In the News this month: a new way to search for exoplanets
Artist’s impression of a baby star still surrounded by a protoplanetary disc in which planets are forming CREDIT: ESO/L. Calçada
Planet searching techniques are continuously being refined and are detecting ever smaller planets at greater and greater distances from their parent stars. But a team of astronomers have discovered a link between planetary systems and lithium abundance that could provide a new tool in the search for exoplanets. Most methods of searching for planetary systems around other stars are best suited to finding large planets orbiting very close to their host stars. But what if there was a way to determine the likelihood of a particular star hosting planets, without actually detecting the planets at all? A team, led by Garik Israelian of the Instituto de Astrofisica de Canarias in Tenerife, think they have found a link between whether stars host planets and how much lithium is observed.
Lithium is one of the lightest chemical elements and is present in detectable quantities in most stars. The surface abundance of lithium on the Sun is 140 times less than what it was in the protostellar cloud from which the Sun formed. The surface of the Sun consists of a convective layer where material is constantly circulated in large convection cells, but the temperature at the base of this layer is not high enough to burn lithium, so where did it go?
The team studied spectra of 451 stars, some of which host planets while some do not. All the stars in the sample were similar to the Sun with surface temperatures between 4900 and 6500 degrees Kelvin. When they compared the lithium abundances, they found that for stars with surface temperatures in the range 5600 to 5900 Kelvin, the majority of stars known to host planets were severely depleted in lithium, whereas the non-planet hosting stars showed a much lower level of depletion. This indicates a possible link between lithium depletion and planet formation which could be used to pre-screen stars for planet searches.
However, the link is only seen for stars in a particular temperature range: above 5900 Kelvin the convective layer is too shallow to reach a depth where the temperature is high enough to burn helium, while below 5700 Kelvin the convective layers penetrate deeper and all stars show significant lithium depletion. Within this range, the amount of lithium depletion seen in star hosting planets is independent of both the star's surface temperature, metallicity and age, indicating that the presence of a planetary system is related.
While there is currently no model that explains this apparent link, the authors suggest a few ideas. The existence of a planetary system may result in a variation in the rotation rate of the star, increasing the mixing of material within layers; planetary migration may also alter the stellar rotation resulting in a similar effect. Another suggestion is that an interaction of the proto-star with the surrounding accretion disk may lead to a large variation in rotation speeds of different layers within the star, with the outer layers slowed down by the disk compared to the core, again resulting in greater mixing of material and more lithium being dragged down to layers where the temperature is high enough to destroy it.
While more observations and detailed modeling is required to determine the physical process causing lithium depletion in planetary systems, these results suggest that an understanding of the Sun's lack of lithium may be best understood by looking at other planetary systems.
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