I have a lot of respect for planet hunters. It’s not an easy job. Discovering and confirming the existence of a planet requires a lot of patience, and I’d imagine it must be frustrating at times. So I’m sure a few planet hunters are rejoicing at the news that knowing which stars to look at just became a lot easier. All you have to do is look for lithium.
Lithium is one of the rarest elements in the Universe. It’s rare because unlike the elements that make up you and I, it’s actually destroyed in stellar fusion. As stars burn, they fuse together elements like carbon and oxygen from helium and hydrogen. Lithium, on the other hand tends to absorb a stray proton and be split apart into helium atoms. As a result, the Sun contains far less lithium today than it did when it first formed. But there’s something not quite right here… The surface of the Sun contains 140 times less lithium than it did originally, but that’s actually lower than it should be! The surface of the Sun isn’t hot enough to burn lithium. Temperatures in the Sun are only hot enough to burn lithium properly much deeper inside the star, and there’s no way for the surface material to circulate down that deep. Which poses a riddle of how exactly the Sun lost as much lithium as it did. Seemingly, there’s a good chance it has to do with the fact that the Sun has planets!
While sample sizes of stars with planets are limited by the number of planets discovered so far, this team based in Tenerife looked at a set of 451 stars, of which 70 are known to have planets. They found that roughly 50% of the solitary stars had 10 times as much lithium in their skins than did their planet-bearing kin. The logical conclusion is that for one reason or another, planets must increase the convection inside their host stars. This would allow material inside the star to circulate more, allowing lithium to reach depths where it may be fused.
Interestingly, the effect is only really apparent for sun-like stars. Hotter stars are more massive, and hence their surfaces are less convective anyway. Past a certain temperature, planets don’t seem to do much to promote lithium burning. The authors make no comment on stars much cooler, though they probably think the reason would be obvious. Red dwarfs are highly convective. Fully convective at the lower end of the mass range. As a result, their lithium would be depleted in any case.
A few other mechanisms are considered for why these stars could be lithium poor. For example, older stars will have had more time to burn up their lithium. But the authors considered how active these stars are at their surfaces, and how fast they rotate (both of which decrease with age). The idea is dismissed. Apparently, it’s nothing to do with age. Nor is there any correlation with any other metals residing in these stars. The main cause for this lithium depletion can only be the planets these stars host!
Planets around stars, as I mentioned previously, may promote convection within the star itself by increasing the star’s angular momentum. Alternatively, it could go back further, to when the star’s still forming. By the time they’re fully formed, planet bearing stars may rotate more slowly than solitary stars do, due to tidal effects. This could cause increased differential rotation within the stars — the core and the outer layers may rotate at very different speeds. Could this too, promote instabilities and cause mixing?
Whatever the exact cause is, the exciting upshot is that this suddenly makes stars with planets easy to spot. It’s especially interesting to me that this effect works exclusively for Sun-like stars. It’s almost as if by some quirk of fate, we have a physical effect tailor-made to help us look for Solar system analogues elsewhere in the galaxy. Suddenly, the search for exo-Earths looks slightly easier!
If it’s confirmed that planets have a direct influence on their parent star’s spectrum this way, I wonder how this will influence future exoplanet searches. Perhaps some future searches might specifically target stars which are lithium-poor in order to maximise their efficiency. Perhaps this effect might help us to find planets which are so far undetectable to us.* In the hunt for exoplanets, this is quite an exciting turn of events. I, for one, can’t wait to see what might come out of it!
*If the current planet hunter’s arsenal were in orbit around Alpha Centauri, it could just about detect Earth — but Mars and Venus would be invisible!
Image credit: NASA
Israelian, G., Mena, E., Santos, N., Sousa, S., Mayor, M., Udry, S., Cerdeña, C., Rebolo, R., & Randich, S. (2009). Enhanced lithium depletion in Sun-like stars with orbiting planets Nature, 462 (7270), 189-191 DOI: 10.1038/nature08483