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White Dwarf Debris Suggests a Common Destiny

An iron and nickel-rich planetesimal is apparently all that survives of a planet following the death of its star, SDSS J122859.93+104032.9. We are talking about an object in an orbit around a white dwarf so tight that it completes a revolution every two hours. Significantly, spectroscopic methods were used to make the identification, the first time a solid body has been found around a white dwarf with spectroscopy. Variations in emitted light were used to identify the gases generated by the planetesimal, with data from the Gran Telescopio Canarias in La Palma.

Lead author Christopher Manser (University of Warwick) notes the advantages of the method the team developed to study a white dwarf 400 light years away:

“Our discovery is only the second solid planetesimal found in a tight orbit around a white dwarf, with the previous one found because debris passing in front of the star blocked some of its light — that is the “transit method’’ widely used to discover exoplanets around Sun-like stars. To find such transits, the geometry under which we view them has to be very finely tuned, which means that each system observed for several hours mostly leads to nothing. The spectroscopic method we developed in this research can detect close-in planetesimals without the need for a specific alignment.”

Image: A planetary fragment orbits the star SDSS J122859.93+104032.9, leaving a tail of gas in its wake. Credit & copyright: University of Warwick/Mark Garlick.

This is an extreme environment, the white dwarf in question being surrounded by a debris disk through which the object passes in its orbit. The star itself is about 70 percent of the mass of the Sun and, like all white dwarfs, this one — roughly the size of Earth — is quite dense, a survivor of the star’s red giant phase. An object moving this close to the white dwarf will be under extreme gravitational stress; the gravity of SDSS J122859.93+104032.9 is fully 100,000 times that of the Earth. The fact that the team could identify a planetesimal deep within the gravitational well indicates it must be an object of great density, probably made up of iron and nickel.

On where the object came from, the paper offers this intriguing possibility:

This object may be the differentiated iron core of a larger body that has been stripped of its crust and mantle by the tidal forces of the white dwarf. The outer layers of such a body would be less dense and would disrupt at greater semimajor axes and longer periods than those required for core disruption. This disrupted material would then form a disc of dusty debris around SDSS J1228+1040, leaving a stripped corelike planetesimal orbiting within it.

Manser’s colleague and co-author Boris Gaensicke adds if the assumption that we are dealing with a planetary core is correct, then the original body would have been at least hundreds of kilometers in diameter, because it is only at this size that planets begin to differentiate, with heavier elements sinking to form a metal core. It could, of course, have been much larger.

Thus the survival of a planetesimal here, actually orbiting within the original radius of its star, suggests a large object ultimately shredded by gravitational forces. We are glimpsing what our own Solar System may resemble in 5 to 6 billion years, when it will be a white dwarf orbited by the outer planets along with asteroids and comets. Our star’s expansion into a red giant will savage the inner system, perhaps leaving debris like what we see around SDSS J122859.93+104032.9. Bear in mind, too, that the vast majority of the stars known to host planets will end their lives as white dwarfs, so we are looking at a common destiny.

The debris disk of the white dwarf is rich in magnesium, iron, silicon and oxygen, and it is within that disk that the scientists found gas streaming from the evidently solid body. The object appears to be about a kilometer in size but could be as large as a few hundred kilometers in diameter. Whether it is the source of the gas or simply the cause of the gaseous ‘tail’ as it collides with debris in the disk is not yet known. Learning more will involve studying other debris disks similar to SDSS J122859.93+104032.9 (eight gaseous white dwarf debris discs are currently known), where the spectroscopic method will perhaps find other instances of planetesimals orbiting near or within the parent star’s debris disk.

The paper is Manser et al., “A Planetesimal Orbiting Within the Debris Disc Around a White Dwarf Star,” Science April 4 2019 (abstract).


{ 16 comments… add one }
  • Ron S. April 5, 2019, 12:45

    Although I haven’t read the paper a few things strike me about this system. First, this body could have been very large and much further out when the star either went nova or became a red giant. Even if it were Jupiter class it’s likely that all its volatiles would have been lost. Small inner planets would have been vaporized entirely, with only the heavier rock, but fractured, remaining.

    Second, the larger body would have spiraled in to its present tight orbit due to debris friction or expelled gas from the red giant phase. Only once it reached its present orbit and the star settled down to become a white dwarf would there be tidal disruption. That is, if it was still large enough to be disrupted.

    Third, the drawing is misleading. All material at the same position would have similar velocities, varying only due to a small range of orbital parameters. The large body would be equally likely to run into small debris as the debris would be to hit it from behind. There should be wakes both fore and aft.

  • John walker April 5, 2019, 14:51

    Apropos destruction, while not purely in keeping with today’s blog, I’d like to recommend an entertaining video on the topic of cosmic destruction. A work of beauty, the film offers its viewers a vision the future of the cosmos from the present through to the unimaginably distant end of the black hole era. The film is 30 minutes with a clock rate doubling 350 times to make it possible to envisage the 1e100 years to maximum entropy. https://m.youtube.com/watch?v=uD4izuDMUQA

  • wdk April 5, 2019, 16:05

    This metal rich object suggests a history of spiraling into the parent star that must have had several phases:
    1. the gas giant phase where the outer envelope of the star stretched out to planetary distances ( at least by our solar system’s measure) maybe an AU or two.
    2. The explosive phase when the outer envelope was blasted away.
    3. The white dwarf phase in which the core remains of the planet are being torn apart.

