Planets

There are two main categories of planets: Terrestrial Planets and Gas Giants. Some types of Terrestrial Planets support lifeforms or meet the conditions for terraforming. Ships equipped with the Planetary Approach Suite may land on the surface of Terrestrial Planets with 0 to tenuous (0.01-0.10) atmospheres. Some planets have a planetary ring system.

The category has a list of notable planets.

Description

The term planet refers to a celestial body that orbits a star with a gravity in a specific range. The gravity of a planet must be strong enough to hold fluids and plastic solids in a stable state (hydrostatic equilibrium) which prevents them from escaping the planet and it makes the body round, and low enough to prevent stellar nuclear processes to start.

Astronomical objects that meet the official criteria for planet status yet orbit a planet are instead classified as natural satellites or moons. When the barycentre of the planet-moon system falls outside the planet, then both bodies are considered planets and part of a double-planet system.^[1]

Terrestrial Planets

Type	Description	Rarity
Icy body	Ice world composed mainly of water ice. Worlds like this will not have had much heating in the past, forming in the cooler regions of a star system and have retained many volatiles as solids in their crust.	Very Common
Rocky body	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.	Very Common
Rocky Ice World	Typically formed in the cooler regions of a 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.	Common
High metal content world	High metal content world with a metallic core. Worlds like this can have metallic ores near the surface in places, especially around areas of past volcanism.	Common
Metal-Rich Body	Metal-rich 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.	Uncommon
Ammonia World	Terrestrial ammonia world with an active ammonia-based chemistry and carbon-ammonia based life.	Rare
Water World	Terrestrial water world with an active water-based chemistry and carbon-water-based life.	Uncommon
Earth-like world	Outdoor world with a human-breathable atmosphere and indigenous life. The atmosphere is far from chemical equilibrium as a result.	Rare

Gas Giants

Type	Description	Rarity
Class I gas giant	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.	Common
Class II gas giant	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.	Uncommon
Class III gas giant	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.	Common
Class IV gas giant	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 the brighter colours. The temperature of their upper cloud layers is typically above 900 K.	Rare (Common in systems with stars of class A or higher)
Class V gas giant	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.	Rare (Common in systems with stars of class A or higher)
Helium-rich	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.	Very Rare
Helium gas giant	Helium gas giants have primarily a helium atmosphere. They have lost most or all of their hydrogen because they have insufficient mass to hold on to it over time. It may also be because temperatures in their past were much higher, driving off the hydrogen at a greater rate.	Extremely Rare (Report discoveries to explorer groups)
Gas giant with ammonia-based life	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 a small quantity of liquid ammonia, extracting their energy from the intense radiation flux.	Uncommon
Gas giant with water-based life	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 a small quantity of liquid water, extracting their energy from the intense radiation flux.	Uncommon
Water Giant	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.	Rare

Features

Type	Description	Rarity
Dwarf planet	A dwarf planet orbits a star and has sufficient mass and gravity for a round shape, but it does not clear its orbital neighborhood of debris. Their orbital path is shared with other space objects and rocks. Dwarf planets are very common in the galaxy. Examples of dwarf planets in Sol: Haumea, Makemake, Persephone, Pluto, Salacia, Sedna.	Common
Tenuous atmosphere	Planets with tenuous atmospheres have surface pressures well below Earth, ranging from thin exospheres (where gases rarely collide) to denser, transient envelopes. Tenuous atmospheres are common for rocky and icy bodies, because they are a natural equilibrium between atmospheric generation and atmospheric loss.	Common
Thick fog effect	Rocky bodies can have a thick fog effect that covers the surface.^[2] For example "HIP 87621 2 B B" has around 400 meter high, thick fog.^[2]^[3] This effect is caused by flying inside the eclipse zone of a rocky body with a water atmosphere.^[4] The fog is an aerosol made of tiny liquid water droplets or ice crystals in the air. These conditions are rare.^[2]	Rare
Tidally locked	Tidally locked planets have a matching rotation period with the host star. One hemisphere has perpetual daylight while the other has only night. The Moon is also tidally locked to Earth.	Rare