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Planetary Annihilation around White Dwarfs

Can we tell something about the planets around another star by examining that star’s atmosphere? A new study out of the University of Warwick makes a strong case for the method in the study of white dwarfs, following up on a landmark 2007 paper by Benjamin Zuckerman (UCLA) that looked at pollution in white dwarf photospheres. ‘Pollution’ as in metals that shouldn’t be there, which suggests an accretion disk of material feeding the star, which itself would have collapsed from a red giant stage and is perhaps now absorbing planetary material around it.

What we would expect to find in the atmosphere of a white dwarf is little more than hydrogen and helium — heavy elements should quickly sink to the core and not be observable. But white dwarfs with metal-contamination in their atmospheres have been observed for almost a century now. Let me Boris Gänsicke and colleagues on this, from the paper on the University of Warwick work (internal references deleted for brevity):

…the rapidly growing number of white dwarfs that are accreting from circumstellar discs… unambiguously demonstrates that debris from the tidal disruption of main-belt analogue asteroids or minor planets… or Kuiper-belt like objects…, likely perturbed by unseen planets…, is the most likely origin of photospheric metals in many, if not most polluted white dwarfs.

In a study of more than 80 white dwarfs using the Cosmic Origin Spectrograph on the Hubble Space Telescope, the researchers found four that showed not only oxygen, magnesium, iron and silicon, but a small amount of carbon in their photospheres, closely matching the composition of rocky planets, including the Earth, that orbit close to our Sun. The evidence is that all four stars once had at least one rocky planet orbiting them which has now been destroyed. And because heavy elements like these would be pulled into the core in short order, the researchers believe they are observing the final phase of the destruction of these worlds, an inflow of material falling into the stars at a rate of up to 1 million kilograms every second.

Image: A white dwarf sits in the centre of the remnant of a planetary system. Asteroid sized debris is scattered inwards by interaction with the remaining planets and is tidally disrupted as it approaches the white dwarf forming a disc of dust some of which is raining down onto the star. The researchers have found that the composition of the debris that has just fallen onto the four white dwarfs matches the composition of Earth-like rocky worlds. Credit: Mark A. Garlick.

The white dwarf PG0843+516 turns out to be particularly interesting because of the amount of iron, nickel and sulphur in its atmosphere — the study refers to it as ‘extremely polluted’ — strongly suggesting the star is swallowing the core of a rocky planet that had undergone the same kind of differentiation that occurred in the Earth. Gänsicke sees this as a glimpse of the processes that will one day play out long after our Sun has left its red giant phase:

“What we are seeing today in these white dwarfs several hundred light years away could well be a snapshot of the very distant future of the Earth. As stars like our Sun reach the end of their life, they expand to become red giants when the nuclear fuel in their cores is depleted. When this happens in our own solar system, billions of years from now, the Sun will engulf the inner planets Mercury and Venus. It’s unclear whether the Earth will also be swallowed up by the Sun in its red giant phase – but even if it survives, its surface will be roasted.”

Not a pretty picture, but the rest of the Solar System will be likewise disrupted:

“During the transformation of the Sun into a white dwarf, it will lose a large amount of mass, and all the planets will move further out. This may destabilise the orbits and lead to collisions between planetary bodies as happened in the unstable early days of our solar system. This may even shatter entire terrestrial planets, forming large amounts of asteroids, some of which will have chemical compositions similar to those of the planetary core. In our solar system, Jupiter will survive the late evolution of the Sun unscathed, and scatter asteroids, new or old, towards the white dwarf. It is entirely feasible that in PG0843+516 we see the accretion of such fragments made from the core material of what was once a terrestrial exoplanet.”

All of the more than 80 white dwarfs in the study are within several hundred light years of Earth, offering us a glimpse into deep time, a reminder that our own system formed long after many nearby stars were fully mature and doubtless orbited by planets of their own. The paper is Gänsicke et al., “The chemical diversity of exo-terrestrial planetary debris around white dwarfs,” accepted for publication in the Monthly Notices of the Royal Astronomical Society (preprint). The Zuckerman paper cited above is “Externally Polluted White Dwarfs with Dust Disks,” Astrophysical Journal 663 (2007), p. 1285 (preprint). A University of Warwick news release is also available.

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Comments on this entry are closed.

  • ljk May 7, 2012, 9:58

    We should also be checking stellar spectrums for artificial lines due to an advanced ETI dumping their technological waste into their sun. Or they could be deliberately trying to get the attention of other suitable intelligences by dumping certain elements into their star that would be recognized as not natural for that stellar class and thus get the attention of observers.

