"For almost 10 years we have tried to find out what distinguishes（区别，辨别） stars with planetary systems（行星系） from their barren（不育的，贫瘠的） cousins," says Garik Israelian, lead author of a paper appearing this week in the journal Nature. "We have now found that the amount of lithium（锂） in Sun-like stars depends on whether or not they have planets." Low levels of this chemical element have been noticed for decades in the Sun, as compared to other solar-like stars, and astronomers¹ have been unable to explain the anomaly（异常，反常）. The discovery of a trend among planet-bearing stars provides a natural explanation to this long-standing mystery. "The explanation of this 60 year-long puzzle is for us rather simple," adds Israelian. "The Sun lacks lithium because it has planets."

This conclusion is based on the analysis of 500 stars, including 70 planet-hosting stars. Most of these stars were monitored for several years with ESO's High Accuracy Radial Velocity² Planet Searcher. This spectrograph（摄谱仪）, better known as HARPS³, is attached to ESO's 3.6-metre telescope and is the world's foremost exoplanet（外部恒星） hunter. "This is the best possible sample available to date to understand what makes planet-bearing stars unique," says co-author Michel Mayor.

The astronomers looked in particular at Sun-like stars, almost a quarter of the whole sample. They found that the majority of stars hosting planets possess less than 1% of the amount of lithium shown by most of the other stars. "Like our Sun, these stars have been very efficient at destroying the lithium they inherited（遗传） at birth," says team member Nuno Santos. "Using our unique, large sample, we can also prove that the reason for this lithium reduction is not related to any other property of the star, such as its age."

Unlike most other elements lighter⁴ than iron, the light nuclei⁵（轻核） of lithium, beryllium（铍） and boron（硼） are not produced in significant amounts in stars. Instead, it is thought that lithium, composed of just three protons（质子） and four neutrons⁶（中子）, was mainly produced just after the Big Bang, 13.7 billion years ago. Most stars will thus have the same amount of lithium, unless this element has been destroyed inside the star.

This result also provides the astronomers with a new, cost-effective way to search for planetary systems: by checking the amount of lithium present in a star astronomers can decide which stars are worthy⁷ of further significant observing efforts.

Now that a link between the presence of planets and curiously⁸ low levels of lithium has been established, the physical mechanism⁹ behind it has to be investigated. "There are several ways in which a planet can disturb the internal motions of matter in its host star, thereby¹⁰ rearrange（重新整理） the distribution of the various chemical elements and possibly cause the destruction of lithium. It is now up to the theoreticians（理论家） to figure out which one is the most likely to happen," concludes Mayor.