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Structure and Composition of a White Dwarf Planet

Given everything we’re learning about planets around other suns, it’s frustrating that we have so little information about the chemical composition of the rocky planets we’ve found thus far. Now we have a new study, announced at the San Diego meeting of the American Astronomical Society, that offers data on a ‘planet-like body’ whose surface layers are being consumed by the white dwarf SDSSJ1043+0855. Although it’s been known for some time that the star has been devouring rocky material orbiting around it, the new work offers a striking view of the accretion process and the composition of what was once a differentiated body.

At least, that’s the best interpretation of the data taken from the Keck Observatory’s HIRES spectrometer (installed on the 10-meter Keck I instrument) and the Hubble Space Telescope. White dwarf stars are the remains of stars like the Sun — this one was once a few times the Sun’s mass — that have gone through their red giant phase and expelled all their outer material. The ‘planet-like body’ the researchers refer to is likely the remnant of a surviving planet.

To study the chemical composition of such a world, Carl Melis (UC-San Diego) and Patrick Dufour (Université de Montreal) used the spectra of the rocky accretion material as filtered through the atmosphere of the star. The researchers believe that using these methods, they are able to analyze specific layers of the body undergoing accretion. We learn that the object shows large amounts of carbon, combined with smaller amounts of calcium and oxygen.

We may be looking, Melis and Dufour suggest, at calcium carbonate (CaCO3), a mineral widely found in shelled marine organisms here on Earth. As this news release from Keck Observatory points out, incorporating carbon in the surfaces of rocky objects is difficult, which is why the terrestrial planets in our own Solar System are sometimes described as being in a ‘carbon desert.’ The surface being accreted by SDSSJ1043+0855 shows perhaps as much as several hundred times the amount of carbon found on the surface of the Earth.

Is it possible that life played a role in this object’s history? Melis comments:

“…the presence of such high levels of carbon is unique and really needs to be explained. Our choice of calcium-carbonate as a potential carrier of the carbon provides a natural way for it to be locked up in the planet and eventually delivered to the white dwarf star, is entirely consistent with the observations in hand, and of course is suggestive. That’s really the hidden subtext. When people think about finding extra-terrestrial life, they think about Hollywood dramatizations. But the first evidence of life outside of our Solar system will probably come in a much subtler form. More likely than not, it’s going to come as a nuanced signature that may not be immediately recognizable.”


Image (click to enlarge): Artist’s impression of the surface of the massive, planet-like body being devoured by the white dwarf SDSSJ1043+0855. The Keck Observatory and Hubble Space Telescope data (shown in inset) show calcium and carbon, the presence of which can be explained with a model suggesting the surface of the planet may have been encrusted in limestone (calcium carbonate). This material was removed from the surface of the massive rocky body, probably through large-scale collisions, subsequently shredded into a disk of material, and accreted by the white dwarf star (ringed object seen in the planet’s sky). Credit: A. Hara/C. Melis/W. M. Keck Observatory.

But calcium carbonate isn’t always the result of biology, and the current work examines only the accretion materials that have been absorbed by the parent white dwarf. Melis and Dufour would like to look next at surrounding dust before it falls into the star, using the James Webb Space Telescope if possible to confirm whether calcium carbonate is present. This would allow a better estimate of whether the amount of calcium carbonate is consistent with natural processes.

Centauri Dreams’ take: Calcium carbonate or not, it’s striking that using accreted material in the region of a white dwarf and in its atmosphere can help us understand the structure of an exoplanet. We move beyond bulk chemical composition to differentiate between the layers of the body being accreted. That’s a highly useful tool for studying planetary structure.

The presentation is Melis and Dufour, “The Surface of a Limestone-Rich World?” American Astronomical Society 20 June 2016, AAS Meeting #228, id.#201.03 (abstract).


Comments on this entry are closed.

  • Marshall Eubanks June 16, 2016, 11:36

    If there was life on this world, it is gone now, killed by the evolution of its host star.

  • Torbjörn Larsson June 16, 2016, 12:17

    The very first putative biosignature (apart from a martian paper)! One for the history books, even if it likely is a dud. (The amount of carbon is suspicious.)

    An earlier paper on the star and the debris disk here: http://www.astro.keele.ac.uk/jkt/pubs/2007MNRAS.380L..35G.pdf

    I can’t find an estimate of the age of the star in the main sequence, which would be interesting to compare to Earth. I have log g and Teff [ibid], but it would give me the age for a main sequence star. If its location gives more info (say, being part of a cluster), I dunno.

    • Harry R Ray June 17, 2016, 9:35

      Which “martian paper” are you talking about? Three papere immediately come to mind; the “martian meteorite paper, Nora Noftke’s paper on microbial mats at Gillespie Lake Member is another, and the recent one on “cauliflower formations” is the LATEST one. ALSO: The “excess carbon” issue goes away if what we are seeing here are the COMPLETELY FRAGMENTED REMAINS of a Carbon based EXOMOON that formed via an analog of the Earth-Theia collision. The ‘excess carbon” would be from the mantle AND the core of the now completely destroyed exomoon and its ORIGINAL COMPOSITION would have consisted MAINLY of Silicon Carbide.

    • Harry R Ray June 17, 2016, 12:38

      MY BAD! It was SODIUM Bicarbonate that was discovered in Enceladus’ plums, NOT Calcium Carbonate or Calcium Dicarbonate. Sorry about that.

