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Cryonian

Very cold giant planets

Hackenbush
Image from Steve Bowers
Hackenbush, a Cryonian in the outer parts of the Pen-y-Ghent system
In NoLWoCS, Cryonians (prefix: Cryo-; eg. CryoNeptunian, CryoJovian) are very cold giant planets, with tropopause temperatures between 30 K, below which methane condenses out and the planet becomes a Frigidian, and 70 K, above which ammonium hydrosulfide and ammonia clouds dominate and the planet becomes a Psychronian. Cryonians are characterized by the presence of clouds of hydrogen sulfide (H2S) at lower altitudes, and methane (CH4) at higher altitudes, alongside gaseous methane absorption producing a vivid blue colour. On Cryonians receiving sufficient high energy radiation, photochemical haze generally forms, causing the planet's colour to become paler.

The darkening and clearing of aerosol particles in the hydrogen sulfide cloud layer can produce dark features on Cryonians such as bands or storms, while regional increases in production of photochemical haze can result in paler, brighter bands or polar hoods, and vice versa. Additionally, high-altitude bright clouds of methane ice are often found in variable amounts.

The low temperature required means Cryonians are usually neptunians, although some older subjovians located far from any star may also become cold enough to become Cryonians.
 
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Planetology
 
Development Notes
Text by AstroChara
John M. Dollan and Steve Bowers
Initially published on 31 December 2007.

Overhauled 2025-10-14 by AstroChara
 
 
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