Planets are, by and large, at the mercy of their stars. Not only do stars provide a ready energy source of radiated light and heat, but the mass and gravitational pull of stars flat-out dwarfs the summed masses and pulls of any orbiting companions. In our solar system, which has more planets—regardless of where one stands on the Pluto debate—than any other planetary system we know of so far, the sun still makes up more than 99.8 percent of its system's mass.
But a new survey of stellar chemistry in solar-type stars reveals at least one way that pip-squeak planets can strike back, affecting the evolution of their parent stars. A paper in the November 12 issue of Nature shows that lithium is greatly depleted in stars known to host planetary systems compared with otherwise similar stars that appear to be barren of planets. (Scientific American is part of the Nature Publishing Group.)
A correlation between stellar lithium abundances and the presence of planetary systems had been suspected for years—our lithium-weak sun, for one, certainly fits the bill. But the catalogue of stars with such extrasolar planets, or exoplanets, was too small to evaluate the relationship with statistical confidence. In the past dozen years, however, numerous exoplanetary discoveries have been announced, including a suite of 30 new planets unveiled in October by the European Southern Observatory's HARPS planet-finding collaboration that boosted the full set of known exoplanets to more than 400.
Study co-author Nuno Santos, an astrophysicist at the Center for Astrophysics at the University of Porto in Portugal, and his colleagues took chemical-abundance data, derived from precision light spectra, on 133 stars of roughly sunlike temperature from the HARPS survey, 30 of which are known to harbor planets. (They also added more than two dozen other stars to the population to boost the sample size.) The vast majority of stars with planets were excessively depleted in lithium, whereas most "single" stars were only partly depleted. And in a subset of the 84 stars closest to the sun's temperature, the correlation was even stronger.
The researchers suspect that the presence of orbiting planets may increase convective mixing in the host star, plunging the lithium into its hotter regions where nuclear reactions consume the light element as fuel. "We know that lithium depletion in a star is dependent on the history of the star, how it rotates through its history," Santos says. "The presence or formation of planets could change this rotational history of the star."
But might lithium-depleted stars simply be more amenable to planet formation? Not likely, Santos says. "I don't think that's the reason, because actually lithium is not supposed to play any role in planet formation," he says. "There are only very small quantities of lithium, so it's not acceptable that lithium is, by itself, influencing planet formation. The idea is that things come the other way around. By some process, the planet-formation process is influencing the depletion of lithium in the atmosphere of the stars."
Whatever the reason, a simple lithium measurement—in concert with characteristics such as stellar mass and other chemical abundances—might aid future exoplanet hunters in pegging the stars that are most likely to bear planetary fruit.
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Add CommentCausal affects cannot be directly inferred from statistical correlations. As mentioned, planets’ gravitational affects on their stellar host are likely most often negligible, although large mass planets orbiting near a star would have more affect than does Mercury on the Sun. The depletion of stellar Lithium may still occur as a byproduct of planet formation: Lithium and other material in planetary disks may be included in stars that do not develop planets.
Reply | Report Abuse | Link to thisIn any case, if this correlation holds up it might indicate a good method for identifying stars with planets, especially since smaller planets cannot be easily detected.
Causal affects cannot be directly inferred from statistical correlations. As mentioned, planets’ gravitational affects on their stellar host are likely most often negligible, although large mass planets orbiting near a star would have more affect than does Mercury on the Sun. The depletion of stellar Lithium may still occur as a byproduct of planet formation: Lithium and other material in proto-planetary disks may have been included in stars that do not develop planets. From the report, it seems that there may also be a correlation between stellar temperature and Lithium depletion. It’s interesting that scientists are free to pick and choose among possibilities without supporting evidence.
Reply | Report Abuse | Link to thisIn any case, if this correlation holds up it might indicate a good method for identifying stars with planets, especially since smaller planets cannot be easily detected. However, in this case there should be a significant sample of Lithium depleted stars without detected planets. A study identifying lithium depleted stars without detected planets to determine whether planets may exist would be more useful.
There is an alternative cosmochemical interpretation. During the accretion of the refractory chondrules (millimetre size) in the condensing stellar nebulae the alkaline metals, including Li, are preferentially fractionated into lithophiles and siderophiles, essentially of chondritic meteoritic composition. The more "dustier" protostellar clouds fractionate Li preferentially as well as a host of other heavy elements. The enhanced Li abundance in carbonaceous chondrites and the selective fractionation of the stable lithium isotopes ( sup6Li and the more mobile sup7Li in aqueous phase) was reported in the late 1960's.
Reply | Report Abuse | Link to thisThe low Li stellar abundances would then correlate with those stars that form with terrestrial type "rocky planets", from clouds with high dust content rather than gas rich giant or Jovian type planets, or those with normal Li abundance and no planets at all.
