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Planets Survive Red Giant Expansion

The most interesting thing about the worlds known as KOI 55.01 and KOI 55.02 is not just the fact that they are — if current thinking holds — the smallest planets yet detected around an active star other than our Sun, but that they are evidently survivors of the most extreme kind of experience. KOI 55, their host star, is of subdwarf B class, the exposed core of a red giant that has lost most of its gaseous envelope. The two planets that circle it are in such tight orbits that they would have been engulfed when the central star went through its red giant expansion.

What a scenario, one we’ve often contemplated in these pages as we look toward the future of our own Sun. We tend to think in terms of planets that survive the red giant phase by orbiting far enough from the primary not to be swallowed up in it — smaller worlds like Mercury, Venus and the Earth would not survive the experience. But KOI 55.01 and KOI 55.02 evidently were swallowed, and probably represent the remains of gas giants that underwent the plunge. At present, they are thought to have radii 0.76 and 0.87 times the Earth’s radius.

Elizabeth Green (University of Arizona) explains:

“When our sun swells up to become a red giant, it will engulf the Earth. If a tiny planet like the Earth spends 1 billion years in an environment like that, it will just evaporate. Only planets with masses very much larger than the Earth, like Jupiter or Saturn, could possibly survive.”

Green participated in this work as a member of a team led by Stephane Charpinet (Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse-CNRS). The team’s findings make an intriguing case for the proposition that planets may directly influence the development of their host stars in this late phase of stellar evolution. Here they seem to have done it by contributing to the mass loss that is necessary for a subdwarf B star to form.

Here’s Green again on how this would work:

“As the star puffs up and engulfs the planet, the planet has to plow through the star’s hot atmosphere and that causes friction, sending it spiraling toward the star. As it’s doing that, it helps strip atmosphere off the star. At the same time, the friction with the star’s envelope also strips the gaseous and liquid layers off the planet, leaving behind only some part of the solid core, scorched but still there.”

Finding remnant planets around a stellar core, planets that have passed through the maelstrom of red dwarf expansion, is surprising enough, but the work also came from an unexpected direction. The team’s objective had been to use Kepler data to study astroseismology, examining the rhythmic expansions and contractions that pressure and gravitational forces produce inside a star as it burns. It’s useful stuff because with enough data, astronomers can estimate the star’s mass, temperature and size, as well as learning something about its internal structure.

Astroseismology takes time as researchers accumulate information about the star’s variations in brightness and compare these to theoretical models of stellar interiors. Green had already been studying hot subdwarf stars in the galactic plane and had accumulated spectra of KOI 55 with instruments on Kitt Peak before the Kepler mission was launched. Kepler was able to show the star’s pulsational modes with great clarity and it was in the midst of examining the Kepler data that periodic modulations occurring every 5.76 and 8.23 hours began to turn up. The researchers were able to show that the modulations could not have been caused by internal pulsations.

Two planets were the best explanation, each orbiting closer to their star than Mercury is to the Sun. Conditions on these worlds today must be hellish, as KOI 55 is much hotter than the Sun, but their tight orbits tell us that things were once worse still, when the planets would have been engulfed during the star’s expansion.

“I find it incredibly fascinating that after hundreds of years of being able to only look at the outsides of stars, now we can finally investigate the interiors of a few stars – even if only in these special types of pulsators – and compare that with how we thought stars evolved,” Green said. “We thought we had a pretty good understanding of what solar systems were like as long as we only knew one – ours. Now we are discovering a huge variety of solar systems that are nothing like ours, including, for the first time, remnant planets around a stellar core like this one.”

The paper is Charpinet, “A compact system of small planets around a former red-giant star,” Nature 480, 496–499 (22 December 2011). Abstract available. This University of Arizona news release offers further details.


Comments on this entry are closed.

  • James Pailly December 22, 2011, 15:51

    It’s really exciting to see all the extra stuff the Kepler Mission discovers in addition to the exoplanets it was looking for. I take it KOI 55.01 and 55.02 aren’t confirmed planets yet, or they wouldn’t be listed as KOI’s. Am I correct on that, and do we know how long before NASA can confirm them?

  • jkittle December 22, 2011, 16:22

    as so many stars have close in giant planets this is a previously unexplored issue. Imagine visiting a world post apocalypse !
    a great “end of the world” scenario
    how do they know the planets did not just migrate in was the gas envelop dissipated due to the tremendous drag ?

  • Tom Baty December 22, 2011, 21:25

    J Kittle….was I thinking the same thing as I read the article.

