Exoplanets Clue to Sun's Curious Chemistry

Nov 11, 2009
Artist’s impression of a baby star still surrounded by a protoplanetary disc in which planets are forming. Using ESO’s very successful HARPS spectrograph, a team of astronomers has found that Sun-like stars which host planets have destroyed their lithium much more efficiently than planet-free stars. This finding does not only shed light on the low levels of this chemical element in the Sun, solving a long-standing mystery, but also provides astronomers with a very efficient way to pick out the stars most likely to host planets. It is not clear what causes the lithium to be destroyed. The general idea is that the planets or the presence of the protoplanetary disc disturb the interior of the star, bringing the lithium deeper down into the star than usual, into regions where the temperature is so hot that it is destroyed. Credit: ESO/L. Calçada

(PhysOrg.com) -- 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 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 in Sun-like stars depends on whether or not they have ."

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 . 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 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.

More information: This research was presented in a paper that appears in the 12 November 2009 issue of Nature (Enhanced lithium depletion in Sun-like stars with orbiting planets, by G. Israelian et al.).

Source: ESO (news : web)

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yyz
4 / 5 (4) Nov 11, 2009
This could indeed be an efficient, low cost method of detecting planetary systems around other stars if this finding can be replicated and/or expanded upon. Time for theorists to investigate the probable mechanism(s) involved here and possibly come up with a few testable hypotheses.
omatumr
2.2 / 5 (5) Nov 11, 2009
The explanation is not that "The Sun lacks lithium because it has planets."

The Sun lack lithium because the Sun formed on a supernova remnant.
Lithium has low nuclear stability (i.e., M/A is high for lithium).
Lithium is quickly destroyed inside an evolved star.

With kind regards,
Oliver K. Manuel
yyz
3.7 / 5 (3) Nov 11, 2009
@Oliver Manuel, do you have links to any published papers that directly discuss and explain the reason for this lithium depletion observation in planetary systems as opposed to stars without planets?
gmurphy
2 / 5 (1) Nov 11, 2009
This is an interesting correlation but the actual cause is still unknown. What Oliver M says makes sense, planets tend to form around certain types of stars, these stars in turn are formed from specific processes (remnants of supernovas).
dirk_bruere
3 / 5 (1) Nov 11, 2009
Numbers! What % of stars have low Lithium content?
Husky
3 / 5 (2) Nov 11, 2009
seems like a sensible explanation, supernova remnants would contain more irregular blobs of heavier elements, that opposed to a pristine hydrogen cloud would allow to contract into planetsized bodies, before the whole cloud would contract into a giant star. Higher densities would not only aquire planetmass through sheer gravitational attraction, but also capture by its lower viscosity, slowing other dust down and more local induced disturbances due to magnetism and mass sorting caused by filtering of polarised light, i would go as far to say that we might find a strong correlation of planetforming systems in relation to the strentgh & orientation of magnetic fieldlines acting upon it by its host hostgalaxy
Husky
2.5 / 5 (2) Nov 11, 2009
Btw The article seems to imply that the lithium would somehow be burned by our sun, while the rise from the ashes theory assumes the lithium was already burned by an exploding star, that means it would be interesting to look what kind of stars are the seeds that create a nova with such specific lithium signature, what would be the telltale leftover object, a white dwarf, a neutron star, a black hole?, start looking at the surrounding stars of such associated objects if they were seeded by the nova. Typical candidates would be stars whos began their life about the time the shockwave of the nova reached and compressed the surrounding molucular gascloud.
Nik_2213
4 / 5 (3) Nov 11, 2009
"On a supernova remnant..." ??
I thought helioseismology etc would have shown some oddity if there was a 'compact core'...

If you meant that the s/n shockwave had both seeded and collapsed a gas cloud into a protostar, that would do nicely. Perhaps a 'seeded' origin might need a lower-mass Bok than otherwise...
Arikin
1 / 5 (1) Nov 11, 2009
So assuming the correlation is true. Does the number and mass of planets orbiting a star affect the amount of Lithium? More gravity influence means a faster burning of Lithium?
omatumr
2 / 5 (4) Nov 11, 2009
I would expect low lithium (Li) in any star with rocky, Earth-like planets.

D, Li, Be and B have low nuclear stability (high value of M/A), would not survive inside an evolved star, and are likely produced by spallation type reactions that B2FH called the x-process near the stellar surface.

Earth, ordinary meteorites, and the interior of the Sun consist mostly of Fe, O, Si, Ni, S, Mg and Ca - elements made in the deep interior of the star that gave birth to the solar system.

With kind regards,
Oliver K. Manuel
Sonhouse
1 / 5 (1) Nov 11, 2009
My hats off to you mr Manuel! Nice bio, very impressive:
http://www.thesun...ress.asp
Take a look if anyone doubts his creds.
BTW Oliver, I had the honor to have at least held a moon rock in my hand, inside the vault at Goddard, when I was working as an Apollo tech (Apollo timing and tracking, atomic clocks and the distance measuring transponder) in 1970. A man lived next door to my place in Alexandria Va and we got to talking and it turned out he was the geologic tech who actually cut the samples into thin slices, I told him about my job and he invited me to his lab where the massive vault was and let me hold one of the rocks. It was almost a religious moment for me! I remember thinking 'why are they letting me hold this precious rock with my bare hands?' and they told me the rocks were all contaminated on the surface and the real work of analysis was done on the core where there was less chance at contamination. What a day for me! Don.
out7x
not rated yet Nov 12, 2009
Its hard to understand how a star could form without planets.
NotAsleep
5 / 5 (1) Nov 12, 2009
A lot of the comments here seem to be coming from people with a thin grasp of star/solar system formation. Start with wikipedia and work your way out if you're still interested:

Stars:
http://en.wikiped...ormation

Solar Systems:
http://en.wikiped...ormation

And thank you, Dr. Manuel, for continuing to express your opinions here (although I still find them hard to believe!)
gwrede
not rated yet Nov 15, 2009
I think that a star that has planets doesn't "destroy" the lithium. It would be much easier to assume that the lithium originally ends up in the planets instead of the star, just like silicon ended up in our inner planets, instead of the sun or the outer planets.

That's a lot easier to accept than our planets having some effect on the sun that causes the lithium to disappear.
Ronan
not rated yet Nov 16, 2009
Gwrede: I don't know, Jupiter's got quite a bit of heft to it, and it actually tugs the Sun enough that 'ol Sol wobbles measurably as Jupiter travels around it. That any other planets would have much of an effect, I agree, seems a bit unlikely, but don't discount the King of Worlds so quickly...

At any rate...Yes, I know it's irresponsible, and that I'm probably overlooking all sorts of pesky inconveniences and pratfalls of reality, but...This sounds fantastic! If this method really does work so well, then we should be able to find stars that, at the very least, have gas giants just by looking at their spectrum and figuring out whether or not they've got sizable quantities of lithium. Goodness, I hope more is heard of this in the future; this could be really, really useful. Such an...EASY method to get such interesting, interesting information...

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