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Shedding Light on the Sun’s “Lithium Mystery”

Artist’s impression of a baby star still surrounded by a protoplanetary disc in which planets are forming.  Credit: ESO
For decades, astronomers have known our Sun contains a low amount of lithium, while other solar-like stars actually have more. But they didn’t know why. By looking at stars similar to the Sun to study this anomaly, scientists have now discovered of a trend: the majority of stars hosting planets possess less than 1% of the amount of lithium shown by most of the other stars. “The explanation of this 60 year-long puzzle is for us rather simple,” said Garik Israelian, lead author on a paper appearing in this week’s edition of Nature. “The Sun lacks lithium because it has planets.”

This finding sheds light not only on the lack of lithium in our star, but also provides astronomers with a very efficient way of finding stars with planetary systems.

Isrealian and his team took a census of 500 stars, 70 of which are known to host planets, and in particular looked at Sun-like stars, almost a quarter of the whole sample. Using ESO’s 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.

“For almost 10 years we have tried to find out what distinguishes stars with planetary systems from their barren cousins,” Israelian said. “We now have found that the amount of lithium in Sun-like stars depends on whether or not they have planets.”

These stars have been “very efficient at destroying the lithium they inherited at birth,” said 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,” said co-author Michael Mayor. ” It is now up to the theoreticians to figure out which one is the most likely to happen.”

Read the team’s paper.

Source: ESO

About 

Nancy Atkinson is Universe Today's Senior Editor. She also works with Astronomy Cast, and is a NASA/JPL Solar System Ambassador.

Comments on this entry are closed.

  • Jon Hanford November 11, 2009, 2:06 PM

    I like the possibility that this technique could be used as a ‘quick-and-dirty’ method of discriminating planetary systems as opposed to stars without planets.

  • DrFlimmer November 11, 2009, 2:22 PM

    It will be interesting to find the theoretical background of this research. I mean, if and especially how/why these things are correlated.

  • Greg November 11, 2009, 4:01 PM

    This is potentially some great news. At this point I would hesitate to accept this correlation until the mechanism can be understood. Additionally it may be a marker for a star having gas giants only and may have nothing to do with whether one has smaller terrestial planets. Also I would like to see if low levels of lithium correlate with higher levels of heavier elements or not. This could potemtially point to the composition of the nebula from which the star formed as being important towards planet formation.

  • Nexus November 11, 2009, 4:19 PM

    Stars with planets should also have slow rotations, because much of the angular momentum is in the orbits of the planets. This is what we see in our Solar System- generally, stars like the Sun rotate much quicker than the Sun. Perhaps there is some mechanism whereby slowly rotating stars can dispose of their lithium, but quickly rotating ones can’t.

  • stargeezer November 11, 2009, 4:46 PM

    I wonder if these results will affect Keplers observations. Can Kepler ‘filter’ stars with a high lithium content? One wonders.

  • Jon Hanford November 11, 2009, 4:47 PM

    Greg points out important caveats as to the usefulness of this single observed correlation, so of course confirmatory data is needed to help confirm or refute this relationship.

  • Lawrence B. Crowell November 11, 2009, 5:23 PM

    Nexus: As an exercise you can determine if your hypothesis is reasonable. The sun rotates about every month. The moment of inertia of a solid sphere is I = (2/5)Mr^2, M = mass of sun, r = radius. The angular momentum is then L = 2pi*I*freq. Now consider the angular momentum of Jupiter. It moment of inertial in its orbit is just mr^r, r = radius of orbit. The frequency is 1/(11.8year). Then use the same formula for the angular momentum to see if this is at all comparable to the angular momentum of the sun.

    It is curious that planet producing stellar systems would have little lithium in the star. It is in line with the question of why comets have different concentrations of deuterium than Earth does.

    LC

  • Nexus November 11, 2009, 5:59 PM

    A very rough calculation suggests that Jupiter has about 150 times more angular momentum than the Sun. I’ve read somewhere that the planets contain about 99.5% of the angular momentum of the Solar System, and that Jupiter’s got the lion’s share of it, so that sounds about right.