    To quote from the following article by Guillermo A. Lemarchand from here:

    http://www.coseti.org/lemarch1.htm

    There have also been independent suggestions by Drake and Shklovskii (Sagan and Shklovskii, 1966) that the presence of a technical civilization could be announced by the dumping of a short-lived isotope, one which would not ordinarily be expected in the local stellar spectrum, into the atmosphere of a star.

    Drake suggested an atom with a strong, resonant absorption line, which may scatter about 10 to the 8 power photons sec to the -1 power in the stellar radiation field. A photon at optical frequencies has an energy of about 10 to the -12 power erg or 0.6 eV, so each atom will scatter about 10 to the -4 power erg sec to the -1 power in the resonance line.

    If we consider that the typical spectral line width might be about 1 Angstrom and if we assume that a ten percent absorption will be detectable, then this “artificial smog” will scatter about (1 Angstrom/5000 Angstrom)x10 to the -1 power = 2×10 to the -5 power of the total stellar flux.

    Sagan and Shklovskii (1966) considered that if the central star has a typical solar flux of 4×10 to the 33 power erg sec to the -1 power, it must scatter about 8×10 to the 28 power erg sec to the -1 power for the line to be detected. Thus, the ETC would need (8×10 to the 28 power)/10 to the -4 power = 8×10 to the 32 power atoms. The mass of the hydrogen atom (mH) is 1.66×10 to the -24 power g, so the mass of an atom of atomic weight (mu) is approximately mu.mH grams.

    Drake proposed the used of Technetium (Tc) for this purpose. This element is not found on Earth and its presence is observed very weakly in the Sun, in part because it is short-lived. Tc’s most stable form decays radioactively within an average of twenty thousand years. Thus, for the case of Tc, we need to distribute some 1.3×10 to the 11 power grams, or 1.3×10 to the 5 power tons, of this element into the stellar photosphere. However, technetium lines have not been found in stars of solar spectral type, but rather only in peculiar ones known as S stars.

    We must know more than we do about both normal and peculiar stellar spectra before we can reasonably conclude that the presence of an unusual atom in an stellar spectrum is a sign of extraterrestrial intelligence.

    Whitmire and Wright (1980) considered the possible observational consequences of galactic civilizations which utilize their local star as a repository for radioactive fissile waste material. If a relatively small fraction of the nuclear sources present in the crust of a terrestrial-type planet were processed via breeder reactors, the resulting stellar spectrum would be selectively modified over geological time periods, provided that the star has a sufficiently shallow outer convective zone.

    They have estimated that the abundance anomalies resulting from the slow neutron fission of plutonium-239 and uranium-233 could be duplicated (compared with the natural nucleosynthesis processes), if this process takes place.

    Since there are no known natural nucleosynthesis mechanisms that can qualitatively duplicate the asymtotic fission abundances, the predicted observational characteristics (if observed) could not easily be interpreted as a natural phenomenon.

    They have suggested making a survey of A5-F2 stars for (1) an anomalous overabundance of the elements of praseodymium and neodymium, (2) the presence, at any level, of technetium or plutonium, and (3) an anomalously high ratio of barium to zirconium. Of course, if a candidate star is identified, a more detailed spectral analysis could be performed and compared with the predicted ratios.

    Following the same kind of ideas, Philip Morrison discussed (Sullivan, 1964) converting one’s sun into a signaling light by placing a cloud of particles in orbit around it. The cloud would cut enough light to make the sun appear to be flashing when seen from a distance, so long as the viewer was close to the plane of the cloud orbit. Particles about one micron in size, he thought, would be comparatively resistant to disruption. The mass of the cloud would be comparable to that of a comet covering an area of the sky five degrees wide, as seen from the sun.

    Every few months, the cloud would be shifted to constitute a slow form of signaling, the changes perhaps designed to represent algebraic equations. Reeves (1985) speculated on the origin of mysterious stars called blue stragglers. This class of star was first identified by Sandage (1952). Since that time, no clear consensus upon their origins has emerged. This is not, however, due to a paucity of theoretical models being devised. Indeed, a wealth of explanations have been presented to explain the origins of this star class.

    The essential characteristic of the blue stragglers is that they lie on, or near, the Main Sequence, but at surface temperatures and luminosities higher than those stars which define the cluster turnoff. A review of current thinking about these stars in the light of recent visible and ultraviolet Hubble Space Telescope observations assigns an explanation to stellar mergers occurring in the dense stellar environment of globular clusters (Bailyn, 1994).

    Reeves (1985) suggested the intervention of the inhabitants that depend on these stars for light and heat. According to Reeves, these inhabitants could have found a way of keeping the stellar cores well-mixed with hydrogen, thus delaying the Main Sequence turn-off and the ultimately destructive, red giant phase.

