PLANET
CLASS J: JOVIAN
TYPE
Gas giant
CORE
Iron, silicate rock, ice
MANTLE
Liquid metallic hydrogen and helium
ATMOSPHERE
Hydrogen, helium, ice
LIFE
None
Class J: Jovian planets are among the most common and familiar sights in any star system. Unlike terrestrial worlds that are primarily composed of rock, these vast giants are made of liquid and gaseous forms of hydrogen and helium (along with various trace elements that are often responsible for the planet's color).
The outer atmosphere is the most dynamic and tends to vary in composition between planets, but in general, contains several layers of visible clouds. These clouds are often home to violent and long-lived storms that might endure for many centuries with winds in excess of 650 kph. The inner atmosphere (or mantle) constitutes nearly three-quarters of the planet and generally constitutes a dense layer of liquid metallic hydrogen and helium. At the center is a rocky core; comparatively speaking, it is a small fraction of the planet's mass, but oftentimes even this "small size" is equivalent to an entire terrestrial world.
Ring systems are not uncommon around gas giants, particularly those planets in the far reaches of a star system.
CLASS J1: AMMONIA
Planets in this class are often shrouded in clouds of ammonia and water vapor. They tend to lurk in the outermost reaches of a star system. In the Solar System, both Jupiter and Saturn fall into this category.
CLASS J2: WATER
Planets in this class are too warm for ammonia clouds, and instead have clouds made of water vapor and methane.
CLASS J3: CLOUDLESS
Planets with equilibrium temperatures lack the chemicals necessary to form clouds and appear as little more than a plain blue sphere. Though similar in appearance to Neptune, they are structurally very different and tend to form in the inner portion of a star system.
CLASS J4: ALKALINE
These hot worlds have outer atmospheres of alkali metals, sodium, and carbon monoxide, with cloud decks of silicates and iron. They form in the inner regions of a star system.
CLASS J5: SILICATE
Planets in this class tend to have orbits very close to a star, in the innermost regions of a star system... or lurk as ultra-giants in the outermost part. The atmosphere is primarily iron and silicates.
SUBCLASS JD: DWARF
Not all gas giants are... giant. Smaller than average planets are deemed dwarfs.
SUBCLASS JM: MOON
In very rare instances, a Jovian world can form as a moon around another much larger gas giant.
SUBCLASS JP: PEGASID
Also known as a "Hot Jupiter," these planets are parked in orbit perilously close to a star. Many form in the outer regions of a star system and spiral inward. A Pegasid is doomed to have its atmosphere stripped away by solar winds and radiation (thus becoming the terrestrial subclass "Chthonian") before spiraling into the star to meet its demise.
SUBCLASS JR: ROGUE
A planet that has been ejected from its star system and is adrift in interstellar space.
SUBCLASS JS: SUPER GIANT
A planet that is significantly larger than average. They tend to stay in the outermost regions of a star system and act as a "shield" to the inner planets, as their powerful gravitational fields tend to divert comets and asteroids away from the interior of a star system.
SUBCLASS JU: ULTRA GIANT
These titanic worlds represent the upper limits of planetary masses. Most are content to loom in the cold zone of a star system, but their immense size occasionally forces them to assume eccentric orbits. This
causes them to spiral inward toward the heart of the star system and become a Pegasid. This destructive process disrupts the entire star system, ejecting smaller planets into interstellar space.
SUBCLASS JX: IRRADIATED
A planet that is heavily irradiated by its parent star.