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Exoplanet Geochemistry: The White Dwarf Factor

I continue to be fascinated by small stars. My earliest passion for such involved red dwarfs, which appeared to make habitable planet possibilities that would be of great interest to science fiction authors, assuming such environments could survive tidal lock and stellar flaring. But white dwarfs have a weird seductiveness of their own, because we’re learning how to extract from them information about planets that orbited them before being consumed.

Thus a new paper out of UCLA, which focuses on an unusual way of determining the geochemistry of rocks from beyond our Solar System. We can do this because white dwarfs, the remnants of normal stars that have gone through their red giant phase and collapsed into objects about the size of the Earth, have strong gravitational pull. That means we would expect heavy elements like carbon, oxygen and nitrogen to vanish into their interiors, utterly out of view to our instruments. We should see little more than hydrogen and helium, making what actually does show up in their atmospheres intriguing.

Image: An artist’s concept showing debris falling into a white dwarf star. Credit: NASA/JPL-Caltech.

According to the UCLA researchers, spectroscopic studies reveal that the atmospheres of up to half of white dwarfs with effective temperatures below 25,000 K are polluted by elements heavier than helium. With their own heavy elements hidden within the stars’ interiors, white dwarf atmospheres are clearly collecting something external, presumably debris from rocky bodies that once orbited the stars and became disrupted by their gravitational pull.

We are talking about stellar objects a long way off. The closest examined in the study led by graduate student Alexandra Doyle is some 200 light years out, but Doyle’s UCLA team went to work on six white dwarfs in all, the farthest being at a distance of 665 light years. Doyle likens the work to a tool we’re familiar with in other spheres of inquiry. “Observing a white dwarf,” she says, “is like doing an autopsy on the contents of what it has gobbled in its solar system.”

Image: An artist’s rendering shows a white dwarf star with a planet in the upper right. Credit: Mark Garlick.

A key issue here is what is known as fugacity, which is helpfully defined in the issue of Science in which the paper ran: “The oxygen fugacity of a rock, fO2, is a measure of how oxidizing or reducing its surroundings were when the rock formed. Different minerals form at different fO2 and have different physical properties, so the internal structure of an exoplanet depends on this value.”

The importance of oxidation — in which iron shares its electrons with oxygen — is clear when we consider its impact on our own planet. Co-author Edward Young (UCLA):

“All the chemistry that happens on the surface of the Earth can ultimately be traced back to the oxidation state of the planet. The fact that we have oceans and all the ingredients necessary for life can be traced back to the planet being oxidized as it is. The rocks control the chemistry.”

The question under consideration: Are rocks in our Solar System typical of those around other stars? Doyle’s results show oxygen fugacities within range of what we find on Earth and Mars, as well as asteroids. This would suggest that there are geochemical similarities between Earth and the rocky exoplanets whose disintegration is measured around the white dwarfs.

It’s hard to see how else we might go to work on geochemistry around other stars than by studying white dwarf atmospheres. The paper notes that estimating the composition of exoplanets from their host star abundances or from planet mass-radius relationships is unreliable. White dwarfs provide a more direct alternative for studying extrasolar rocks.

Image: UCLA researchers Benjamin Zuckerman, Beth Klein, Alexandra Doyle, Hilke Schlichting, Edward Young (left to right). Credit: Christelle Snow/UCLA..

The UCLA scientists homed in on the six most common elements in rock: iron, oxygen, silicon, aluminum, magnesium and calcium, as found in white dwarf atmospheres. They compared their calculated results with Solar System materials, with findings that imply, according to co-author Hilke Schlichting (UCLA), that a planet bearing such rocks would have similar plate tectonics and potential for magnetic fields as the Earth. Let the paper conclude the story:

Our results show that the parent objects that polluted these WDs had intrinsic oxidation states similar to those of rocks in the Solar System. Based on estimates of their mass, the bodies accreting onto WDs were either asteroids that represent the building blocks of rocky exoplanets, or they were fragments of rocky exoplanets themselves. In either case, our results constrain the intrinsic oxygen fugacities of rocky bodies that orbited the progenitor star of their host WD. Our data indicate that rocky exoplanets constructed from these planetesimals should be geophysically and geochemically similar to rocky planets in the Solar System, including Earth.

