PLANET
CLASS H: HELIUM
TYPE
Gas Giant
CORE
Iron, silicate rock, ice
MANTLE
Liquid metallic helium
ATMOSPHERE
Helium, carbon monoxide, carbon dioxide
LIFE
None
Class H: Helium worlds are gas giants with an atmosphere dominated by helium. They often begin their lives as Class J: Jovian planets that are located perilously close to their parent star. Over the ages, the hydrogen is effectively boiled out of the atmosphere, leaving behind heavier elements such as helium.
Class H planets can also form from a low-mass white dwarf star that is part of a binary system with a second, massive object such as a neutron star. The larger object strips the white dwarf of its atmosphere, effectively demoting the star to a planet.
In either case, the loss of atmosphere will continue over the eons until the planet's rocky core is exposed. At that point, a terrestrial Chthonian world is born.
Helium planets are functionally similar to Jovian worlds, with thick clouds and violent atmospheres prone to long-lived storms.
CLASS H1: HELIUM
Unlike their Jovian counterparts, Helium worlds are not very diverse. They are exclusively pale in color and located only in the inner-most portions of a star system. As a result, nearly all Class H planets are of the Pegasid subclass.
SUBCLASS HD: DWARF
Not all gas giants are... giant. Smaller than average planets are deemed dwarfs.
SUBCLASS HP: PEGASID
Also known (erroneously, in this case) 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 HS: 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 HU: 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 HX: IRRADIATED
A planet that is heavily irradiated by its parent star.