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Massive Planets Might Escape Stellar Engulfment Largely Undiminished

The views expressed are those of the author and are not necessarily those of Scientific American.


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exoplanets, red giant, stellar evolution

Artist's conception of the planets orbiting KIC 05807616. Credit: S. Charpinet

Having your planet swallowed by a star is no fun. But some planets might be able to run the astrophysical gauntlet and make it through more or less intact.

When a star comparable to or somewhat larger than the sun enters advanced age, it swells up into a red giant, expanding far beyond its original radius. In the process, the star’s ballooning atmosphere will consume any nearby planets—such is the fate awaiting the planets of the inner solar system, Earth most likely included.

Mercury, Venus and Earth are all too small to endure engulfment, and will quickly spiral in toward the sun due to drag forces from the surrounding stellar atmosphere. Larger planets or substellar objects called brown dwarfs, however, can actually dispel the star’s bloated exterior and survive. But they may emerge somewhat worse for wear. The density of a star’s expanded atmosphere can strip away the outer layers of an orbiting planet, so what comes out may be very different from what went in.

A new study by Jean-Claude Passy of the American Museum of Natural History in New York City and the University of Victoria in British Columbia and his colleagues in the Astrophysical Journal Letters seeks to uncover what a handful of engulfment survivors may have looked like before their close encounter with a star.

In recent years astronomers have found several planets or brown dwarfs in close orbits around aged stars that should already have passed through their swollen phase. Some of the planetary and brown dwarf survivors, unsurprisingly, are giants—one weighs 55 times as much as Jupiter. But some are considerably smaller—the two planets discovered last year orbiting the former red giant star KIC 05807616 are each less massive than Earth.

The researchers concluded that the modest-size planets orbiting KIC 05807616 must have undergone severe changes during engulfment by their star. These planets, Passy and his colleagues estimate, are remnants of a much larger world—or worlds—a few times the mass of Jupiter. In other words, the progenitor planet was hundreds of times as massive as the survivor.

The larger planets and brown dwarfs known to circle other evolved stars were a different story altogether. Those objects probably escaped engulfment more or less unscathed, the new study found. One caveat: Passy and his colleagues did not consider the effects of planetary evaporation under the star’s heat. The researchers note that evaporation could help diminish planets during stellar engulfment but report that the efficiency of the process is not yet well understood.

About the Author: John Matson is an associate editor at Scientific American focusing on space, physics and mathematics. Follow on Twitter @jmtsn.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. jtdwyer 5:03 pm 10/24/2012

    The late life changes in stars is definitely presents challenges for their planets.

    However, when a star like the Sun transitions from a main sequence star to a Asymptomatic Giant Branch star, they eventually expel around 30% of their total mass. It’s overly simplistic to assume that the observed location of, especially multiple, planets of a red giant or eventual white dwarf represents their prior locations. As mass distribution changes it is likely to affect the orbits of planets: they cannot sit like ducks in their ‘original’ orbit awaiting engulfment by star whose atmosphere is expanding…

    Link to this
  2. 2. Acoyauh2 6:45 pm 10/24/2012

    Agree with jtdwyer. In addition. the actual friction a planet would suffer from the star’s atmosphere would depend on the relative speeds of the star’s rotation vs the planet’s orbital velocity. Both a modified orbit and a ‘worn down’ planet could be the result if the friction was not enough to slow the planet into a dive.

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  3. 3. alan6302 9:01 pm 10/24/2012

    The predictions of the bible and Nostradamus indicate that our sun is very close to a solar event. 700 % for 7 days is the prediction. An event that size will destroy most of the food etc. The destruction of earth by sun expansion is only thousands of years away ,according to Nostradamus.

    Link to this
  4. 4. rloldershaw 10:22 pm 10/24/2012

    Given that trillions of unbound planetary-mass “nomad” objects have been discovered recently (Sumi et al, Nature, 19 May 2011), and that planets can be ejected from stellar systems, is it not about time for scientists to give far more attention to the sensible and evidence-based idea that planets can be captured by stars from the unbound planetary populations. Especially in dense molecular cloud environments.

    Scientists seem to be almost blind to planet capture models because they are mentally stuck in the old Laplacian “cloud collapse” model.

    Maybe they should study nature instead of decades-old theories that are poorly tested and have recently been put in doubt.

    Exoplanet systems have little in common with what was predicted on the basis of the old assumptions, and some of their characteristics defy explanations within the old assumptions.

    Time for new ideas!!!!

    Robert L. Oldershaw
    Discrete Scale Relativity
    http://www3.amherst.edu/~rloldershaw

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  5. 5. petehix 12:33 pm 10/25/2012

    @jtdwyer
    I’m almost certain that those behind the research of this did account for orbital separation when the bodies have changing mass, as this is the first thing my supervisor told me to do in my 3rd year undergraduate research project on the fate of planetary systems! :)
    I believe orbital separation increases only very, very slowly, even on the time scale of star lifetimes.

    @rloldershaw
    Let me say first that I do agree with fresh ideas! That’s exactly how science develops (very obvious example: Einstein vs Newton, or Quantum vs Classical)
    In the above project, the first article I was asked to read was studying a white-dwarf with a very close orbit brown-dwarf companion, and there was a section of the article discussing that the brown-dwarf may have been captured into the system, but it was ruled – I can’t remember why, forgive me I don’t have the article to hand.

    I’m very interested in this article, what with it being so closely related to my project. Maybe my research might even end up helping somewhere, eh? ;)

    Link to this
  6. 6. jtdwyer 1:56 pm 10/25/2012

    petehix – you’re making an assumption, since this article (like most referring to the fate of the Earth) makes no mention that orbits will vary. Perhaps if this and similar articles briefly explained this, your supervisor wouldn’t have had to make certain that you understood this, as a 3d year undergraduate… I’m simply doing the readers the same favor your supervisor did for you!

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  7. 7. thorargent 3:16 pm 10/25/2012

    Has the concept of chthonian planets escaped the notice of the American Museum of Natural History? This is a well-known process that strips the outer atmosphere from a gas giant world and leaves a remnant that could be very like a supermassive terrestrial world. This article is certainly interesting but it is not new material by any stretch of the imagination.

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  8. 8. Bird/tree/dinosaur/etc. geek 7:37 pm 10/25/2012

    alan6302, have you taken your meds yet today? Because if not, someone should notify your caretaker.

    Great article. The idea that a super-super Earth could have fragmented and vaporized to form several smaller planets is cool, if strange.

    Link to this
  9. 9. Ungolythe 8:34 pm 10/30/2012

    rloldershaw, you throw around the word “scientists” in a way belies your pedigree. Given that we have only relatively recently discovered exoplanets and the study that you cite was published only a year and a half ago how would the idea gain much traction? I’m assuming that you exclude Hagai Perets and Thijs Kouwenhoven from the monolithic body of “scientists” since they have recently published a paper on this very topic.

    Link to this
  10. 10. Acoyauh2 8:39 pm 10/30/2012

    @thoragent,
    They mention KIC 05807616 which seems to be chthonian. Seems to be. No chthonian planets have been confirmed so far, and only about 3-4 candidates exist. Chthonians are officially still theoretical. So ‘well known process’ is a bit of an overstatement, I think.

    To make this even more fun, we have Mercury next door, and some scientists have suggested it’s a chthonian planet because of its dense composition and other characteristics. Spit distance and still cannot say for sure. No, definitely not well known…

    Link to this
  11. 11. ABlack 1:11 pm 12/3/2012

    hopefully the earth will never be swallowed, the idea however unlikely is quite frightening. http://buysteroidsuk.co/

    Link to this

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