The paper is Doyle et al., “Oxygen fugacities of extrasolar rocks: Evidence for an Earth-like geochemistry of exoplanets,” Science Vol. 366, Issue 6463 (18 October 2018), pp. 356-359 (abstract/full text).

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{ 17 comments… add one }
  • Alex Tolley October 22, 2019, 13:20

    Does this imply that the rocky worlds of these stars were never water worlds, but much more similar to our own 4 rocky worlds in terms of water content?

    • stefan October 23, 2019, 12:57

      No, the atmospheres of the white dwarf are in the process of separating. There should be an increasing amount of hydrogen over time. The white dwarf will have had a hydrogen atmosphere when the planet or asteroids crashed into it. That adds a lot of uncertainty to the oxygen hydrogen ratio. The study just gives us the ratio of hydrogen to oxygen, iron silicon, aluminum, magnesium and calcium. Iron and oxygen could be rust or a separated iron core and water surface.

      • Alex Tolley October 23, 2019, 18:25

        Stepfan, thanks for your reply. These snippets from the paper were the reason for my question. I’m still not sure I have a good answer though.

        From the paper:

        In cases where there is both more oxygen than
        required to oxidize all other major elements and
        a commensurate amount of hydrogen, water in
        the accreting body is implied and the partitioning
        of oxygen between ice and Fe can be ambiguous
        (33, 34).

        and:

        Raising delta IW by 5 log units, from solar
        to rock-like values, requires the gas to acquire
        an H2O/H2 ratio ~400 times that of a solar gas
        (28). This enrichment factor is greater than
        can be explained by simply transporting water
        in the form of ice particles from the outer to
        the inner Solar System (28). If dust/gas ratios
        control the oxidation states during rock formation,
        we conclude that the Solar System and the
        planetary systems around these six polluted
        WDs had similar ratios.

        I don’t really understand how to interpret these statements. However, naively to me it seems that the amount of water can be inferred from the Fe:FeO ratio. The authors seem to suggest the values are similar to our system. If there were water worlds in the white dwarf system, there would have been more H2O in the rock which in turn would have contributed to a much higher water content of the rocky worlds. Maybe.

        It does also make me wonder is the Fe was more hydrated and hence oxidized, whether this would mean that watery worlds would have depleted Fe cores and all that implies for planetary evolution.

  • Geoffrey Hillend October 22, 2019, 17:41

    A water world still has a rocky core and a star near a solar mass has to become a red giant before it becomes a white dwarf. As a result, the water gets boiled off and planets that get swallowed by the red giant might get broken apart. Water would still oxidize the sea floor rocks and remove some oxygen from the water. As a result, the above elements would not indicate there could not be any water worlds.

  • Robin Datta October 22, 2019, 22:57

    So even social studies subjects such as history, when pursued far enough in Deep Time (astrohistory?) demand STEM proficiency. Would stellar systems that were near such white dwarfs at their red giant stage bear any evidence of colontzation?

  • Andrew LePage October 23, 2019, 2:12

    This piece reminds me that the first evidence for extrasolar planets (at least in retrospect) was obtain in 1917 when Van Maanen’s Star was found to be the first white dwarf to have an enhancement of heavy elements in its atmosphere which is now taken as evidence that it is accreting debris from a planetary system it once had.

    https://www.drewexmachina.com/2014/10/21/the-first-observational-evidence-of-extrasolar-planets/

  • Geoffrey Hillend October 23, 2019, 14:41

    The outer planets would still be intact so one might find bases or stations on white dwarf systems.

  • Ivan Vuletich October 24, 2019, 19:06

    Looks like accretion onto white dwarfs might not be the only way to find out about extra solar geology.
    https://www.livescience.com/exoplanet-smash.html

    The future has been getting decidedly retro lately. First we have shiny stainless steel space ships. Now we have crashing planets….

  • Michael October 26, 2019, 8:25

    Should be quite nice place to live around when the universe goes into retirement.

  • wdk October 27, 2019, 11:08

    While this sounds like a pleasant surprise, I am going to drag my feet some. Maybe some of the issues I would like to raise were already resolved somewhere in the paper or someone has some background on them. And maybe some others might be curious as well.

