There are three main categories of planets: Terrestrial planets, Ocean Planets (water worlds) and Gas Giants.
The term planet refers to celestial bodies orbiting a star with a gravity in a specific range. The gravity of a planet has to be strong enough to hold fluids in a stable state (hydrostatic equilibrium) which prevents them from escaping the planet and low enough to prevent nuclear processes to start as in stars.
Some astronomical bodies, we call planets, are not considered as planets by this definition. Moons are natural satellites orbiting a planet and do not fall into the category of planets. However, some natural satellites are considered as planets because they fit their definition.[1]
Terrestrial Planets
| Image | Class | Colour | Rarity | In game description |
|---|---|---|---|---|
| Icy Planet | Atmospheric: White.
Airless: Blue, orange, green, yellow, pink (Rare). |
Very Common | Ice world composed mainly of water ice. Worlds like this will not have much heating in the past, forming in the cooler regions of a star system and have retained many volatiles as solids within their crust. | |
|
Rocky Planet |
Atmospheric:
Yellow, black, brown. Airless: Orange, grey, brown, red, green, black. |
Common | Rocky world with little or no surface metal content. Worlds like this have lost most of their volatiles due to past heating, and any metallic content will form a small central core. | |
| Rocky Ice Planet | White - Grey - Light blue - Light brown | Uncommon | Rocky ice world. Typically formed in the cooler regions of star system these worlds have a small metal core and thick rocky mantle with a crust of very deep ice. Geological activity is common in these worlds because of the large quantities of volatiles in the crust, often creating a thin, sometimes seasonal atmosphere. | |
| High-Metal Content Planet | Atmospheric:
Yellow, black, brown. Airless: Orange, grey, brown, red, green, black. |
Common (Rare if orbiting M, L, T, S class stars) | High metal content worlds like this have a large metallic core, with plentiful metallic ores even at the surface. In places, especially around areas of past or current volcanism or liquid erosion, some higher metals can be found in their elemental form too. Mining is therefore very efficient, so these worlds are highly valued. | |
| Lava Planet, Metal-Rich Planet | Atmospheric: Black, brown, visible lava.
Airless: Typically deep red/orange due to proximity to star. |
Rare | Metal-rich world with a metallic core. Worlds like this can have metallic ores near the surface in places, especially around areas of past volcanism. | |
| Water World | Atmospheric
Blue, often with white (ice). Airless: Blue. |
Rare (Uncommon if orbiting K, G, F class stars) | Terrestrial water world with an active water-based chemistry and carbon-water-based life.
Water Worlds may occasionally possess a small cluster of islands. This does not alter its classification, however. | |
| Earth-like World | Blue with green, brown and white |
Extremely Rare (Mainly orbit K, G, F class stars) |
Outdoor world with a human-breathable atmosphere and indigenous life. The atmosphere is far from chemical equilibrium as a result. | |
| Ammonia World | Atmospheric:
Light brown, brown, beige. Airless: Brown. |
Very Rare | Terrestrial ammonia world with an active ammonia-based chemistry and carbon-ammonia based life. | |
| Nonspherical Bodies | Atmospheric:
N/A Airless: Grey. |
Rare | These are planets or moons with a nonspherical shape, with a radius of less than 1 000 KM. It's elongated and can resemble a potato. Typically do not retain any atmosphere. | |
| Terraformed World | Blue with green, brown and white |
Extremely Rare (Mainly orbit K, G, F class stars) |
Outdoor world with a human-breathable atmosphere and indigenous life. The atmosphere is far from chemical equilibrium as a result. |
Ocean Planets
| Image | Rarity | In game description |
|---|---|---|
| Very Rare | Water giant. Worlds like this have a large atmosphere made mainly of water vapour. It most likely formed when a large icy body warmed up enough to evaporate a large amount of its surface ice, this would in turn trigger a runaway greenhouse effect leading to a very thick atmosphere made of the evaporated ices. |
Gas Giants
| Image | Class | Rarity | In game description |
|---|---|---|---|
| I | Uncommon | Class I or Jovian gas giants have primarily hydrogen and helium atmospheres. Colouration comes from clouds in the upper atmosphere of ammonia, water vapour, hydrogen sulphide, phosphine and sulphur. The temperature at the top of their upper cloud layers is typically less than 150 K. | |
| II | Rare | Class II gas giants have primarily hydrogen and helium atmospheres. Water vapour in the upper cloud layers gives them a much higher albedo. Their surface temperature is typically up to or around 250 K. | |
| III | Uncommon | Class III gas giants have primarily hydrogen and helium atmospheres without distinctive cloud layers. Their surface temperature typically ranges between 350 K and 800 K. They are primarily blue in colour because of optical scattering in the atmosphere - with the chance of wispy cloud layers from sulphides and chlorides. | |
| IV | Very Uncommon | Class IV gas giants have primarily hydrogen and helium atmospheres with carbon monoxide and upper clouds of alkali metals above lower cloud layers of silicates and iron compounds, hence he brighter colours. The temperature of their upper cloud layers is typically above 900 K. | |
| V | Very Rare | Class V gas giants have primarily hydrogen and helium atmospheres, with thick clouds of silicates and iron compounds, even metallic iron.They are the hottest type of gas giants with temperatures at their upper cloud decks above 1400 K, and much hotter in the lower layers, often emitting a dull glow from the internal heat within. | |
| Helium-rich | Very Rare | Helium-rich gas giants have a greatly inflated percentage of helium compared to the hydrogen in their atmosphere. Much of their hydrogen has been lost over time because they have insufficient mass to hold on to it. It may also be because temperatures in their past were much higher, driving off the hydrogen at a greater rate. | |
| Gas Giant with ammonia-based life | Very Rare | Gas giant with ammonia-based life. This is primarily a hydrogen and helium-based atmospheric gas giant, but a little below the surface cloud layers,life exists based in the ammonia-cloud layer. The chemistry of this gaseous region is far from equilibrium, with a surprising excess of oxygen and many carbon-based compounds giving it some vivid colours. As with many such gaseous living systems, it is underpinned by vast quantities of free-floating radioplankton - tiny carbon-based algae, each retaining small quantity of liquid ammonia, extracting their energy from the intense radiation flux. | |
| Gas Giant with water-based life | Very Rare | Gas giant with water-based life. This is primarily a hydrogen and helium based atmospheric gas giant, but not far below the surface exists life based in the water-cloud layer just below the atmospheric surface. The chemistry of this gaseous region is far from equilibrium, with a surprising excess of oxygen and many carbon-based compounds giving it some vivid colours. As with many such gaseous living systems, it is underpinned by vast quantities of free-floating radioplankton - tiny carbon-based algae, each retaining small quantity of liquid water, extracting their energy from the intense radiation flux. |