    Beech (1990) made a more detailed analysis of Reeves’ hypothesis and suggested an interesting list of mechanisms for mixing envelope material into the core of the star. Some of them are as follows:

    * Creating a “hot spot” between the stellar core and surface through the detonation of a series of hydrogen bombs. This process may alternately be achieved by aiming “a powerful, extremely concentrated laser beam” at the stellar surface.

    * Enhanced stellar rotation and/or enhanced magnetic fields. Abt (1985) suggested from his studies of blue stragglers that meridional mixing in rapidly rotating stars may enhance their Main Sequence lifetime.

    If some of these processes can be achieved, the Main Sequence lifetime may be greatly extended by factors of ten or more. It is far too early to establish, however, whether all the blue stragglers are the result of astroengineering activities.

  • Brett Bellmore May 8, 2012, 7:37

    “Whitmire and Wright (1980) considered the possible observational consequences of galactic civilizations which utilize their local star as a repository for radioactive fissile waste material. ”

    Strikes me as remarkably unlikely. “Radioactive fissile waste” is just fuel it’s politically incorrect to utilize. By the time we’re capable of processing amounts of fissile fuel great enough that the ‘waste’ could be detected in a stellar atmosphere, it’s almost certain we’ll have gotten past this irrationality about not using 95% of the fuel. We’d have to have gotten past the irrationality in order to have developed the capacity!

  • spaceman May 8, 2012, 8:13

    Interesting work. I have a couple of questions: Why so few (4/80) stars with these chemical signatures? Is this a detection limit issue or is it evidence that a smaller fraction of this earlier generation of stars had terrestrial planets?

  • ljk May 8, 2012, 11:40

    Brett Bellmore said on May 8, 2012 at 7:37:

    “Whitmire and Wright (1980) considered the possible observational consequences of galactic civilizations which utilize their local star as a repository for radioactive fissile waste material. ”

    Strikes me as remarkably unlikely. “Radioactive fissile waste” is just fuel it’s politically incorrect to utilize. By the time we’re capable of processing amounts of fissile fuel great enough that the ‘waste’ could be detected in a stellar atmosphere, it’s almost certain we’ll have gotten past this irrationality about not using 95% of the fuel. We’d have to have gotten past the irrationality in order to have developed the capacity!

    LJK replies:

    I think both you and Whitmire and White (and me and most other people who talk on the subject) are guilty of assuming an intelligent alien species will progress with their technology just as we may, or that they would dismiss nuclear energy as soon as possible because there are factions of humanity that think it is bad.

    Until about the early 1960s, nuclear power was seen by most people as the savior of civilization and progress, ironically at a time when the US and USSR had thousands of nuclear weapons pointed at each other – a number that would not peak until 1990 with an estimated 55,000 such devices worldwide.

    As just one example from the late 1950s, Ford Motor Company seriously considered making a car called the Ford Nucleon that would run on nuclear fuel cells that would not need to be changed out for five thousand miles.

    Ford assumed that gasoline stations would be replaced by nuclear fuel depositories where a Ford Nucleon owner would just drive up with his car and the man at the station would switch out the old fuel rod for a new one. And presumably wash the windshields, too.

    http://www.damninteresting.com/the-atomic-automobile/

    Imagine an alien species where nuclear energy is not only plentiful and relatively cheap but through evolution they are much more resistant to radiation than we are, to say nothing of less skittish. Why should such beings want to switch from fission power? Since we are speculating here on alien motives and needs.

    And while I do not see sustained nuclear fusion as impossible, it has been promised for decades now with little more than the hydrogen bomb and some really brief experimental results to show for it. As for cold fusion, I take that about as seriously as the Dean Drive.

    In any event, how much would it hurt to have some folks look through (and make their own) stellar spectra for anything unusual? It would not surprise me in the least if there are ETI activities going on right in front of our collective faces out there and we are just unable (or perhaps unwilling) to recognize their true natures.

  • A. A. Jackson May 8, 2012, 19:54

    “Following the same kind of ideas, Philip Morrison discussed (Sullivan, 1964) converting one’s sun into a signaling light by placing a cloud of particles in orbit around it. The cloud would cut enough light to make the sun appear to be flashing when seen from a distance, so long as the viewer was close to the plane of the cloud orbit. Particles about one micron in size, he thought, would be comparatively resistant to disruption. The mass of the cloud would be comparable to that of a comet covering an area of the sky five degrees wide, as seen from the sun.”

    I have never heard this one. I wonder what these particles would be composed of? Because , it takes a while, but Poynting–Robertson drag would eventually disrupt such a cloud unless replenished. Even then distinguishing it against the natural interplanetary dust medium would be hard. It would be brightest in the infrared , also masked by the solar system dust distribution.

