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Solar lithium shortage blamed on planets
KEITH COOPER
ASTRONOMY NOW
Posted: November 12, 2009


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New findings from one of the most sensitive spectrographs in the world look to have finally answered the decades-long mystery as to why the Sun contains less lithium than many other stars. The resolution to the puzzle appears to be the fact that our Sun has planets orbiting it.

Planet-forming discs are more likely to be found around stars with low lithium abundance. Image: ESO/L Calçada.

Using the world-renowned HARPS (High Accuracy Radial Velocity Planet Searcher) spectrograph on the 3.6-metre telescope at the European Southern Observatory in Chile, a team led by Garik Israelian of the Institutos de Astrofisica de Canarias in Tenerife sampled 500 stars with ages between six and nine billion years. Seventy of these 500 stars are known to have planets. Because HARPS is also one of the most prolific and sensitive planet-finding instruments that we have, it has ruled out there being any large planets around the other 430 stars. “Given the superb precision of HARPS, we are confident that it could detect all solar analogue stars with gas giant planets and even Neptune-sized planets,” he says.

Israelian found that the stars with planets contained less lithium than those without; indeed, their lithium abundance was a mere one percent of the abundance in stars that apparently don’t have large planets. This is irrespective of any other property of the star, such as its age. Because lithium was created in the big bang, it stands to reason that all stars should contain equal amounts, relative to their masses. The fact that some have a shortage, including the Sun, has been a mystery for over 60 years, but not any longer according to Israelian.

“The Sun lacks lithium because it has planets,” he says, but understanding why the presence of planets results in the destruction of lithium within the star is still not completely explained. One possibility is the transfer of angular momentum from gas giant planets to the star; hence if the 430 stars do turn out to have smaller, rocky planets with minimal angular momentum but no gas giants, they would not affect the lithium abundance.

“Giant planets can migrate into closer orbits during their first 1–2 million years,” Israelian tells Astronomy Now. “This migration transfers angular momentum to the stellar atmosphere, which creates ‘friction’ between the upper atmosphere and the inner radiative layer, triggering mixing.” Lithium near to the star’s surface suddenly finds itself being dragged deeper into the hotter regions of the star by convection currents. At temperatures of 2.5 million degrees Celsius, lithium is destroyed.

The upshot of the findings, which are published in 12 November issue of Nature, is that astronomers will be able to target stars with low lithium abundances to help narrow down their search for exoplanets.

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