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Space NASA

First Planet Known To Orbit a White Dwarf Is Falling Apart (nasa.gov) 67

Posted by Soulskill from the possible-death-star-targets dept.
schwit1 writes: It's virtually certain that some white dwarfs still have planets in orbit despite their violent histories, but seeing those planets has proven difficult... at least, until now. Astronomers using the Kepler space observatory have spotted a planet circling around WD 1145+017, a white dwarf 570 light years away. Not that it's in great shape, mind you. The unusual light signature (PDF) from the dying star hints that the planet is disintegrating under the star's gravitational pressure, leaving behind a giant dust cloud. Researchers suspect it fell into its fatal orbit after the star's rapid change in mass triggered a planetary collision.

You should see more discoveries like this in the future, since the weaker light of a white dwarf is less likely to obscure planets. There's even a chance (however small) that collisions have bumped some planets into habitable zones, giving scientists an unusually clear view of worlds that could support life. Either way, it's evident that planetary systems don't vanish simply because their host stars are running out of time.
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First Planet Known To Orbit a White Dwarf Is Falling Apart More

First Planet Known To Orbit a White Dwarf Is Falling Apart

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  • Star's rapid change in mass? (Score:3)

    by sinij ( 911942 ) on Friday October 23, 2015 @11:46AM (#50788415)
    Can someone explain to non-physicist how this rapid change in mass happens?
    • I was wondering the same thing. Maybe its referring to a nova or something like that so there's not much less.

      I would think that in a red giant or even black hole stage it would still have the same mass.

    • Re: (Score:1)

      by Anonymous Coward

      From TFA:

      As stars like our sun age, they puff up into red giants and then gradually lose about half their mass, shrinking down to 1/100th of their original size to roughly the size of Earth. This dead, dense star remnant is called a white dwarf.

      • Still doesn't change mass. Yes, density is a lot greater, but gravity doesn't care about that.

        • Re: (Score:2, Insightful)

          by Anonymous Coward

          "gradually lose about half their mass,"

          "Still doesn't change mass"

          So losing half the mass doesn't change the mass?

        • Re: Star's rapid change in mass? (Score:4, Insightful)

          by Anonymous Coward on Friday October 23, 2015 @12:30PM (#50788767)

          The mass of the star has changed, in that perhaps half of its mass has been expelled into a planetary nebula. That expelled mass is beyond the planet's orbit, and the orbit of the planet is subsequently determined only by the remaining stellar mass. Also, that expelled mass is moving away from the remaining white dwarf, and is being diluted in interstellar space. I've forgotten enough college physics to be unable to say what the expected effect on the planet is due to the combined effects of the force of expulsion and the reduced mass of the central star on the planet's orbit should be (does it move in or out, etc).

          • Re: (Score:3)

            by cyn1c77 ( 928549 )

            I've forgotten enough college physics to be unable to say what the expected effect on the planet is due to the combined effects of the force of expulsion and the reduced mass of the central star on the planet's orbit should be (does it move in or out, etc).

            Ah, I still remember that extremely painful integration in BC Calculus.

            The planet's orbit increases due to the decrease in stellar mass: F = G M1 M2 / r^2 . (The stellar mass expanded around the planet will asymptotically cancel itself out as the mass expansion radius grows large relative to the planet's orbit.)

            It's tricky to mentally model the effect of force expulsion on the planet. But if it interacted significantly with the planet, it would have forced it radially outwards with a relatively short imp

        • Comment removed based on user account deletion
    • It doesn't. The media have confused surface gravity of a much denser object with gravity at orbit distance, which will be unchanged.

      • Well, the actual Nasa article talks about changes in mass, and stars pretty constantly eject material.

        And this [uoregon.edu] says:

        This also means the massive stars (with masses greater than 1.4 solar masses) must shed most of their mass as planetary nebula or the final contraction to a white dwarf cannot be stopped by the degenerate electrons.

        So, I'm more inclined to believe there is loss of actual mass going on.

        It certainly sounds like changes in mass are part of the explanation for the mechanics of this. (Not that I c

      • Re: (Score:1)

        by Anonymous Coward

        This. A typical main-sequence star (like our Sun), will eventually burn through (fuses) it's hydrogen, it then fuses the resulting helium and other elements and expands in a red-giant (for low and medium mass stars, high mass stars will form supergiants). Eventually the star will shed its outer layers (forming planetary nebula) and the core of the star made up of carbon and oxygen will collapse into a very dense white dwarf. White dwarves cannot sustain fusion, and as a result succumb to immense gravita

    • Re: (Score:2)

      by ceoyoyo ( 59147 )

      When a star starts to run out of hydrogen in it's core fusion slows and the core contracts, and gets hotter. If it gets hot enough, it will start fusing helium, then carbon. The core ends up small, hot and producing a lot of energy. That energy causes the outer layers of hydrogen and helium to expand and the star becomes a red giant. Material from the outer layers eventually gets blown entirely off the star to form a nebula. That's where the mass goes.

      I guess it's fairly rapid in the context of a star'

    • Main sequence stars die in different ways depending on start mass. Near the end of its life the star, like our sun will expand to huge sizes. The outer layers are sheded and eventually the fusion process stops. What is left collapses. Our sun is not big enough to become a neutron star, so it eventually becomes a white dwarf. Electron degenrate matter basically.

