Two More Earth-Sized Planets Discovered by Kepler, Orbiting Former Red Giant Star

Article written: 26 Dec , 2011
Updated: 24 Dec , 2015


Amid all of the news last week regarding the discovery by Kepler of two Earth-sized planets orbiting another star, there was another similar find which hadn’t received as much attention. There were two more Earth-sized planets also just discovered by Kepler orbiting a different star. In this case, however, the star is an old and dying one, and has passed its red giant phase where it expands enormously, destroying (or at least barbecuing) any nearby planets in the process before becoming just an exposed core of its former self. The paper was just published in the journal Nature.

The two planets, KOI 55.01 and KOI 55.02, orbit the star KOI 55, a subdwarf B star, which is the leftover core of a red giant star. Both planets have very tight orbits close to the star, so they were probably engulfed during the red giant phase but managed to survive (albeit “deep-fried”). They are estimated to have radii of 0.76 and 0.87 that of Earth, the smallest known exoplanets found so far orbiting an active star.

According to lead author Stephane Charpinet, “Having migrated so close, they probably plunged deep into the star’s envelope during the red giant phase, but survived.”

“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,” added Elizabeth ‘Betsy’ Green, an associate astronomer at the University of Arizona’s Steward Observatory. “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.”

The discovery was also unexpected; the star had already been the subject of study using the telescopes at Kitt Peak National Observatory, part of a project to examine pulsating stars. For more accurate measurements however, the team used data from the orbiting Kepler space telescope which is free of interfering atmospheric effects. According to Green, “I had already obtained excellent high-signal to noise spectra of the hot subdwarf B star KOI 55 with our telescopes on Kitt Peak, before Kepler was even launched. Once Kepler was in orbit and began finding all these pulsational modes, my co-authors at the University of Toulouse and the University of Montreal were able to analyze this star immediately using their state-of-the art computer models.”

Two tiny modulations in the pulsations of the star were found, which further analysis indicated could only come from planets passing in front of the star (from our viewpoint) every 5.76 and 8.23 hours.

Our own Sun awaits a similar fate billions of years from now and is expected to swallow Mercury, Venus, Earth and Mars during its expansion phase. “When our sun swells up to become a red giant, it will engulf the Earth,” said Green. “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.” The discovery should help scientists to better understand the destiny of planetary systems including our own.

This finding is important in that it not only confirms that Earth-size planets are out there, and are probably common, but that they and other planets (of a wide variety so far) are being found orbiting different types of stars, from newly born ones, to middle-age ones and even dying stars (or dead in the case of pulsars). They are a natural product of star formation which of course has implications in the search for life elsewhere.

The abstract of the paper is here, but downloading the full article requires a single-article payment of $32.00 US or a subscription to Nature.

Paul Scott Anderson is a freelance space writer with a life-long passion for space exploration and astronomy and has been a long-time member of The Planetary Society. He currently writes for Universe Today and His own blog The Meridiani Journal is a chronicle of planetary exploration.


11 Responses

  1. Aaron Oswald says

    ” Both planets have very tight orbits close to the star, so they were probably engulfed during the red giant phase but managed to survive (albeit “deep-fried”).”

    Whoawhoawhoa…. can that actually happen?

    • Torbjörn Larsson says

      It is the 2nd time this has been observed (if indeed this is) IIRC.

      Remember the exoplanet that orbits a star that originated in a dwarf galaxy outside the Milky Way? It too was a survivor of a similar deep-fry interaction resulting in a subdwarf B star with no companion star as the usual culprit.

      Apparently subdwarf B stars form if they can’t go to white dwarf state by starting to burn helium because they loose too much hydrogen. They loose helium with a companion, which in many or most cases can be companion stars. (Most stars are twins.)

      But now we have another pathway that indirectly should predict the frequency of exoplanets. Neat!

      • magnus.nyborg says

        Your overall explanation is good, but the following quote from the Wikipedia article lost a bit of ‘zist’ in your explanation:

        “These stars represent a late stage in the evolution of some stars, caused when a red giant star loses its outer hydrogen layers before the core begins to fuse helium.”

        The start is that these stars loose the hydrogen envelope at a too early stage, basically exposing deeper and therefor hotter parts. And to this part i suspect you are correct that orbiting close planets may aswell aid in this process, by basically slingshooting the hydrogen layer out of the system.

        “During the 1960s spectroscopy discovered that many of the sdB stars are deficient in helium, with abundances below that predicted by the big bang theory.”

