To Find New Planets, Look for the Lithium?

November 11, 2009

Sunlike stars that harbor planets are low on lithium, according to a recent study that may offer a new tool in the hunt for planets beyond our solar system.

(See what could be the first picture of a planet orbiting a sunlike star.)

Stars are made almost entirely of hydrogen and helium. A small percentage of a star's mass comes from heavier elements, which astronomers refer to as metals.

Young, yellow stars like our sun usually have more metals than older, redder stars, although the exact mix of those metals can vary.

But astronomers have been unable to explain why otherwise similar sunlike stars have widely different lithium levels.

The new study suggests that the answer lies with the presence of planets.

"There're stars with and without planets, and the scatter of lithium abundance is very large," noted lead study author Garik Israelian, of the Instituto de Astrofísica de Canarias on Tenerife, one of the Spanish-run Canary Islands.

"But for planet-host stars, we find that they all have low abundances of lithium."

Mixed-up Stars

It's been a puzzle for decades why our sun has significantly less lithium on its surface than other stars of the same age, mass, and metal abundance.

For the new study, Israelian and colleagues monitored 451 sunlike stars, 70 of which were already known to harbor planets.

(Related: "32 New Planets Found Outside Our Solar System.")

Observations made over several years revealed that all the known planetary hosts had similarly low lithium levels.

By contrast, stars that have been scrutinized for years, without any planets having been found, had almost ten times more lithium than stars with known planets.

Why, exactly, planets would cause lithium depletion remains unknown.

One possibility is that, early in a star's life, gravitational motion from orbiting planets or a planet-forming disk stirs up the star's elements. This mixing causes surface materials such as lithium to get shuttled into the star's interior and burned away.

More research is required, Israelian said, "but at least we have a clear signature that planets do change [a star's] surface abundance of lithium."

Israelian cautions that low lithium does not guarantee the presence of planets around a particular star. Astronomers would have to follow up with existing methods for planet detection.

But, he said, "if we have ten planet-host candidates, those ones with very low lithium will be the best ones to [check for] planets."

Findings appear in this week's issue of the journal Nature.

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