    Let’s assume that these local White Dwarfs are about the same age as our sun. So, if they got where they are, they started as more massive bodies, perhaps up to 2 or 3 x solar mass, and in route to WDdom released a considerable amount of mass, especially during their transit off the main sequence to their current state (stellar winds planetary nebulae). And in the course of this evolution, beside hydrogen to helium burning, in the interior there were nuclear reactions further up the periodic table scale. Some of those involve carbon, nitrogen and other species with atomic weights in the teens, yet in astronomical terminology are referred to as “metals” when they are identified in stellar spectra or are estimated as abundances relative to those found in the sun. Iron (Fe) abundance would be difficult to adjust ( I would think ) in the low or middle main sequence in a surviving white dwarf. So maybe high abundance of that could be attributed to infall.

    We do have white dwarfs closer to the Sun, notably in the two Dog Stars, Procyon and Sirius, but their histories are complicated by being binaries. Whatever happened to their brighter, nearby partners could/would tend to spill over, just as the remains of planets; so having a white dwarf in isolation would probably be a better test for studying post mortem studies of planets.

    But what about the transition from Red Giant to White Dwarf? Some of the mass loss is due to stellar winds during the Red Giant phase, but also due to the blow off of what have been called planetary nebulae. I stress the name, because it is not related to planets in theory or observation, but to the a large scale blow off of outer layers revealing the white dwarf inner shell. Some of it might even fall back.

    Now if the surface layers blow off, you have to wonder what kind of “metal” abundances existed within them. That becomes a question of studying Red Giant convection and mixing processes – as well as that of the White Dwarf. And there are presumably concentric shells of predominant nuclear reactions, but the higher atomic number reactions are going on closer to the core. Abundances of various isotopes build up in catalytic reactions for the primary ones.

    So that is about as detailed as I can get without doing a counter study, which I not really capable of doing. But I have to wonder: with all the complexity of this environment and the presence of stellar generated abundances of the isotopes in question, does it not seem rather difficult to reverse engineer a planetary system? From my own uninformed perspective, regarding fugacity, it would seem that a key would be a coincidental ratio of iron and oxygen abundance in the white dwarf
    upper atmosphere, elicited from spectral analysis, which is no mean feat itself.

    It sounds to me like a planetary system finger print method has been invented, but I am far from clear how it is supposed to work considering the background noise. Were there White Dwarf cases where the existence of a planetary system were ruled out because…?

    • Alex Tolley October 27, 2019, 14:26

      IOW, you are suggesting that it would be good to have a control where the WD was known to have been a system without other planets in order to determine whether the elements seen in the atmosphere of the WD in this example are from planets alone.

      • wdk October 28, 2019, 0:55

        AT,
        Yes, something of that nature. Though I suspect the most likely outcome would be review of White Dwarf models assuming an absence of planets and what the upper layers would entail.

        It was pointed out that Van Maanen’s star, a White Dwarf, was one of the earliest spotted due to proximity. I suppose a black body temperature estimate would give a clue as to how old the star is.

        An old cold star one would expect a diminished abundance of astronomical metals. A new hot star vice- versa. In a star similar to the sun, you get a lot of iron lines, disproportionate to its actual abundance. But I suspect that atomic oxygen is greater in abunance. Adding rusty planets with considerable silicate, carbonate and dissociated water to a white dwarf’s atmosphere…
        The mass is about the same as the sun, but the surface area is a lot lower, what with terrestrial diameter or so. Is it suggested that the terrestrial planet contents are practically all that is seen in the upper layers?

  • Geoffrey Hillend October 28, 2019, 17:47

    @Alex Tolley. Stars like our Sun and with 2 to 3 solar masses don’t make iron. Only very large stars can make Iron at over three and one half billion degrees, the temperature where silicon burns. Fusion in stars end with silicon, so after silicon comes Iron which cannot be fused due to too much proton repulsion. The reason is the larger stars cores have a stronger gravity due to their larger masses, so their of cores burn hotter due to faster particle collisions within the core from the greater gravity, The hotter burning corresponds to a faster burning of fuel and a shorter life the larger the star.

