Wednesday, November 11, 2009

Low on Lithium? If the Star's the Size of the Sun: It's Probably Got Planets

"60-Year-Old Solar Mystery Finally Explained" (November 11, 2009)

"The search for planets beyond our solar system may be a little easier, thanks to a new comparison of sun-like stars that has revealed a key difference in the chemistry of stars that have planets and their barren cousins and solved a long-standing mystery about our own sun's chemistry.

"The solar mystery revolves around the abundance of the element lithium in the sun.

"Unlike most elements lighter than iron, lithium is not readily produced in stars. Instead, it is thought that this element was mainly produced just after the Big Bang, 13.7 million light-years ago. Most stars will thus have the same amount of lithium, unless this element has been destroyed inside the star.

"Models of stellar evolution predict that stars of about the sun's mass and age should have a relatively high level of lithium because their atmospheres aren't hot enough or thick enough to burn off the element. But astronomers see a range of lithium levels in sun-like stars, with some having a lithium abundance of about 10 percent of that estimated for the primordial universe, and others --such as the sun -- having only 1 percent of the primordial abundance....

One explanation for the discrepancy might have been that the models of stellar evolution were wrong. That wasn't too likely, since those models were based on a whole lot of data that's been gone over with a fine-tooth comb over the decades.

So, assuming that existing models of stellar evolution are at least vaguely correct, there's some other explanation. Back to collecting and correlating data.

"...But by looking at the sun-like stars in the HARPS survey, about one-fourth of the total sample, astronomer Garik Israelian, of Spain's Instituto de Astrofisica de Canarias in Tenerife, and his colleagues found that those that had an orbiting planetary system had lithium levels similar to the sun's, while those that were barren had higher levels.

" 'The explanation of this 60 year-long puzzle is for us rather simple,' Israelian said. 'The sun lacks lithium because it has planets'...."

The next question is why having planets makes a star like the sun lithium-deficient. Israeleian and others have an idea, but no data to support - or disprove - it. Yet.

Given time, and effort, I figure scientists will tweeze out details of the connection between low lithium levels and planets.

Meanwhile, the correlation between low lithium levels in stars like the sun, and having planets, will make looking for other planetary systems easier. It's easier to measure chemical abundances in stars, than see if they've got itty-bitty flecks of ice and rock spinning around them.

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