  • Torbjörn Larsson June 16, 2016, 12:32

    Ah, I get it now. The dwarf mass is too small to indicate main sequence star lifetime, even if I know that the dwarf itself has only cooled for 0.1 billion years. A 0.67 solar mass star would have a lifetime of 50 billion years, older than the universe. [ https://books.google.se/books?id=jmriAwAAQBAJ&pg=PA142&lpg=PA142&dq=age+of+star+with+0.67+solar+mass&source=bl&ots=KnLkmMaFuA&sig=glTGdnnATgjdXNkFRTMYorKSi1I&hl=sv&sa=X&ved=0ahUKEwiK3tGf-6zNAhWEF5oKHdl6AcsQ6AEIHjAA#v=onepage&q=age%20of%20star%20with%200.67%20solar%20mass&f=false ]

    So it throw out a lot of mass before it went WD. And so we likely can’t know its MS age.

    Oh, well.

  • Harry R Ray June 16, 2016, 12:42

    A question for Melis and Dufour? Is there any way that you can tell from the existing data, the ratio of the carbon isotopes C12 and C13? A biogenic process would almost certainly produce a different from a non-biogenic process. Also. has this white dwarf been observed for transits? The reason I ask this is that if the progenator is an exoplanet. we may be seing something akin to KIC12557548, whereas if the progenator was an exomoon, we may be seeing something akin to WD1145+017. In the latter case, we may be seeing not just the outer layers polluting the white dwarf, but a mantle and a core of Silicon Carbide, emblimatic of a Carbon-based exomoon.

    • Alex Tolley June 16, 2016, 17:07

      How would they get the baseline ratio to compare the carbon in the carbonate to?

  • Harry R Ray June 16, 2016, 12:44

    Oops, I meant to say “…produce a different ratio…”.

  • Michael June 16, 2016, 18:17

    Something is weird here, how can it be calcium carbonate? If it was that close to the WD it would have been turned to CO2 during the RG phase. Did it wonder in from the far reaches of the system or did it turn into CC after the RG stage when it cooled down.

  • Eniac June 16, 2016, 22:11

    I agree with Michael, here. My next question is: Where is the silicon? Is it absent or did they not measure it? If the latter, most likely we are seeing CaO, SiC, and SiO2, maybe elemental carbon, also. Unlike CaCO3, those are all stable solids under high temperature. If the former, maybe it’s just CaO and elemental carbon. The phrasing “large amounts of carbon, combined with smaller amounts of calcium and oxygen” certainly is explained much better by either of those than by CaCO3. CaCO3 would result in “large amounts of carbon, Ca, and oxygen”, instead, or not?

    • Harry R Ray June 17, 2016, 9:40

      Smaller(than the Carbon)amounts of BOTH Silicon AND Oxygen were detected. MY OPINION IS: What we are LIKELY seeing are the REMAINS of Calcium Carbonate AND Silicon Carbide.

      • Michael June 17, 2016, 21:32

        There is also the possibility of calcium carbide which is quite stable at high temperatures near the WD, it reacts dangerously with water though.

        CaO + 3 C → CaC2 + CO

  • Christopher Phoenix June 17, 2016, 2:07

    The technique used for these observations alone is fascinating enough, but the possibility that we are observing calcium carbonate delightfully suggestive. Might we have just observed the remnants of ancient alien sea life falling into a dead sun?

    When people think about finding extra-terrestrial life, they think about Hollywood dramatizations. But the first evidence of life outside of our Solar system will probably come in a much subtler form. More likely than not, it’s going to come as a nuanced signature that may not be immediately recognizable.

    This is very true! When we envision finding alien life, we tend to an alien starship entering a parking orbit around Earth, or humans going visiting and encountering alien animals on some habitable exoplanet. Or, maybe, we envision intercepting a message from an advanced culture in one of our radio telescopes.

    But quite possible that the first sign of alien life (or even civilizations) will come in some subtle enigmatic signature that initially interests only the researchers. Perhaps the first indications of alien life have already been found. And even if we have found it, we may not be able to determine what that signature really means for a while. Tabby’s Star comes to mind!

    • Harry R Ray June 17, 2016, 9:47

      KEEP IN MIND: Calcium Carbonate(AND Calcium Dicarbonate) were ALSO detected in the plumes of Enceladus, but no far-out fantastical claims of multi-cellular sea-life came out of THAT detection. A “Stardust”-like sample and return mission to Enceladus has been proposed, and if we can retrieve A DECENT AMOUNT OF CARBON, we should be able to determine the c12/c13 ratio. EXCITING TIMES!

      • Eniac June 21, 2016, 0:05

        It is quite expected and not a biosignature to find CaCO3 in water.

  • Phil June 17, 2016, 4:36

    If the progenitor star really was ‘a few times the sun’s mass’, then its main sequence lifetime would have been fairly short – surely too short for any life to develop?


    • Harry R Ray June 17, 2016, 9:51

      Correct for LP475-242(the OTHER White Dwarf upon which Calcium was detected), but unsure for SDSSJ104143,53+085558.2.

  • Eniac June 21, 2016, 0:13

    Let me point out again that no CaCO3 was detected, here. What was detected were the elements Ca, C, and O, plus many others. The ratio in which they were detected is nowhere near anything that would suggest CaCO3. This planet is close to its star, so it is hot and CaCO3 would have long since baked into CaO and CO2. Methinks their comment suggesting CaCO3 is criminally unfounded speculation.

    Move on, folks, no biosignatures here.

  • Harry R Ray June 21, 2016, 9:47

    There appears to be a very interesting way to TEST whether the progenitor is STILL a monolithic(albeit somewhat REDUCED)”exoplanet”, or; simply the REMAINS of an exomoon. “On the Detection of Non-Transiting Exoplanets with Dusty Tails.” by J. Devore, S. Rappaport, R. Sanchis-Ojeda, K. Hoffman, and J. Rowe is currently up on exoplanet.eu. ALSO: If a dusty tail IS detected, telescopes on the ground may be able to get a leg up on JWST!