    Just enumerating these phases to suggest that a planet such as Earth hurdling around our sun at 30 km/sec, could possibly have been braked
    in a decelerating spiral to a high speed but low radius orbit through successive non vacuum media. In the case of the first extra solar planets discovered, a pulsar was observed to have sizeable planets with periods of many days. These objects evidently survived a nova or super nova event.

    Decades ago when I first heard discussions of these stellar “evolutionary” events, planets such as Earth were assumed simply to have reached the end of their histories. Usually the discussion got any further than our particular example. But judging from this and other
    evidence, there is still a more to tell about. An instantaneous vaporization sounds like an inaccurate representation. But given that, there is not any less reason to want to pack your gear at first warning.
    Centauri Dreams is not about to lose some of its rationale.

  • David April 6, 2019, 11:16

    Science Times did an article about future solar system engineering that a super technology could use to save Earth. It mentioned Edward Teller said it coulld be done but he said we had a billion years we should worry about more immediate threats like asteroids.
    and the recent palentological study sure showed that. ,,,,We can prevent that with current technology. A billion years who knows.

  • Bruce D. Mayfield April 6, 2019, 12:24

    I agree that what has happened to this exoplanet orbiting a white dwarf would be a common destiny of many planets, but there are good reasons to expect that at least our planet could be spared this fate. As David wrote above, technology could be used to save the Earth.

    If a planet possesses intelligent life that reaches the point we have the civ will realize that life on the planet will end at some point as the homeworld’s star swells. If they appreciate how precious life is they should be highly motivated to take steps to preserve it.

    We already know how to do solar system engineering on paper; place a third body into a solar orbit that periodically robs momentum from Jupiter and then exchanges it with Earth. Such methods chould be used to keep life bearing planets in the sweet spot of their sun’s HZs for several extra billions of years. If we or they know how and could do so, why wouldn’t we or they do it?

  • Gary Wilson April 6, 2019, 14:49

    I think we had better worry about much more current existential threats David. Here I go again. Climate change is the key threat to our entire civilization. Given our current CO2 levels of over 400 ppm we are only a couple of decades away from a runaway greenhouse effect of 5 C or more of increased average temperatures. That would mean extreme weather events of unprecedented scale all over the globe, reduced arable land and huge migrations of people. Think about it the next time you do something that emits carbon. Let’s work together before it’s too late for our children and grandchildren. Pass the word. Make an effort for the future.

    • Bruce D. Mayfield April 6, 2019, 18:13

      Very valid comment Gary, (and here I go again too). Sadly our “civilization” is too fractured, too uncivil to make the needed corrections to divert disaster. Your call to action is all well and good, but realisticly today’s worldwide soical (dis)order is too selfish for enough individuals to make a large enough difference to be able to ward off climate shift and/or other end of the world catastrophies. We have no benevolent global dictator who can make everyone do the right things.

      But post-apocalyptically, I’m an optimist.

    • Ron S. April 7, 2019, 12:23

      There are over 7 billion of us. We can think about and work on more than one thing at a time.

    • Patient Observer April 7, 2019, 21:26

      Not just climate warming – the potential for a nuclear war seems to be increasing. But, an interest in astronomy can help to maintain a mental/emotional balance. How? The grandeur of the cosmos places our dramas in the proper perspective. With a good balance, things like global warming and the threat of nuclear war can be tamed. In other words, politicians and the wealthy should develop an interest in astronomy!

      • Gary Wilson April 8, 2019, 13:03

        Thank you for all the great comments and words of encouragement. I hope we can all make a difference. I do come on here to think about all the fantastic things we’ve learned about our place in the universe. It’s truly a great blog with great people on it. Keep up the tremendous work Paul.

  • David April 7, 2019, 13:52

    We have the technology to save ourselves now or soon will. There is clean meat which was first developed for NASA which has great enthusiasm in Asia though reaction to the Impossible Whopper which is plant makes me optimistic. And there is 4th generation nuclear which is in todays Times and we can use for Starshot and Asteroids. So we can get to billions of years if we have the will we gave the way. Then there are Dyson and Tipler who looked at Universe engineering. That seems to depend on what the dark energy is …..but a long way off. …..In the meantime evidence of solar system engineering is what we look for in strange transits.

  • Michael April 7, 2019, 17:13

    All those metals and materials to build colonies nice and close to the star for warmth, the future is indeed bright !

    • ljk April 9, 2019, 9:09

      Not that close, unless you like melting and having your planet torn out from under your feet!

      • Michael April 9, 2019, 10:13

        The temperature can be quite tolerable and the shear forces managable but getting into and out of the gravity well would require a bit of effort.

  • Harry R Ray April 16, 2019, 11:15

    Discovery of a Meteor of Interstellar Origin.” by Amir Siroj, Abraham Loeb. The authors conclude that this 0.45 meter object detected on 17:05:34 UTC, January 8, 2014(PREDATING `Oumuamua, and thus laying claim to being the EARLIEST, albeit NOT the first interstellar object ever detected)was of natural origin(i.e. NOT part of a defunct lightsail),entered Earth’s atmosphere, and completely burned up before reaching the surface. Could this have been debris from a white dwarf?

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