  • We'll probably not see many of these (Score:1)

    by Anonymous Coward
    regardless of what the summary says, because
    A) The star itself is smaller than a 'normal' star, making an occlusion more unlikely, and
    B) Most inner planets are likely gone after the red giant phase, leaving only planets further away which are less likely to occlude the line of sight from Earth, and
    C) Any configurations like this particular one are fleeting and on a cosmological timescale it is exceedingly unlikely we'll catch it at the right moment.

  • habitable zone? (Score:4, Insightful)

    by phayes ( 202222 ) on Friday October 23, 2015 @11:57AM (#50788493) Homepage

    Nope.

    White dwarfs are stars that have gone through an expansion to red giants & then shrink back down once they run out of low atomic level fuel like hydrogen & helium.

    All planets close enough to be in a white dwarf's "habitable zone" would have been well inside the star during the star's red giant phase.

    Unless someone comes up with a mechanism for the planets to escape from the red giant & then migrate even further inward to the white dwarf's now much smaller & closer "habitable zone", its extremely implausible.

    Somebody please reassure me that this is once again a "journalist" attempting to talk of matters that far outstrip his comprehension & not an astrophysicist gone barking mad.

    • Re: (Score:1)

      by Anonymous Coward

      Unless someone comes up with a mechanism for the planets to escape from the red giant & then migrate even further inward to the white dwarf's now much smaller & closer "habitable zone", its extremely implausible.

      The mechanism is in the summary, along with the disclaimer that it's very unlikely but possible. I get not RTFA, but at least read to the end of the summary.

      • Re: (Score:2)

        by phayes ( 202222 )

        The posited mechanism is implausible to the point of being ridiculous but then your post was probably typed by a bunch of monkeys so I suppose that implausible is no longer a problem...

    • Re: (Score:1)

      by Anonymous Coward

      Nope.

      White dwarfs are stars that have gone through an expansion to red giants & then shrink back down once they run out of low atomic level fuel like hydrogen & helium.

      All planets close enough to be in a white dwarf's "habitable zone" would have been well inside the star during the star's red giant phase.

      Unless someone comes up with a mechanism for the planets to escape from the red giant & then migrate even further inward to the white dwarf's now much smaller & closer "habitable zone", its extremely implausible.

      Somebody please reassure me that this is once again a "journalist" attempting to talk of matters that far outstrip his comprehension & not an astrophysicist gone barking mad.

      The red giant phase causes the start to swell up to massive sizes, albeit at insanely low density. This actually could have an aero-braking effect on planets further out, causing them to spiral inward, and they may survive this phase and end up close enough to be in the habitable zone BUT...
      Because the luminosity of white dwarves is so much lower than main sequence stars, the habitable zone is much much closer than a main-sequence star. This causes two issues. One is that as the habitable zone get closer, i

      • Re: (Score:3)

        by Rei ( 128717 )

        On the other hand, when you're subjecting planets to increased tidal forces, you're also unlocking a new source of energy: tidal flexure heating. You're bending a massive chunk of rock into a new shape, there's a tremendous amount of heat released in the process (you're probably also tidal locking it if it wasn't already).

        Counterproductive if the body ends too close to the star, but useful if the body ends up too far from the star. Unless it's to the extremes covered in this article where the tidal forces

    • Re: (Score:1)

      by Anonymous Coward

      All planets close enough to be in a white dwarf's "habitable zone" would have been well inside the star during the star's red giant phase.

      Unless someone comes up with a mechanism for the planets to escape from the red giant & then migrate even further inward to the white dwarf's now much smaller & closer "habitable zone", its extremely implausible.

      Why have you assumed that a planet must escape from the "habitable zone" before the red giant phase, and then migrate back in afterwards? There's no need for the planet to have been close to the star before the red giant phase. If a planet begins well away from the star, and only comes near it after it's become a white dwarf, you avoid half the problem: you need to explain it migrating inwards, but you don't need a mechanism for it to migrate outwards earlier.

      An inward migration is the easier problem to s

  • >> You should see more discoveries like this in the future

    I'm sorry, I don't currently have access to the Kepler space observatory. Perhaps YOU'LL see...

  • We don't call them "White Dwarf" any more. Now, they're known as "White Little People".

  • the planet is disintegrating under the star's gravitational pressure

    I'm guessing that's not the actual scientific term for whatever's happening to it. So what is? Is it a tidal forces thing?

    • See the Roche limit, this term describes the boundary where you are at risk of disintegrating. A small moon around a gas giant would end up as big Saturnian rings.
      Yes ocean tides would be a very tiny version, or Jupiter melting Io is more dramatic but not quite disintegrating.

      If the moon came much closer I'm sure we'd have no danger of the Earth disintegrating but perhaps we would all be dead from earthquakes and tsunamis (or worse)

  • Considering white dwarfs live an insanely long time, the star isn't "running out of time."

    If a habitable planet was orbiting a white dwarf, life on that planet could potentially go on for billions and billions of years, barring any planet-killing catastrophes. The star would slowly cool, but life once formed might be able to adapt to the cooling temperatures over billions of years.

    A red dwarf would be better, though. They're practically immortal and keep a steady output over their lifetime. Only problem

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