A likely explanation, the research team suggests, is that red giants mix up far more of their contents than hydrogen-burning stars, diluting the metal content.
So "pollution" from heavy elements falling into a star would be more readily apparent on dwarfs than gas giants.
Alternatively, the findings may suggest that the red giants simply started off their lives as stars inherently less rich in metal.
If so, the findings suggest that giant planets like Jupiter, Saturn, Uranus, and Neptune can form readily around a star with little metal. (See Saturn pictures from National Geographic magazine).
A paper on Pasquini's research is slated to appear in the journal Astronomy & Astrophysics.
New Chance for Life?
In a few billion years our own sun will also run out of hydrogen and expand into a red giant.
So understanding the nuances of such stars—both how metals influence planet formation and how expanding red giants affect existing planetary systems—holds vital clues about our solar system.
Unraveling such mysteries could explain why the sun, for instance, mysteriously contains more metals than many of its neighboring stars.
Experts speculate that either our solar system is a rarity, or that it formed in a more metal-rich part of the inner galaxy and then somehow migrated to the Milky Way's outer suburbs.
The studies could also offer us a glimpse of life in the future.
"For me, the main motivation is to actually see what happens to planetary systems when the star evolves," said Alex Wolszczan, an astronomer at Pennsylvania State University who is also leading a red giant planet-hunting survey.
Aside from finding planets around these aging stars, Wolszczan hopes that his survey will help theorists better understand what happens to solar systems when red giants expand.
"Orbits may cross and there may be collisions, but the habitable zone will expand as the star expands. So does life have a chance in a somewhat wild environment like that?" he asked.
"Planets that could have been dead for billions of years will all of a sudden find themselves suitable for life," he added.
"An example would be Jupiter's moon of Europa. That's just water ice, so if that melts you will have an ocean planet."
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