J:
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This is a cold rocky planet with a thin atmosphere (or none at all). Temperature are below freezing, any water (and there may be none or very little) will be frozen, and there will be no recognizable biosphere.
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| Class-K:
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This is a desert world with a breathable atmosphere and working biosphere, such as Vulcan. There will be some small oceans, lakes, or seas, but these will be limited and brackish.
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| Class-L:
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This is a world similar to Earth, but with substantially less water. Oceans will cover less than a third of the surface, but there will be a breathable atmosphere and a working biosphere.
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| Class-M:
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This planet is Earthlike.
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| Class-N:
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Like Earth, but oceans cover 90% of the surface and humidity is high.
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| Class-O:
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Like Earth, but much colder, with extensive glaciers trapping most of the water.
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| Class-P:
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Most of the atmosphere is frozen into glaciers of carbon-dioxide or other gases.
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| Class-Q:
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A frozen iceball. The orbit may be erratic; these are typically large Kuiper Belt Objects.
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| Class-R:
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A nitrogen-ammonia atmosphere on an otherwise Class-J planet, such a Q’Nabb (ISC sector).
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| Class-S:
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Frozen methane oceans with floating continents, such as Hydrax.
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