Hence, a cosmochemical fractionation is regarded as more likely occurring prior to stellar ignition in the condensing accretion disk.
Wouldn't the lack of lithium in the original gas cloud from which a star forms, be why planets form or not,not the other way around?
Reply | Report Abuse | Link to thiscool article. love it. perfect for my presntation thursday.
Reply | Report Abuse | Link to thisJack-only a trace of lithium was synthesised in the Big Bang;about 1 lithium atom per 10^10 H atoms. No higher elements were formed- no stable isotopes occur with atomic masses 5 or 8. All heavier elements were formed subsequently in stars.
Reply | Report Abuse | Link to thisOur Sun is depleted in lithium compared to its cosmic abundance ie. 1 in 10^10 in many stars that lack debris from former supernova. The article cites a depletion of ~10^2.
However, meteorites collected on Earth contain much more Li isotopes than the average cosmic abundance, infact ca. 10^2 more. Thus Li in meteorites is ca. 10,000 fold concentrated in our solar system than what is found in the solar atmosphere.
At the time of the formation of the solar system and prior to the ignition of the Sun the high temperatures in the inner disk, where high temperature siilicates, aluminates and iron were forming like rain drops in a cloud. It seems more likely, at least to me, that the alkali metals including lithium were chelating to these molten chondrules. So the cloud prior to formation of these chondrules and then planets had the original Big Bang complement of Li. The first stars would have this composition at a time prior to the origin of the heavier elements in the first giant stars which then became red giants and finally supernova explosions which distributed heavier debris to the still "clean" hydrogen/helium clouds.
So lithium doesn't promote planetary formation at all. Its long term fate is destruction in stars. However, chemically speaking, since it is an alkali metal and as an ion at high temperature it would react with the aluminosilicates in the incandescent cloud and be sequestered in rocky material. Stars that lack original rocky dust in the proto nebula would not lose Li to forming chondrules.
Our Sun's planetary system is stable. were a rocky planet to be perturbed and plunge into the a sun then the vapourisation of the planet could boost the chromosphere Li composition. It is not a likely event, otherwise the clear relationship empirically observed wouldn't happen.
Graham-Thanks for the information.So most of the lithium in a given area came from a supernova or nebula?Thus those stars deceided whether the next stars would or would not have planets?
Reply | Report Abuse | Link to thisHello again Jack. By The chronology of events is as follows.
Reply | Report Abuse | Link to thisAfter about four minutes post Big Bang the composition of the universe is Hydrogen(75%) (actually as protons or ions) and Helium(25%) nuclei (that is 2 protons bound to 2 neutrons) with trace deuterium isotope, ca 20 parts per million(one proton bound with one neutron). Lithium, which is 3 protons bound with 3 or 4 neutrons was present as an even lower trace, only one atom of the isotopes per 10^10 protons , 1 part in ten billion!
So the gas or plasma composition from which all stars and then galaxies that were to form later was hydrogen and helium, as ions, and electrically neutral and charge balanced by electrons. Of course, atoms couldn't form because the temperature of the gas was too hot for atoms to form.
By the time the radiation(photons ie. light) had cooled, ca. 380,000 years post BB, atoms could form ie. combine . This is the era called recombination, at a universal temperature of ca. 3000 Kelvin when the universe became transparent to light and is present day the source of the cosmic background radiation or CMB.
The sun's surface atmosphere temperature is ca. 5800Kelvin. At higher temperatures prior to the CMB era atoms couldn't form and gravitate to form stars.
The first stars formed from this primordial gas that was exclusively Hydrogen and Helium atoms with a tiny trace, 1 part in ten billion of Li atoms. This represents the Li cosmic abundance. Some globular clusters contain stars low in heavier elements, that represent this low cosmic abundance of Li isotopes.
Some of the more massive first stars evolved rapidly and generated the heavier elements; supernova generated those elements beyond the atomic mass of iron, Fe. Ten billion years later the Sun condensed from a nebula which was essentially the initial H/He cosmic abundance with the tiny trace Li content, but which included the stellar debris of former stars that had destructed in former supernova events.
However, only ca. 1 per cent of the original cosmic gas abundance has been processed in stars. Many stars still form from gas which lacks debris from later stars. The more debris that is available, the more likely that rocky planets will form. The molten chondrules that form, like rain drops in a terrestrial cloud, fractionate out the heavier elements from the dirty proto stellar cloud.
So purposivity doesn't enter into the process of planet formation. The lithium content from the BB is conserved, in that it isn't destroyed but is fractionated into what becomes rock here on Earth.
I WOULD LIKE TO READ AN ARTICLE ON CHEMISTRY AND THE ARTICLE TO CONTAIN AT LEAST ONE DESCRIPTIVE DIAGRAM OR GRAPHIC ILLUSTRATION.
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