The Sciences

Want a planet? You might want to avoid lithium

Bad AstronomyBy Phil PlaitNov 11, 2009 4:20 PM


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A science joke: A woman is out walking and sees a kid on his hands and knees looking at the sidewalk. She asks the boy what he's doing, and he says, "Looking for a quarter I lost." She asks him where he lost it, and he points across the street. Quizzically, she asks, "Then why are you looking here?" He replies, "The light's better over here." What's this got to do with astronomy? I'm glad I asked.

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. It's possible to think of simple ways that a planet could affect the lithium abundance of a star. Maybe the gravitational tugging of the planet helps mix up the star's interior, letting the lithium get close enough to the core to get destroyed. Shortly after the star and planets form, the planets can migrate slowly toward the star over long periods of time, which might affect how rapidly the star rotates. That in turn will affect how deeply the star's outer convection layer can penetrate the interior (bringing lithium down with it, destroying the element; in fact, this is one scenario proposed by the team that made this discovery. Or maybe it's something else. Or maybe there is a third thing we haven't thought of yet, something that both destroys lithium and allows the star to make planets. The presence of planets and depletion of lithium might be related, but not directly. It's a mystery, but astronomers love mysteries. More observations will no doubt uncover more clues, give us more data we can analyze to uncover yet more correlations. And that brings me back to my joke at the start. The press release for this news story makes an interesting statement:

This finding does not only shed light on the lack of lithium in our star, but also provides astronomers with a very efficient way of finding stars with planetary systems.

I disagree with the philosophy of this conclusion. Sure, if you want to find stars with planets, it might make sense to concentrate on stars with depleted lithium abundance. But I think that's not a great idea: you're only looking where the light's good. When planets were first discovered around sun-like stars, we were all surprised to find them very close to their parent stars, orbiting in days, not years. The reason they'd been missed for so long is that no one had thought to look for them in orbits that small! We'd been looking where the light was good (literally) and not where the planets really were. I'm not saying it's wrong to only look to lithium-poor stars when seeking planets, but I am saying that if you're a planet hunter, you might want to open your criteria a bit. This lithium finding is very interesting, and may well play out to be a hard-and-fast law, but I think it's still a little early to rule anything out just yet. As always, the Universe knows what it's doing. It's our task to figure out just what that is. Image credit: ESO/L. Calçada

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