 Atmospheric metallicity of a giant planet

The atmospheric metallicity of a giant planet describes its atmospheric inventory of elements heavier than hydrogen and helium. It must specify a convention: an elemental abundance ratio relative to hydrogen, an enhancement relative to a reference composition, or a total heavy-element mass fraction are related but different quantities. A quoted enrichment also needs to identify its reference, such as the host star.

Atmospheric retrieval estimates molecular abundances, which must then be connected to elemental inventories through a model of chemical equilibrium or disequilibrium chemistry in an exoplanet atmosphere. Atmospheric condensate rainout can hide elements in particles below the observed levels. Thus an atmospheric enrichment need not directly equal the heavy-element fraction of the whole exoplanet interior.