A moving group is distinguished by its members having about the same age, composition (or metallicity) and motion through space.
The explanation of their underluminosity lies in their low metallicity: these stars are unenriched in elements heavier than helium.
The lower metallicity decreases the opacity of their outer layers and decreases the radiation pressure, resulting in a smaller, hotter star for a given mass.
Stars of higher mass and metallicity tend to have more numerous and more massive planets.
The unit often used for metallicity is the dex which is a (now-deprecated) contraction of decimal exponent.
In practice, corrections are required to account for variations in age or metallicity from galaxy to galaxy.
Metallicity tends to play a large role in the formation of planets, determining if they form, and what kind of planet they will form.
Observations of extrasolar planetary systems indicate that the presence of giant extrasolar planets at small distances from their host stars is strongly correlated with high metallicity of the host stars.