        Many are also deficit in helium, but not all. Im not certain i understand the process here since exposing underlying layers seems to mean a more helium-dominant layer. But perhaps it is part of some dredge-up process or something other.

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    • Member
      Anonymous says

      I’m surprised too – I would have expecting the planet to spiral inwards in the same way (and for the same reasons) as an object might fall to Earth if its orbit takes it into Earth’s atmosphere.

      I can only imagine that the stellar pressure (the same thing that keeps a star from collapsing ever inward) keeps the planet ‘bouyant’, so that it can continue to orbit within the star’s envelope. Maybe its orbit migrates a little. I don’t know. Certainly I would like to know more.

      • magnus.nyborg says

        I believe I understand what your implicit model is, but I have a slightly different model that might explain this better. Bare with me, I am speculating:

        The model requires one or more sufficiently large planets initally orbiting relatively close to the border that the star would swell to if it did swell up to a red-giant the way a lone star would. In our solarsystem i would suspect Jupiter is a little to far out for this process, but if jupiter was orbiting in Earths place i can definitely see it work (I might be wrong)

        As the star is swelling, the planet(s) keeps the orbit relaively clean by gravitational interaction, meaning slowly the very loose hydrogen is slingshot (well, not exactly that dramatic, more like gently sent) out of the system. This does decrease the planets orbit, but as the processes are so slow it simply keeps migrating slowly inwards, while continuing to keeping its orbit ‘clean’ Some of the material slingshot might not even leave the system, its just ‘pushed’ into a wider disc.

        During the red-giant phase (or during the pre-red-giant phase) the outer layers are extremely loosely bound, and even a gentle effect will remove it. This means that the loss in angular momentum of the planet is relatively small.

        Since this would be a balanced process, with the planets migrating gradually inwards until there is no more material expelled, the process can end up in two ways.
        1. Planets incorporated with the bulk of the star.
        2. Planets stops migrating somewhere before, likely the migration stops or is reduced at the point where helium fusion is initiated in the core, because at that point the star will shrink overall. I am pretty convinced a jupiter would loose the atmosphere in this process, leaving only its core – but in this particular case the remaining planets are even less massive. Neptunes migrating inwards perhaps.

        I am very unsure of the more exact requirements on the part of the planets, but overall I believe this can be a working process. A planet too small would not be able to keep the orbit clean, and would end up spiralling inwards within the atmosphere as you indicated, but it would then also not be able to help expelling the hydrogen atmosphere of the star.

        Does this seem plausible?

    • Member
      Anonymous says

      I’m surprised too – I would have expecting the planet to spiral inwards in the same way (and for the same reasons) as an object might fall to Earth if its orbit takes it into Earth’s atmosphere.

      I can only imagine that the stellar pressure (the same thing that keeps a star from collapsing ever inward) keeps the planet ‘bouyant’, so that it can continue to orbit within the star’s envelope. Maybe its orbit migrates a little. I don’t know. Certainly I would like to know more.

  2. magnus.nyborg says

    Earthlike planets could survive in this manner.

    But: This could also be the remaining cores of Neptunian-like planets that lost the atmosphere in this hot encounter.

  3. David Frankis says

    You can download supplementary information from the Nature website. Although it’s a tough read (and I’m no expert), it’s clear that these planets *don’t* transit. As far as I can tell, they are detecting the changing brightness due to the changing phases of the planets as they go round their star.

    As well as being the smallest measured exoplanets (PSR 1257 12 b is almost certainly less massive but its radius is unknown), they also have the shortest periods for (reasonably) ordinary planets – only the ‘diamond’ planet PSR 1719-14 b has a shorter period. In many ways this is a much more interesting discovery than Kepler-20.

  4. Torbjörn Larsson says

    This article hits some good points but still misses points of interest here:

    – These are not confirmed exoplanets as Kepler-20, these are candidate exoplanets as the lingering KOI designation implies. (Thanks, David Frankis!)

    – They use a slightly different observation technique than the usual. (Again thanks to David.)

    – This is, IIRC, the 2nd time this has been observed. The former exo-galactic exoplanet HIP-13044b seen way back was a similar star + deep fry planet. Former, since it was observed in the Milky Way, but inside a remnant star stream from a torn asunder dwarf galaxy in the process of absorption by MW.

    – The observed stars subdwarf B status in combination with it being a single star puts up the hypothesis that also planets can strip stars from hydrogen and ushering in the subdwarf state. (IIRC an earlier press release.)

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