Exoplanets Clue to Sun's Curious Chemistry

[C1ay]

A ground-breaking census of 500 stars, 70 of which are known to host planets, has successfully linked the long-standing “lithium mystery” observed in the Sun to the presence of planetary systems. Using ESO’s successful HARPS spectrograph, a team of astronomers has found that Sun-like stars that host planets have destroyed their lithium much more efficiently than “planet-free” stars. 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.

 

"For almost 10 years we have tried to find out what distinguishes stars with planetary systems from their barren 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 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 astronomers have been unable to explain the anomaly. 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 Velocity Planet Searcher. This spectrograph, better known as HARPS, is attached to ESO's 3.6-metre telescope and is the world’s foremost exoplanet 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 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 lighter than iron, the light nuclei of lithium, beryllium and boron are not produced in significant amounts in stars. Instead, it is thought that lithium, composed of just three protons and four neutrons, 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 worthy of further significant observing efforts.

 

Now that a link between the presence of planets and curiously low levels of lithium has been established, the physical mechanism 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, thereby rearrange the distribution of the various chemical elements and possibly cause the destruction of lithium. It is now up to the theoreticians to figure out which one is the most likely to happen," concludes Mayor.

 

Source: ESO

[HydrogenBond]

Another way to interpret this data, is planet formation makes use of lithium, since the correlation is so clear cut. The H-bombs use the solid chemical lithium deuteride as the fuel source. The lithium was originally thought to be just a place holder for the deuterium. But after the first tests of the H-bomb, when the yield exceeded theoretical prediction, they realized the lithium took part in the nuclear burn.

 

This turbo output effect from lithium, in the presence of deuterium, might prevent the mass from the forming planets from falling into the star. Alternately, the extra burn kicks off matter from a star from which planets will form. Stars with high lithium never had that extra turbo burn so they either absorbed their forming planetary mass, or never kicked any out any mass from the star for planets.

[CraigD]

Another way to interpret this data, is planet formation makes use of lithium, since the correlation is so clear cut.

Though this is a logical interpretation, a little arithmetic using usual data sources, such as the wikipedia data page “abundance of the elements” shows it to be incorrect. The approximate mass of lithium in the Sun is 2.588e19 kg. The approximate mass of lithium in all of the rest of the solar system is 2.003e16 kg. So if all of the greater and lesser bodies of the solar system were consumed by the Sun, it would increase the amount of lithium in to about 2.590e19 kg, less than a 1/1000th increase.

 

According to the paper cited by this news article (Enhanced lithium depletion in Sun-like stars with orbiting planets, converting from its “Log N(Li)” units), lithium-rich Sun-like stars have about 6.290e19 kg of lithium, more than double that of the Sun.

 

The conclusion of the paper is that Sun-like stars with planets are gravitationally “stirred up” by their planets, increasing the size of their convection zones and amount of mixing of their layers so that they “burn” (transmuted to helium due to collisions with protons) lithium at a greater rate than Sun-like stars without planets. :phones:

 

How in detail this happens is uncertain. According to comments by one of the authors, Michel Mayor

“There are several ways in which a planet can disturb the internal motions of matter in its host star, thereby rearrange the distribution of the various chemical elements and possibly cause the destruction of lithium. It is now up to the theoreticians to figure out which one is the most likely to happen.,”

 

:phones: I find it amazing how sensitive stars are to perturbations by external sources such as these. While I imagine stars to be very “sturdy” compared to everyday things such as human bodies, this appears to be a somewhat inaccurate imagining. By rough analogy, the “enhanced lithium depletion” described in Mayor et al’s paper is like a human contracting a metabolic disease due to the gravitational effects of a few flies in the room with him!