  • VitalStatistic63 November 11, 2009, 10:23 PM

    I wonder if the lithium that is lost from the sun is to be found in the planets instead.

    At the formation of the solar system, heavier elements probably made their way to the centre of the system, while lighter elements tended to drift outwards where they could have been trapped by the dust clouds that became planets.

    This might be tested by comparing lithium concentrations of planets relative to their distance from the sun. If the theory holds, you would expect Mercury to have very little lithium, and Neptune to have a much higher concentration than closer planets.

  • SteveZodiac November 12, 2009, 1:44 AM

    This is good science and will save many man/machine hours of sifting evidence. Well l done Israelian’s team!

  • Lawrence B. Crowell November 12, 2009, 5:51 AM

    A rough estimate could consider the ratio of the angular momenta of the sun to Jupiter

    (L/L’) ~ MR^2*freq/mr^2*freq’

    = (M/m)(R/r)^2*(freq/freq’).

    So the ratio of masses M/m ~ 1000, the ratio of solar radius to Jupiter’s orbital radius is R/r ~ 1/1000, and the frequency ratio is about ~ 144. So we put it into the ratio of angular momenta amd L/L’ ~ .14 . If I restore the 2/5 for the momentu of intertia of a solid sphere I get the angular momentum of Jupiter is about 17 times that of the sun.

    LC

  • Dark Gnat November 12, 2009, 7:43 AM

    Stars with planets come from supernova remnants (which is howwe get heavier elements) Could the supernova have destroyed more lithuim, or fused it into heavier elements somehow?

    It an interesting corrolation, assuming it’s not coincidence.

  • Curious November 12, 2009, 9:48 AM

    Would brown dwarf companions have the same lithium-destroying effects as gas giant planet companions?

  • Lawrence B. Crowell November 12, 2009, 1:14 PM

    The question is how it is that lithium ends up in stars without planets preferentially over those with planets. Curiously there is a significant amount of lithium on Earth as well. It is a bit odd if this trend is universal.

    LC

  • Torbjorn Larsson OM November 12, 2009, 2:39 PM

    Nice!

    So now we know how the Ganaeans did it.

    [Obscure reference to A.E. van Vogt’s “Resurrection” – actually the Ganaeans claimed their technique to localize suns with planets was “unique and coincidental”.

    Also, look at the populations in the paper, they overlap. Its not useful for individual stars.]

    This is one of the interesting cases where correlation is a strong contender with causation. Causation is the parsimonious case. (E.g. with correlation you have to add an object and a process compared to causation.) But as there are many more possibilities for correlations they can win out. (E.g. biology, with its many contingencies.)

    I’ll bet on causation though, there are many possible mechanisms as well here.

    Btw, thanks for the link to the paper, Nancy!

    It an interesting corrolation, assuming it’s not coincidence.

    From the paper:

    “We performed different two-sample statistical tests using ASURV (version 1.2). All tests consistently confirm (at the 3? level) that the planet-host and single star populations are not drawn from the same parent population. [Ref. removed.]”

    So if it was just an ad hoc observation it would be too close to a coincidence to be useful, you would like at least 5 sigma separation. But as there is a set of likely hypotheses around (see above) that it is testing, it’s sound.

  • RBH November 12, 2009, 4:18 PM

    “The explanation of this 60 year-long puzzle is for us rather simple,” said Garik Israelian, lead author on a paper appearing in this week’s edition of Nature. “The Sun lacks lithium because it has planets.”

    Um, describing a correlation constitutes an “explanation”? Baloney.

  • Aodhhan November 13, 2009, 6:14 AM

    I’m not buying this.

    The Sun lacks lithium because it has planets… sounds more like an affect, not a cause.
    When making a claim, you have to be able to differentiate between the two, and solve for both.

    Also, the sample isn’t completely random and it is imbalanced as well. You can’t take 500 stars, and stick in the values of 70, which are known… against others which are unknown. This is basically akin to “slight of hand”. It is very likely many of the Stars which are currently assumed to have no planets, actually do.
    Basically, I wouldn’t buy this paper as an undergrad thesis.

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