for National Geographic News
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?"
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