    Elements heavier than iron must be formed in super nova explosions since Iron does not liberate energy, so it builds up in the core, and when the Chandrasekhar limit is passed, it collapses and rebounds blowing apart and away the outer layers of the star in a supernova explosion. The higher elements above Iron are formed there due to the excess energy, e.g., Iron captures neutrons which can get past the proton repulsion of electric forces between Iron and the protons of other atoms since neutrons have no charge or are neutral and can go through the electron repulsion into the nucleus. After capturing several neutrons, the nucleus of the atom becomes unstable and one of the extra captured neutrons decays into a proton changing iron into cobalt. Other heavier elements are formed that way. Also elements heavier than iron are formed Kilonova explosions, but these are rarer or there a less of them compared to supernova explosions.

    Consequently, not all the elements in rocky planets and table of elements were formed in stars like our Sun and up to 2 and 3 solar masses especially iron and silicon. Also planetary nebula were given that name because they were first thought to be planets, since they looked like planets, and that name stuck.

    One can infer the mass of the star from the mass of the white dwarf and also it’s chemical composition due to it’s mass.

    • Alex Tolley October 28, 2019, 21:12

      I understand the issue concerning where heavy elements are made. However, since both the star and the planets form from the same material, a rocky planet with heavy elements implies its star also has heavy elements. Is that incorrect?

      I can believe that these elements sink to the center of the star and remain there, ensuring the atmosphere of its WD is mostly light elements but is that the case, or is some mixing still possible to contaminate the atmosphere and hence fugacity estimation of the planets falling into the atmosphere?

      • wdk October 29, 2019, 14:15

        AT,
        Whether we are following the reasoning or rationale of the paper I am not sure, but their notion seems to invite some of this.
        Assuming terrestrial planets are singed by the Red Giant phase, then the remaining material might be cores akin to Mercury. The more volatile materials could have departed with the stellar winds. But then if cataclysmic events continue to originate at the star, then perhaps iron rich planetary cores could rain down on the outer layers of the resulting White Dwarf? Early WDs are very hot, tens of thousands of degrees K without this consideration. The grape shot raining down on the surface – well is it even grape shot? It starts out hot without explosions, experiences a stellar convulsion and then is headed to a very hot surface.

        I believe it was Chandrasekhar who had predicted the interior structure of WDs, describing equation of state degeneracy which made the atoms behave more like a conducting metal. That does not sound to me like a convective environment. But there are processes such as diffusion. So could an iron rich layer remain near the surface boundary of a White Dwarf, assuming it had experience
        a rain of iron and iron oxides coming down on it? Then would it be around long enough to be an observable phenomenon?

        But if we were to take the Earth’s mass vs. Solar mass, the ratio is about 1 to 330,000. Uniform distribution of its iron content in a White Dwarf does not sound to me like a substantial enrichment.
        There have to be limitations on the mixing for this to be much of an observable phenomenon.

  • Geoffrey Hillend October 29, 2019, 16:20

    Yes, the star also has the same heavy elements as the planets.

    Yes to your second question. I like the contamination idea of the planetary fragments accretion of the WD atmosphere. If you imagine how the WD is made and its cross section image, the idea makes sense. White dwarfs have a solid, crystalline core of carbon and oxygen, and an atmosphere of H, and He around it. If fragments of the inner planets or asteroids fall into it, then it would be contaminated with more heavy elements that it would have without the accretion. Some of these metals are originally from the contamination of the solar nebula. I would assume we could see from a spectrometer that there would be a higher percentage of them in the WD atmosphere since it is so small, the WD is only the size of our Earth so it would easy to raise the percentage of metals in it’s atmosphere through accretion. I also assume astrophysicists could estimate what is the original percentage from the solar nebula in the WD and what was gained from accretion.

    The elements do sink to the center of the star during fusion if that is what you mean. If not, then the accreted elements would not sink to the center or surface of the WD since it’s surface temperature is 8000 K. If so, the carbon and oxygen always must remain in the center though, so there is no way for these elements to escape the strong gravity of the core of the WD and contaminate the atmosphere, since the WD has the mass of our Sun, but is only the size of the Earth. The C and O is the ash left over from the fusion process, the triple alpha process. Three alpha particles, which are helium nucleuses must collide with each other to make Carbon which is why it is called the triple alpha process and if another one hits Carbon, it becomes Oxygen. This happens only in the core of the star where the temperatures are high enough so when H fused into He, then He fuses into C and O and that is why we have the C and O in the center core of the WD which is left over from the red giant phase and the temperatures are not high enough to go beyond C and O in the core of small stars near the mass of our Sun.

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