Giant Planets More Common, Star Survey Suggests

Bruce Dorminey
for National Geographic News

August 7, 2007

A new survey of red giant stars that host planets suggests that giant alien worlds may form far more readily than previously suspected.

Planets are found much more commonly around stars like our sun that are rich in iron and other metals, hinting that such elements played an important role in planet formation.

But the new study found little trace of metals in the red giants, suggesting that large amounts of metal aren't necessary for planet formation.

"For all we know now, habitable Earths can form around almost every single type of star," said Alan Boss, a planetary scientist at the Carnegie Institution of Washington and a champion of an alternative, low-metal theory of planet formation.

(Related: "First Proof of Wet 'Hot Jupiter' Outside Solar System" [July 11, 2007].)

In fact, he suggested, the minimum amount of metal needed for planets to be born might be far lower than commonly believed.

Case of the Missing Metal

For the new study, Luca Pasquini, an astronomer at European Southern Observatory, and his team conducted a spectroscopic analysis of 14 planet-hosting red giants.

Red giants occur when certain stars exhaust their main source of fuel—hydrogen—causing the stars to greatly expand in size before fizzling out.

Such dying stars likely gobble up at least some of their planets during this expansion, like half-starved beasts eating their young.

"These giant stars are not metal-rich," Pasquini said.

"So why are the red giants with planets more metal-poor than dwarfs with planets?"

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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