  • Scott G. December 22, 2011, 22:16

    This story reminds me a bit of this recent headline: Comet Lovejoy Survives Brush With the Sun. And from Wikipedia’s entry on the event: “The comet’s perihelion took it through the Sun’s corona on 16 December 2011 at 00:35 UTC, as it passed approximately 140,000 kilometres (87,000 mi) above the Sun’s surface.”

  • djlactin December 23, 2011, 6:27

    I wonder what Jupiter/Saturn (+Uranus/Neptune?) would look like after surviving (if they did) such an ordeal. Perhaps only the core (whatever is it made of, in addition to metallic Hydrogen) would remain. jkittle is correct in concluding that their orbital radii would diminish due to drag. Perhaps this finding is our solar system’s fate.

  • A. A. Jackson December 23, 2011, 7:27

    Asymptotic giant branch (AGB) stars with planet (and brown dwarf) companions have been the subject of some study since the early 1980’s.
    For the Sun and the solar system the evolution is a bit complex. During the Sun’s time on the AGB it can lose more that 30 percent of its mass , the orbits from Mars outward will seemingly widen. While Earth, Venus and Mercury will probably have orbital decay into the Sun. (1,2)

    For AGBs with Jovian planets close in quite a lot of activity is expected.
    Accretion shocks and interaction of the Jovian planet’s magnetic field with the red giant atmosphere.
    There have been predictions of all kinds of observational consequences during this phase, not sure the status.
    This is an interesting case , seems there are some other examples of hot rock , possible Jovian cores, about other post AGBs too.
    All manner of surprises in this Exoplanet story.

    (1) Pogge, Richard W. “The Once and Future Sun”,New Vistas in Astronomy, 1997.
    (2) Hecht, Jeff “Fiery future for planet Earth”. New Scientist , 1994.

  • Ole Burde December 23, 2011, 7:56

    “how do they know the planets did not just migrate in was the gas envelop dissipated due to the tremendous drag ?”
    Thats a VERY good question, so good that it probably cant be anwered !

  • ljk December 23, 2011, 9:51

    Earth may not be engulfed by Sol:


    However, any life on our planet will be in big trouble just about one billion years from now regardless of whether Earth ends up inside the bloated reddish remains of our star or not.

  • ljk December 23, 2011, 11:43

    djlactin said on December 23, 2011 at 6:27:

    “I wonder what Jupiter/Saturn (+Uranus/Neptune?) would look like after surviving (if they did) such an ordeal. Perhaps only the core (whatever is it made of, in addition to metallic Hydrogen) would remain. jkittle is correct in concluding that their orbital radii would diminish due to drag. Perhaps this finding is our solar system’s fate.”

    Some of their moons, like Europa and Titan, may enjoy a thaw for a few millions years. Who knows, maybe even life might get started there, though unlike Earth their evolution will be short-lived.

    All this assumes the Sol system will remain untouched by either cosmic forces or intelligences anytime between now and 5 billion C.E., which I have my doubts.

  • ljk December 23, 2011, 11:45

    It looks like Jupiter may not have a core by the time Sol starts turning into a red giant:


  • stephen December 25, 2011, 20:02

    Poul Anderson had a novel, Mirkheim, with an interesting premise.
    A star which becomes a supernova had a planet at just the right distance; particles from the supernova struck the planet with enough force to create an abundance of transuranic/superheavy elements. Any closer and the planet would have been destroyed. Any farther and the heavy elements wouldn’t be created.
    Has anybody tried simulating this, to see what masses and distances could produce this result?
    Unfortunately the transuranics aren’t as stable as we hoped, but there’d be an abundance of heavy elements.
    Life forms could use radioactivity, or nuclear fission, as an energy source.

  • ljk December 26, 2011, 13:12

    The first exoplanets found were around pulsars, so if they can survive a supernova, a red giant should also be survivable for at least some worlds.

  • johnq December 27, 2011, 13:40

    In my youth, I recall reading red giants being described as “red-hot vacuums.” Unfortunately, I do not recall who came up with the phrase. Clarke, perhaps? Thus planets surviving in such an envelope does not seem that odd to me. One can certainly see such an environment doing a number on the surface and atmosphere of a planet, but completely vaporize the world? I’m unsure that follows as a straightforward conclusion. There are likely a number of factors involved.

  • Randy July 10, 2012, 11:28

    ok if the sun starts to expand how long will it take to it become a fully red giant and how long will it take to come to earth orbit and engulf and kill us and what are signs we need to watch for thank send me a answer on email if u dont mind