Why might the outer regions of the Milky Way lack enough heavy elements to form Earth-like planets?

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Habitable zone The habitable zones (green) for stars that are like the Sun (middle), hotter than the Sun (top), and cooler than the Sun (bottom). The red areas are those in which liquid surface water would be lost as a result of a runaway greenhouse effect, and the blue areas are those in which liquid surface water would be completely frozen.

In the context of the galactic habitable zone, the outer regions of the Milky Way Galaxy may lack sufficient heavy elements to form Earth-like planets because the bulk of a terrestrial planet is composed of chemical elements that were produced within stars.

In these outer regions, there are fewer stars compared to the more densely populated areas near the galactic center. Consequently, the interstellar material available for the formation of new stars and planetary systems may not contain a high enough concentration of the heavy elements necessary to build rocky, Earth-like planets.1