Finding planets by looking for the lack of Lithium

Neat. [Link]

Astronomers took a sample of 500 stars, 70 of which are known to have planets orbiting them, while the rest have no planets detected. They examined the spectra of the stars, looking specifically to see how much lithium was present. What they found, with good statistical significance, is that stars with planets had far less lithium than stars that did not have planets.

Somehow, having planets means a star loses its lithium. How the heck does that happen?

A brief digression. Lithium is a weird element. It’s the third lightest after hydrogen and helium, and unlike every other element after it on the periodic table, we don’t think it’s made inside stars. It’s too fragile; the nuclei get smashed up easily, and so it doesn’t last long in the cores of stars. That means that as far as we can tell, all the lithium in the Universe was created in the Big Bang.

Just because it gets wrecked in the cores of stars does not mean they have no lithium at all. Lithium created in the Big Bang would have been in clouds where stars formed, and if a lithium nucleus can avoid the core of the star by staying nearer the surface, it can survive. The Sun has lithium in it, for example, but at far less abundance (<1%) than what you see out in gas clouds. That means the Sun has destroyed a lot - but not all - of its lithium supply.

When astronomers look at other stars like the Sun, the amount of lithium they possess varies wildly. But now it appears that the amount of lithium in a Sun-like star depends on whether it has planets or not. Stars without planets have, on average, 10 times the lithium as stars with planets in the sample.

Weird.