  • Brett Bellmore May 8, 2012, 21:56

    Ljk, I’m not assuming an intelligent race would abandon nuclear power, I’m merely assuming that an intelligent race with the capacity to process the megatons of fissile material necessary to seed a stellar atmosphere to detectable levels would not throw away 95% of the energy that fissile material would provide. That that level of stupidity would preclude the capability.

    Aliens might have different motivations, but they’re still working with the same laws of physics and chemistry.

  • ljk May 9, 2012, 10:18

    A. A. Jackson said on May 8, 2012 at 19:54:

    [In response to Phillip Morrison’s idea of using a giant cloud of particles to turn Sol into an interstellar signalling lamp.]

    “I have never heard this one. I wonder what these particles would be composed of? Because , it takes a while, but Poynting–Robertson drag would eventually disrupt such a cloud unless replenished. Even then distinguishing it against the natural interplanetary dust medium would be hard. It would be brightest in the infrared , also masked by the solar system dust distribution.”

    How about 480 quadrillion little copper needles ala Project West Ford:

    http://www.damninteresting.com/earths-artificial-ring-project-west-ford/

    Expanding on this idea, it has been mentioned elsewhere here that dropping lots of little sensors on other worlds could be the way to go for future exploration, rather than rely on one big probe that can only scout a few areas at best and end the mission entirely if it fails.

    https://www.centauri-dreams.org/?p=18968

  • ljk May 9, 2012, 11:38

    Brett Bellmore said on May 8, 2012 at 21:56:

    “Ljk, I’m not assuming an intelligent race would abandon nuclear power, I’m merely assuming that an intelligent race with the capacity to process the megatons of fissile material necessary to seed a stellar atmosphere to detectable levels would not throw away 95% of the energy that fissile material would provide. That that level of stupidity would preclude the capability.

    “Aliens might have different motivations, but they’re still working with the same laws of physics and chemistry.”

    LJK replies:

    Maybe when technological ETI find better and safer sources of power for their civilizations and spaceships, they take all their fissionable material and dump it into their suns as a way to get rid of it permanently. Again, we should be on the lookout for such activities since I consider them more likely than deliberate signalling.

  • A. A. Jackson May 10, 2012, 9:17

    On the Westford needles. 500 million were dispersed at 3650 km in 1963. However most did not deploy correctly and formed clumps. 98 of the 147 Westford needle clumps have reentered. At their initial altitude radiation pressure is the most important perturbing force, until they encounter the atmosphere. Poynting Robertson drag has little effect even on these 2 cm dipoles. (By the by cold welding is what caused the clumping.) The remaining clumps remain at very high perigee altitudes , but may some day become orbit debris hazards.
    However there is so damn much LEO orbit debris right now it is becoming a bigger problem all the time. (The Chinese did not help the situation with an anti satellite test.)
    There is an international treaty now to mitigate orbit debris, only a rogue nation could launch a Westford like thing, hope against that.
    The natural OD environment at LEO could get worse, remember the Iridium -Cosmos 2251 collision. More of that and the ISS will have to be abandoned.

  • ljk May 10, 2012, 9:57

    It has long been said that if a nation wanted to take out the satellites in LEO, just launch a bucket of pebbles into orbit. Their 18,000 MPH velocity will do the rest. No fancy bombs or laser cannon platforms needed. How many billions did we spend on Star Wars, aka SDI?

  • Eniac May 11, 2012, 21:45

    It has long been said that if a nation wanted to take out the satellites in LEO, just launch a bucket of pebbles into orbit. Their 18,000 MPH velocity will do the rest. No fancy bombs or laser cannon platforms needed. How many billions did we spend on Star Wars, aka SDI?

    I fail to understand the relevance of the last question. Last I heard SDI was meant to destroy intercontinental missiles in flight. Not even supertanker-sized boatloads of pebbles would accomplish this, much less a bucket. Space is too large, and missiles fly too briefly, and too low.

  • ljk May 16, 2012, 16:37

    The sand in orbit is for taking out military satellites involved in helping with communications. ground monitoring, etc., during a war. It would effectively leave an enemy who relies on satellites for such things deaf and blind – and for the fraction of the cost of a laser weapon or even space-based missiles.

    No realistic defense system could stop all missiles from getting through. SDI also did not protect against suitcase nukes and the like.

  • Eniac May 20, 2012, 10:53

    No nation would be so foolish as to rely only on satellites. Note the incredible advances in spyplane and drone technology, which rival and probably exceed those made in space technology. If, despite of that, you were foolish enough to launch that bucket of sand, you would have each and every one of the other space nations on your case, all at once. No more playing with rockets for you.